Lesson: Chemical Reactions and Equations Do you want to know click on

Question 1

Which of the statements about the reaction below are incorrect?

2 PbO(s)+C(s)→2 Pb(s)+CO2(g)

(a) Lead is getting reduced.

(b) Carbon dioxide is getting oxidised.

(d) Lead oxide is getting reduced.

i. (a) and (b)

ii. (a) and (c)

iii. (a), (b) and (c)

iv. all

Solution:

(i)

Question 2

Fe2O3+2 Al→Al2O3+2 Fe

The above reaction is an example of a

(a) Combination reaction.

(b) Double displacement reaction.

(d) Displacement reaction.

Solution:

(d)

Question 3

What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.

(a) Hydrogen gas and iron chloride are produced.

(b) Chlorine gas and iron hydroxide are produced.

(d) Iron salt and water are produced.

Solution:

(a)

Question 4

What is a balanced chemical equation? Why should chemical equations be balanced?

Solution:

A chemical equation represents a chemical reaction. A chemical equation having an equal number of atoms of each element involved in the reaction, on both the sides of the equation is called a balanced chemical equation.

In a chemical reaction, as per the law of conservation of mass, the total mass of the reactants should be equal to the total mass of the products. For this to happen, the total number of atoms of each element should be equal on both the sides of the equation.

Question 5

Translate the following statements into chemical equations and then balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Solution:

(a) 3 H2(g)+N2(g)→2 NH3(g)

(b) 2 H2S(g)+3 O2(g)→2 H2O(l)+2 SO2(g)

(d) 2 K(s)+2 H2O(l)→2 KOH(aq)+H2(g)

Question 6

Balance the following chemical equations.

(a) HNO3+Ca(OH)2→Ca(NO3)2+H2O

(b) NaOH+H2SO4→Na2SO4+H2O

(d) BaCl2+H2SO4→BaSO4+HCl

Solution:

(i) 2 HNO3+Ca(OH)2→Ca(NO3)2+2 H2O

(ii) 2 NaOH+H2SO4→Na2SO4+2 H2O

(iii) NaCl+AgNO3→AgCl+NaNO3

(iv) BaCl2+H2SO4→BaSO4+2 HCl

Question 7

Write the balanced chemical equations for the following reactions.

(a) Calcium hydroxide+Carbon dioxide→Calcium carbonate+Water

(b) Zinc+Silver nitrate→Zinc nitrate+Silver

(d) Barium chloride+Potassium sulphate

→Barium sulphate+Potassium chloride

Solution:

(a) Ca(OH)2+CO2→CaCO3+H2O

(b) Zn+2 AgNO3→Zn(NO3)2+2 Ag

(d) BaCl2+K2SO4→BaSO4+2 KCl

Question 8

Write the balanced chemical equation for the following and identify the type of reaction in each case.

(a) Potassium bromide(aq)+Barium iodide(aq)

→Potassium iodide(aq)+Barium bromide(s)

(b) Zinc carbonate(s)→Zinc oxide(s)+Carbon dioxide(g)

(d) Magnesium(s)+Hydrochloric acid(aq)

→Magnesium chloride(aq)+Hydrogen(g)

Solution:

(a) 2 KBr(aq)+BaI2(aq)→2 KI(aq)+BaBr2(s); Double displacement reaction

(b) ZnCO3(s)→ ZnO(s)+CO2(g); Decomposition reaction

(d) Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g); Displacement Reaction

Question 9

What does one mean by exothermic and endothermic reactions? Give examples.

Solution:

Chemical reactions that involve release of energy in the form of heat, light or sound are called exothermic reactions.

Example:

Na(x)+12Cl2(s)→NaCl(s)+ 411kJ of energy

Reactions that require absorption of energy in order to proceed are called endothermic reactions.

Example:

6 CO2(g) + 6 H2O(l)−→−−−SunlightC6H12O6(aq) + 6 O2(g) Glucose

Question 10

Why is respiration considered an exothermic reaction? Explain.

Solution:

During respiration, glucose combines with oxygen in the cells and provides energy.

C6H12O6(aq)+6 O2(g)→6 CO2(g)+6 H2O(l)+Energy

This reaction is called respiration. Since energy is released in the process, it is an exothermic process.

Question 11

Why are decomposition reactions called the opposite of combination reactions?

Write equations for these reactions.

Solution:

In a decomposition reaction, a substance breaks to form two or more substances. These reactions require a source of energy to proceed.

On the other hand, in a combination reaction, two or more substances combine to form a new substance with the release of energy. Thus, decomposition reactions are just the opposite of combination reactions.

Decomposition reaction:

AB+Energy→A+B

CaCO3−→−Heat CaO(s)+CO2

Combination reaction:

AB+Energy→A+BCaO(s)+H2O(l)→Ca(OH)2(aq)+Heat

Question 12

Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Solution:

(a) Chemical equation where energy is supplied in the form of heat to the chemical reaction:

CaCO3−→−Heat CaO(s)+CO2

(b) Chemical equation where energy is supplied in the form of light to the chemical reaction:

2 AgCl(s) −→−Light2 Ag(s)+Cl2(g)

2 Al2O3(aq) −→−−−−Electricity4 Al(s)+3 O2(g)

Question 13

What is the difference between displacement and double displacement reactions?

Write equations for these reactions.

Solution:

In a single displacement reaction, a more reactive metal replaces a less reactive metal to from a compound.

A+BX→AX+B; where A is more reactive than B

Example:

Fe(s)+CuSO4(aq)→FeSO4(aq)+Cu(s)

In a double displacement reaction, two atoms or a group of atoms replace each other to form new compounds.

AB+CD→AD+CB

Example:

Na2SO4+BaCl2→BaSO4+2 NaCl

Question 14

In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Solution:

2 AgNO3(aq)+Cu(s)→Cu(NO3)2(aq)+2 Ag(s)Silver Nitrate+Copper→Copper Nitrate+Silver

Question 15

What do you mean by a precipitation reaction? Explain by giving examples.

Solution:

A reaction in which an insoluble solid is formed and settles at the bottom of the vessel is called a precipitation reaction. The insoluble solid is called a precipitate.

Example 1:

Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)Sodium sulphate Barium chloride Barium sulphate Sodium chloride

In this reaction, barium sulphate is obtained as a precipitate when sodium sulphate and barium chloride react.

Example 2:

CuSO4(aq) + H2S → CuS(s) + H2SO4(aq)Copper sulphate Hydrogen sulphide Copper Sulphide Sulphuric acid

In this reaction, copper sulphide is obtained as a precipitate when copper sulphate and hydrogen sulphide react.

Question 16

Explain the following in terms of gain or loss of oxygen with two examples each.

(a) Oxidation

(b) Reduction

Solution:

Oxidation: In a redox reaction, oxidation refers to the addition of oxygen to a reactant or removal of hydrogen from a reactant.

Reduction: In a redox reaction, reduction refers to the addition of hydrogen to a reactant or removal of oxygen from a reactant. The reactant that gains oxygen or loses hydrogen is said to be oxidized.

The reactant that loses oxygen or gains hydrogen is said to be reduced.

Example (i): H2S+Cl2→2 HCl+S

In this example, hydrogen sulphide is oxidised and chlorine is reduced.

Example (ii): ZnO+C→Zn+CO

In this example, zinc oxide is reduced and carbon is oxidised.

Question 17

A shiny brown coloured element ‘X’ on heating in air becomes black in colour.

Name the element ‘X’ and the black coloured compound formed.

Solution:

The equation of the reaction is given below.

2 Cu+O2−→−Heat2 CuO(s)

The element ‘X’ is copper (Cu) and the black-coloured compound formed is copper oxide (CuO).

Question 18

Why do we apply paint on iron articles?

Solution:

When iron is exposed to damp air, it gets covered with a red brownish substance called rust. This is because of corrosion of the surface of the metal due to formation of iron oxide.

We apply paint on iron articles to prevent them from rusting. After painting is done, the iron surface does not come in contact with moisture and air. Thus, the iron is prevented from rusting.

Question 19

Oil and fat containing food items are flushed with nitrogen. Why?

Solution:

Oxygen reacts with fat content in food substances and causes change in taste and smell. This process is called rancidity and it makes food items unsuitable for consumption. Nitrogen does not react with oil and fat. Nitrogen that is flushed in food items acts as an antioxidant and prevents the food items from reacting with oxygen.

Question 20

Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity

Solution:

(a) Corrosion: A process by which a metal oxide is formed on the surface of a metal article due to the action of air, moisture and chemicals is called corrosion or rusting. This process slowly destroys a metal article.

Example: When iron metal is exposed to open air and moisture, it reacts with oxygen and moisture to form hydrated iron oxide called rust.

(b) Rancidity: When food substances containing oil or fat are exposed to air, they get oxidised and become unfit for eating. This can be noticed by the change in colour, taste and smell of the food particles. This process is known as rancidity.

Example: Potato chips fried in oil starts giving unpleasant taste and smell when exposed to open air for long.

Lesson: Chemical Reactions and Equations

Question 1

Which of the following is not a physical change?

(a) Boiling of water to give water vapour

(b) Melting of ice to give water

(d) Combustion of Liquefied Petroleum Gas (LPG)

Solution:

(d)

Question 2

The following reaction is an example of a:

4 NH3(g)+5 O2(g)→4 NO(g)+6 H2O(g)

(i) Displacement reaction

(ii) Combination reaction

(iii) Redox reaction

(iv) Neutralisation reaction

(a) (i) and (iv)

(b) (ii) and (iii)

(d) (iii) and (iv)

Solution:

(c)

Question 3

Which of the following statements about the given reaction are correct?

3 Fe(s)+4 H2O(g)→Fe3O4(s)+4 H2(g)

(i) Iron metal is getting oxidised

(ii) Water is getting reduced

(iii) Water is acting as reducing agent

(iv) Water is acting as oxidising agent

(a) (i), (ii) and (iii)

(b) (iii) and (iv)

(d) (ii) and (iv)

Solution:

(c)

Question 4

Which of the following are exothermic processes?

(i) Reaction of water with quick lime

(ii) Dilution of an acid

(iii) Evaporation of water

(iv) Sublimation of camphor (crystals)

(a) (i) and (ii)

(b) (ii) and (iii)

(d) (iii) and (iv)

Solution:

(a)

Question 5

Three beakers labelled as A, B and C each containing 25 ml of water was taken. A small amount of NaOH, anhydrous CuSO4 and NaCl were added to the beakers A, B and C respectively. It was observed that there was an increase in the temperature of the solutions contained in beakers A and B, whereas in case of beaker C, the temperature of the solution falls. Which one of the following statement(s) is(are) correct?

(i) In beakers A and B, exothermic process has occurred.

(ii) In beakers A and B, endothermic process has occurred.

(iii) In beaker C exothermic process has occurred.

(iv) In beaker C endothermic process has occurred.

(a) (i) only

(b) (ii) only

(d) (ii) and (iii)

Solution:

(c)

Question 6

A dilute ferrous sulphate solution was gradually added to the beaker containing acidified permanganate solution. The light purple colour of the solution fades and finally disappears. Which of the following is the correct explanation for the observation?

(a) KMnO4 is an oxidising agent, it oxidises FeSO4

(b) FeSO4 acts as an oxidising agent and oxidises KMnO4

(d) KMnO4 is an unstable compound and decomposes in presence of FeSO4 to a colourless compound.

Solution:

(a)

Question 7

Which among the following is(are) double displacement reaction(s)?

(i) Pb+CuCl2→PbCl2+Cu

(ii) Na2SO4+BaCl2→BaSO4+2 NaCl

(iii) C+O2→CO2

(iv) CH4+2O2→CO2+2 H2O

(a) (i) and (iv)

(b) (ii) only

(d) (iii) and (iv)

Solution:

(b)

Question 8

Which among the following statement(s) is(are) true? Exposure of silver chloride to sunlight for a long duration turns grey due to

(i) The formation of silver by decomposition of silver chloride

(ii) Sublimation of silver chloride

(iii) Decomposition of chlorine gas from silver chloride

(iv) Oxidation of silver chloride

(a) (i) only

(b) (i) and (iii)

(d) (iv) only

Solution:

(a)

Question 9

Solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by liberation of heat. This process is called slaking of lime. Calcium hydroxide dissolves in water to form its solution called lime water. Which among the following is(are) true about slaking of lime and the solution formed?

(i) It is an endothermic reaction

(ii) It is an exothermic reaction

(iii) The pH of the resulting solution will be more than seven

(iv) The pH of the resulting solution will be less than seven

(a) (i) and (ii)

(b) (ii) and (iii)

(d) (iii) and (iv)

Solution:

(b)

Question 10

Barium chloride on reacting with ammonium sulphate forms barium sulphate and ammonium chloride. Which of the following correctly represents the type of the reaction involved?

(i) Displacement reaction

(ii) Precipitation reaction

(iii) Combination reaction

(iv) Double displacement reaction

(a) (i) only

(b) (ii) only

(d) (ii) and (iv)

Solution:

(d)

Question 11

Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated during electrolysis of water is

(a) 1:1

(b) 2:1

(d) 1:2

Solution:

(b)

Question 12

Which of the following(s) is(are) an endothermic process(es)?

(i) Dilution of sulphuric acid

(ii) Sublimation of dry ice

(iii) Condensation of water vapours

(iv) Evaporation of water

(a) (i) and (iii)

(b) (ii) only

(d) (ii) and (iv)

Solution:

(d)

Question 13

In the double displacement reaction between aqueous potassium iodide and aqueous lead nitrate, a yellow precipitate of lead iodide is formed. While performing the activity if lead nitrate is not available, which of the following can be used in place of lead nitrate?

(a) Lead sulphate (insoluble)

(b) Lead acetate

(d) Potassium sulphate

Solution:

(b)

Question 14

Which of the following gases can be used for storage of fresh sample of an oil for a long time?

(a) Carbon dioxide or oxygen

(b) Nitrogen or oxygen

(d) Helium or nitrogen

Solution:

(d)

Question 15

The following reaction is used for the preparation of oxygen gas in the laboratory

2 KClO3(s)−→−−−CatalystHeat2 KCl(s)+3 O2(g)

Which of the following statement(s) is(are) correct about the reaction?

(a) It is a decomposition reaction and endothermic in nature.

(b) It is a combination reaction.

(d) It is a photochemical decomposition reaction and exothermic in nature.

Solution:

(a)

Question 16

Which one of the following processes involves chemical reactions?

(a) Storing of oxygen gas under pressure in a gas cylinder.

(b) Liquefaction of air.

(d) Heating copper wire in presence of air at high temperature.

Solution:

(d)

Question 17

In which of the following chemical equations, the abbreviations represent the correct states of the reactants and products involved at reaction temperature?

(a) 2 H2(l)+O2(l)→2 H2O(g)

(b) 2 H2(g)+O2(l)→2 H2O(l)

(d) 2 H2(g)+O2(g)→2 H2O(g)

Solution:

(d)

Question 18

Which of the following are combination reactions?

(i) 2 KClO3−→−Heat2 KCl+3 O2

(ii) MgO+H2O→Mg(OH)2

(iii) 4Al+3 O2→2 Al2O3

(iv) Zn+FeSO4→ZnSO4+Fe

(a) (i) and (iii)

(b) (iii) and (iv)

(d) (ii) and (iii)

Solution:

(d)

Question 19

Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.

(a) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773K to form ammonia gas.

(b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.

(d) Ethene is burnt in the presence of oxygen to form carbon dioxide, water and releases heat and light.

Solution:

(a) N2(g)+3 H2(g)−→−−−773 KCatalyst2 NH3(g)

Reaction type: Combination reaction

(b) NaOH(aq)+CH3COOH(aq)→CH3COONa(aq)+H2O(l)

Reaction type: Double displacement reaction/Neutralisation reaction

Reaction type: Double displacement reaction/Esterification reaction

(d) C2H4(g)+3O2(g)→2CO2(g)+2H2O(g)+Heat+Light

Reaction type: Redox reaction/Combustion reaction

Question 20

Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.

(a) Thermite reaction, iron (III) oxide reacts with aluminium and gives molten iron and aluminium oxide.

(b) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride.

(d) Ethanol is burnt in air to form carbon dioxide, water and releases heat.

Solution:

(a) Fe2O3(s)+2 Al(s)→Al2O3(s)+2 Fe(l)+Heat

Reaction type: Displacement reaction/Redox reaction

(b) 3Mg(s)+N2(g)→Mg3N2(s)

Reaction type: Combination reaction

Reaction type: Displacement reaction

(d) C2H5OH(l)+3 O2(g)→2 CO2(g)+3 H2O(l)+Heat

Reaction type: Redox reaction/Combustion reaction

Question 21

Complete the missing components/variables given as x and y in the following reactions:

(a) Pb(NO3)2(aq)+2 KI(aq)→PbI2(x)+2 KNO3(y)

(b) Cu(s)+2 Ag NO3(aq)→Cu(NO3)2(aq)+x(s)

(d) CaCO3(s) →xCaO(s)+CO2(g)

Solution:

(a) x→(s)

y→(aq)

(b) x→2 Ag

y→(g)

(d) x→Heat

Question 22

Which among the following changes are exothermic or endothermic in nature?

(a) Decomposition of ferrous sulphate.

(b) Dilution of sulphuric acid.

(d) Dissolution of ammonium chloride in water.

Solution:

(b) and (c) are exothermic as heat is released in these changes.

(a) and (d) are endothermic as heat is absorbed in these changes.

Question 23

Identify the reducing agent in the following reactions

(a) 4 NH3+5 O2→4 NO+6 H2O

(b) H2O+ F2→HF+HOF

(d) 2 H2+O2→2 H2O

Solution:

(a) Ammonia (NH3)

(b) Water (H2O) as F2 is getting reduced to HF

(d) Hydrogen

Question 24

Identify the oxidising agent (oxidant) in the following reactions

(a) Pb3O4+8 HCl→3 PbCl2+Cl2+4 H2O

(b) 2 Mg+O2→2 MgO

(d) V2O5+5 Ca→2 V+5 CaO

(e) 3 Fe+4 H2O→Fe3O4+4 H2

(f) CuO+H2→Cu+H2O

Solution:

(a) Pb3O4

(b) O2

(d) V2O5

(e) H2O

(f) CuO

Question 25

Write the balanced chemical equations for the following reactions:

(a) Sodium carbonate on reaction with hydrochloric acid in equal molar concentrations gives sodium chloride and sodium hydrogen carbonate.

(b) Sodium hydrogen carbonate on reaction with hydrochloric acid gives sodium chloride, water and liberates carbon dioxide.

(c) Copper sulphate on treatment with potassium iodide precipitates cuprous iodide (Cu2I2), liberates iodine gas and also forms potassium sulphate.

Solution:

(a) Na2CO3+HCl→NaCl+NaHCO3

(b) NaHCO3+HCl→NaCl+H2O+CO2

Question 26

A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of the chemical reaction?

KCl(aq)+AgNO3(aq)→AgCl(s)+KNO3(aq)

Solution:

It is a double displacement and precipitation reaction.

Question 27

Ferrous sulphate decomposes with the evolution of a gas having a characteristic odour of burning sulphur. Write the chemical reaction involved and identify the type of reaction.

Solution:

2 FeSO4(s) −→−HeatFe2O3(s)+SO2(g)+SO3(g)

It is a thermal decomposition reaction.

Question 28

Why do fire flies glow at night?

Solution:

The protein present in fire flies oxidizes in the presence of an enzyme. As a result, light is emitted by the insect which appears as a glow in the darkness.

Question 29

Grapes hanging from the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?

Solution:

When grapes are attached to the plant, oxygen reaches the cells of the grapes and so aerobic respiration takes place.

But when plucked, oxygen cannot reach the cells of the grapes. This facilitates the microbes to grow under anaerobic condition and it leads to fermentation.

It is a chemical change.

Question 30

Which among the following are physical or chemical changes?

(a) Evaporation of petrol

(b) Burning of Liquefied Petroleum Gas (LPG)

(d) Curdling of milk

(e) Sublimation of solid ammonium chloride

Solution:

Evaporation of petrol, heating of an iron rod to red hot and sublimation of solid ammonium chloride are physical changes.

Burning of Liquefied Petroleum Gas (LPG) and curdling of milk are chemical changes.

Question 31

During the reaction of some metals with dilute hydrochloric acid, following observations were made.

(a) Silver metal does not show any change

(b) The temperature of the reaction mixture rises when aluminium (Al) is added.

(d) Some bubbles of a gas are seen when lead (Pb) is reacted with

the acid.

Explain these observations giving suitable reasons.

Solution:

(a) Silver metal does not react with dilute hydrochloric acid. Hence, there is no change.

(b) The reaction between aluminium metal and dilute hydrochloric acid is an exothermic reaction. Thus, the temperature of the reaction mixture rises when aluminium is added to hydrochloric acid.

(c) Sodium is one of the most reactive metals and when it reacts with dilute hydrochloric acid, the reaction is exothermic. Thus, the reaction of sodium metal and dilute hydrochloric acid is found to be highly explosive.

(d) When lead reacts with dilute hydrochloric acid, hydrogen gas is evolved. Given below is the chemical reaction:

Pb+2 HCl→PbCl2+H2

Question 32

A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and also write the chemical reactions involved.

Solution:

The substance X is Calcium oxide.

CaO(s)+H2O(l)→Ca(OH)2(aq)

Question 33

Write a balanced chemical equation for each of the following reactions and also classify them.

(a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution.

(b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas.

(d) Hydrogen sulphide gas reacts with oxygen gas to form solid sulphur and liquid water.

Solution:

(a)Pb(CH3COO)2+2 HCl→PbCl2+CH3COOH; Double displacement reaction

(b) 2 Na+2 C2H5OH→2 C2H5ONa+H2; Displacement reaction.

(c)Fe2O3+3 CO→2 Fe+3 CO2; Redox reaction.

(d) 2 H2S+O2→2 S+2 H2O; Redox reaction.

Question 34

Why do we store silver chloride in dark coloured bottles?

Solution:

On exposure to sunlight, the silver chloride may decompose as per the following reaction.

2 AgCl 2−→−LightAg+Cl2

Therefore, it is stored in dark coloured bottles.

Question 35

Balance the following chemical equations and identify the type of chemical reaction.

(a) Mg(s)+Cl2(g)→MgCl2(s)

(b) HgO(s)→Hg(l)+O2(g)

(d) TiCl4(l)+Mg(s)→Ti(s)+MgCl2(s)

(e) CaO(s)+SiO2(s)→CaSiO3(s)

(f) H2O2(l)−→UVH2O(l)+O2(g)

Solution:

(a) The equation is already balanced; it is a combination reaction.

(b) 2 HgO(s) −→−Heat2 Hg(l)+O2(g); it is a decomposition reaction.

(d) TiCl4(l)+2 Mg(s)→Ti(s)+2 MgCl2(s); it is a displacement reaction.

(e) The equation is already balanced; it is a combination reaction.

(f) 2 H2O2(l)−→UV2 H2O(l)+O2(g); it is a decomposition reaction.

Question 36

A magnesium ribbon is burnt in oxygen to give a white compound X accompanied by emission of light. If the burning ribbon is now laced in an atmosphere of nitrogen, it continues to burn and forms a compound Y.

(a) Write the chemical formulae of X and Y.

(b) Write a balanced chemical equation, when X is dissolved in water.

Solution:

(a) When magnesium ribbon burns in oxygen, the chemical equation of the reaction is:

2 Mg+O2→2 MgO

Here the white compound X is magnesium oxide.

If magnesium ribbon burns in an atmosphere that has only nitrogen, the reaction would be:

3 Mg+N2→Mg3N2

The compound formed would be magnesium nitride.

(b) When X, i.e. magnesium oxide is dissolved in water, magnesium hydroxide is formed. The chemical equation of the reaction is:

MgO+H2O→Mg(OH)2

Question 37

Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why?

Solution:

In the activity series of metals, zinc is placed above hydrogen. It means zinc is more reactive than hydrogen. Thus, zinc reacts with hydrochloric acid and hydrogen gas is evolved.

Zn+HCl→ZnCl2+H2

Copper, on the other hand, is placed below hydrogen. It means copper is less reactive than hydrogen. Hence, copper does not displace hydrogen from dilute hydrochloric acid.

Cu+HCl→No reaction

Question 38

A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.

(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.

(b) Name the black substance formed and gives its chemical formula.

Solution:

a) When kept in the open for a few days, the silver (Ag) reacts with the moisture, oxygen and gases present in the air. This phenomenon is called corrosion. This black coating is formed when silver reacts with hydrogen sulphide.

(b) When silver reacts with hydrogen sulphide (H2S) present in the air, the black substance formed is silver sulphide; and its chemical formula is Ag2S.

Question 39

On heating, blue coloured powder of copper (II) nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas X is formed.

(a) Write a balanced chemical equation of the reaction.

(b) Identity the brown gas X evolved.

(d) What could be the pH range of aqueous solution of the gas X?

Solution:

(a) When cupric nitrate is heated in a boiling tube, copper oxide, oxygen gas and nitrogen dioxide are formed. Balanced chemical equation of the referred reaction is:

2 Cu(NO3)2(s) −→−Heat2 CuO(s)+O2(g)+4 NO2(g)

(b) Nitrogen dioxide is a brown coloured gas evolved in the reaction.

(d) When nitrogen dioxide dissolves in water, it forms nitric acid. Water has a pH value of 7 while pH of an acid is less than 7. Therefore, the pH of the aqueous solution of the gas X will be less than 7.

Question 40

Give the characteristic tests for the following gases:

(a) CO2

(b) SO2

(d) H2

Solution:

(a) On reacting with carbon dioxide (CO2), lime water turns milky. This is due to the formation of insoluble calcium carbonate.

Ca(OH)2+CO2→CaCO3+H2O

(b) Sulphur dioxide (SO2) gas reacts with potassium permanganate solution (purple in colour) and makes it colourless. This is due to the action of the reducing agent sulphur dioxide (SO2)

2 KMnO4+2 H2O+5 SO2→K2SO4+2 MnSO4+2 H2SO4

(c) When we bring a burning candle near the mouth of the test tube containing oxygen (O2), the intensity of the flame increases. This confirms the presence of oxygen, as oxygen supports burning.

(d) When a burning candle is brought near hydrogen, it burns with a pop sound. This is the test for the presence of hydrogen gas.

Question 41

What happens when a piece of:

(a) Zinc metal is added to copper sulphate solution?

(b) Aluminium metal is added to dilute hydrochloric acid?

Also, write the balanced chemical equation if the reaction occurs.

Solution:

(a) When a piece of zinc metal is added to copper sulphate solution, it displaces copper from its solution. This happens because zinc is more reactive than copper. Due to this reaction, a solution of zinc sulphate is obtained.

Zn(s)+CuSO4(aq)→ZnSO4(aq)+Cu(s)

(b) Aluminium is more reactive than hydrogen. So, when a piece of aluminium metal is added to dilute hydrochloric acid, it displaces hydrogen from the dilute hydrochloric acid solution and forms aluminium chloride and hydrogen gas.

2 Al(s)+6 HCl(aq)→2 AlCl3(aq)+3 H2(g)

(c) As per the activity series, silver metal is less reactive than copper. Silver cannot displace copper from its salt solution. Therefore, no reaction takes place when a piece of silver is added to copper sulphate solution.

Ag(s)+CuSO4(aq)→No reaction

Question 42

What happens when zinc granules are treated with dilute solution of H2SO4, HCl, HNO3, NaCl and NaOH, also write the chemical equations if reaction occurs.

Solution:

(a) When zinc reacts with dilute H2SO4, zinc sulphate is formed. The reaction is:

Zn(s)+H2SO4(aq)→ZnSO4(aq)+H2(g)

(b) When zinc reacts with dilute HCl, zinc chloride is formed. The reaction is:

Zn(s)+2 HCl(aq)→ZnCl2(aq)+H2(g)

(c) When zinc reacts with dilute HNO3, zinc nitrate, water and nitrous oxide are formed. Reaction with dilute HNO3 is different as compared to other acids because nitric acid is an oxidising agent and it oxidizes H2 gas to form H2O.

4 Zn(s)+10 HNO3(aq)→4 Zn(NO3)2(aq)+5 H2O(l)+N2O(g)

(d) Zinc does not react with NaCl solution.

Zn(s)+NaCl(aq)→No reaction

(e) When zinc reacts with dilute NaOH solution, sodium zincate is formed. The reaction is:

Zn(s)+2 NaOH(aq)→Na2ZnO2(aq)+H2(g)

Question 43

On adding a drop of barium chloride solution to an aqueous solution of sodium sulphite, a white precipitate is obtained.

(a) Write a balanced chemical equation of the reaction involved.

(b) What other name can be given to this precipitation reaction?

Solution:

(a) When a drop of barium chloride solution is added to an aqueous solution of sodium sulphite, barium sulphite and sodium chloride are formed. The balanced chemical equation is:

Na2SO3(aq)+BaCl2(aq)→BaSO3(s)+2 NaCl(aq)

(b) This reaction is also known as double displacement reaction.

(c) BaSO3 is the salt of a weak acid H2SO3 (sulfurous acid). When a dilute acid such as HCl is added to barium sulphite, it produces a white precipitate of barium chloride (BaCl2), water and sulphur dioxide. Since BaCl2 is soluble in water, the white precipitate disappears.

Question 44

You are provided with two containers made up of copper and aluminium. You are also provided with solutions of dilute HCl, dilute HNO3, ZnCl2 and H2O. In which of the above containers these solutions can be kept?

Solution:

A) In copper container

(a) Dilute HCl: The solution of dilute HCl can be kept in a copper container as it does not react with dilute HCl.

(b) Dilute HNO3: The solution of dilute nitric acid ( HNO3 ) cannot be kept in a copper container as nitric acid acts as a strong oxidising agent and reacts with copper.

(c) ZnCl2: Zinc chloride can be kept in a copper container as zinc is more reactive than copper (Cu) and so there is no possibility of a displacement reaction.

(d) H2O: H2O can be kept in a copper container as copper does not react with water.

(B) In aluminium containers

(a) Dilute HCl: The solution of dilute HCl cannot be kept in an aluminium container as aluminium will undergo reaction with the solution of dilute HCl.

(b) Dilute HNO3: The solution of dilute HNO3 can be kept in an aluminium container as aluminium gets oxidised by dilute HNO3 to form a layer of Al2O3.

(c) ZnCl2: ZnCl2 cannot be kept in an aluminium container as aluminium is more reactive than zinc; so, it can displace zinc ion from the solution.

(d) H2O: Aluminium does not react with cold or hot water. Therefore, water can be kept in an aluminium container.

Lesson: Chemical Reactions and Equations

Question 1

Why should a magnesium ribbon be cleaned before burning in air?

Solution:

Magnesium is a highly reactive metal. It reacts with oxygen to form a layer of magnesium oxide on its surface. This layer is quite stable and prevents further reaction of magnesium with oxygen. Hence, the magnesium ribbon is cleaned to remove this layer.

Question2

Write the balanced equation for the following chemical reactions.

(i) Hydrogen+Chlorine→Hydrogen chloride

(ii) Barium chloride+Aluminium sulphate

→Barium sulphate+Aluminium chloride

(iii) Sodium+Water→Sodium hydroxide+Hydrogen

Solution:

(i) H2(g)+Cl2(g)→2 HCl(g)

(ii) 3BaCl2(s)+Al2(SO4)3(s)→3BaSO4(s)+2AlCl3(s)

(iii) 2 Na(s)+2 H2O(l)→2 NaOH(aq)+H2(g)

Question 3

Write a balanced chemical equation with state symbols for the following reactions.

i. Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.

ii. Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Solution:

(i) BaCl2(aq)+Na2SO4(aq)→BaSO4(s)+2 NaCl(aq)

(ii) NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l)

Question 4

A solution of a substance ‘X’ is used for white washing.

(i) Name the substance ‘X’ and write its formula.

(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Solution:

(i) The substance ‘X’ is calcium oxide. Its chemical formula is CaO.

(ii) CaO(s)+H2O(l)→Ca(OH)2(aq)

Question 5

Why is the amount of gas collected in one of the test tubes in Activity1.7 (electrolysis of water) double of the amount collected in the other? Name this gas.

Solution:

The given activity is electrolysis of water. In this activity, hydrogen and oxygen are collected in separate test tubes. A molecule of water ( H2O ) contains two hydrogen atoms and one oxygen atom. Therefore, the hydrogen and oxygen produced during electrolysis of water is in the ratio 2:1. Thus, the amount of hydrogen gas collected in one of the test tubes is double the amount of oxygen gas collected in the other.

Question 6

Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Solution:

Iron is more reactive than copper. When an iron nail is dipped in a copper sulphate solution, which is blue in colour, iron displaces copper from the solution and forms ferrous sulphate, which is green in colour.

Fe(s)+CuSO4(aq)→FeSO4(aq)+Cu(s)

Therefore, the colour of the copper sulphate solution changes from blue to green.

Question 7

Give an example of a double displacement reaction.

Solution:

Sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium chloride. This is a double displacement reaction.

Na2CO3(aq)+CaCl2(aq)→CaCO3(s)+2NaCl(aq)

Question 8

Identify the substances that are oxidised and the substances that are reduced in the following reactions.

(i) 4 Na(s)+O2(g)→2 Na2O(s)

(ii) CuO(s)+H2(g)→Cu(s)+H2O(l)

Solution:

(i) Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.

(ii) Copper oxide (CuO) is reduced to copper (Cu) while hydrogen ( H2 ) gets oxidised to water ( H2O ).

Questions

Q1	State the law of conservation of mass.

Q2	What happens when a silvery white strip of magnesium ribbon is ignited?

Q3	Name two common effects of oxidation we observe in our daily life.

Q4	How is a decomposition reaction different from a combination reaction?

Q5	Explain photolysis with an example.

Q6	On heating in open air, copper turns into a black coloured compound. Why does copper turn black?

Q7	What are the indications that confirm the completion of a chemical reaction?

Q8	List the main types of chemical reactions.

Q9	Explain a double displacement reaction with an example.

Q10	State the rules for writing symbols of physical states in a chemical equation.

Q11	Why does the colour of copper sulphate solution change when a piece of iron is dipped in it?

Q12	What is precipitation reaction? Give an example.

Q13	Explain the process of rusting.

Q14	Give an account of the shiny finish of the walls after a few days of white washing.

Q15	How can rancidity be prevented?

Q16	Explain precipitation reaction.

Q17	Explain combination reaction.

Q18	Explain redox reaction with an example.

Q19	Write a balanced chemical equation for the following chemical reactions. Also, identify and explain the type of reaction in each of the cases. a) H₂(g) + Cl₂(g) → HCl (g) b) ZnCO₃ (s) → ZnO (s) + CO₂ (g) c) KBr (aq) + BaI₂ (aq) → KI (aq) + BaBr₂ (s) d) Mg (s) + HCl (aq) → MgCl₂ (aq) + H₂ (g)

Q20

How are exothermic reactions different from endothermic reactions?

Questions

Q1	State the law of conservation of mass.

A1	The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.

Q2	What happens when a silvery white strip of magnesium ribbon is ignited?

A2	When a silvery white strip of magnesium ribbon is ignited, it burns with a dazzling white flame and forms a white powder of magnesium oxide.

Q3	Name two common effects of oxidation we observe in our daily life.

A3	a) Rancidity of food b) Corrosion of metal

Q4	How is a decomposition reaction different from a combination reaction?

A4	In decomposition reaction, a compound splits to form two or more substances, and the reaction is preceded by a source of energy. In combination reaction, two or more substances combine to form a new substance, with the release of energy. Thus, they are exactly opposite of each other.

Q5	Explain photolysis with an example.

A5	Photolysis is the decomposition of a chemical compound by the use of light energy. For example, silver chloride, in the presence of light, decomposes to form silver and chlorine gas is released.

Q6	On heating in open air, copper turns into a black coloured compound. Why does copper turn black?

Q7	What are the indications that confirm the completion of a chemical reaction?

A7	A chemical reaction can be confirmed by any of the following common observations: 1. Change in temperature 2. Transformation of colour 3. Evolution of gas 4. Change in state

Q8	List the main types of chemical reactions.

Q9	Explain a double displacement reaction with an example.

A9

A chemical reaction, in which two compounds react by the exchange of ions to form two new compounds is called a double displacement reaction.

Example:

AgNO₃ + NaCl → AgCl + NaNO₃

In this reaction, silver nitrate and sodium chloride react to form two new compounds, silver chloride and sodium nitrate.

Q10	State the rules for writing symbols of physical states in a chemical equation.

A10

The physical states of the reactants and products are written after their chemical formulae.

The gaseous state of reactants and products are represented by the notation (g).
The liquid state of reactants and products are represented by the notation (l).
The solid state of reactants and products are represented by the notation (s).
The word ‘aqueous’ (aq) is written if the reactant or product is present as a solution in water.

Examples:

CaCO₃(s) +heat→CaO(s) +CO₂ (g)

Here CaCO₃ and CaO are in the solid state and CO₂ is in the gaseous state.

Q11	Why does the colour of copper sulphate solution change when a piece of iron is dipped in it?

A11

When a piece of iron is dipped in copper sulphate solution, a brown coating of copper is formed on the surface of the iron piece and the colour of copper sulphate solution changes from blue to light green. This happens because the metal iron, being more reactive than copper, manages to displace the latter from its solution by single displacement reaction and iron sulphate solution is formed. So, the blue copper sulphate solution changes to green ferrous sulphate solution.

Fe (s) + CuSO₄ (aq) → Cu (s) + FeSO₄(aq)

Q12	What is precipitation reaction? Give an example.

A12

Any chemical equation that produces a precipitate can be called a precipitation reaction.

For example, sodium sulphate reacts with barium chloride, both as aqueous solutions, to form aqueous solution of sodium chloride along with white precipitation of barium sulphate in that solution.

Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + 2NaCl (aq)

Q13	Explain the process of rusting.

A13

The corrosion of iron is called rusting. There are two conditions for rusting:

a) Presence of air (oxygen)

b) Presence of water (moisture)

When iron is exposed to damp air for a considerable time, it gets converted into a reddish brown substance called rust. Rusting is an example a redox reaction.

The overall chemical reaction in which rust is formed is represented in the following manner:

4Fe (s) + 2H₂O (l) + 3O₂ (g) → 2Fe₂O_3*H₂O (s)

Here, iron combines with oxygen in the air, in the presence of water to form hydrated iron oxide (2Fe₂O_3*H₂O).

Q14	Give an account of the shiny finish of the walls after a few days of white washing.

A14

When quick lime (calcium oxide) is dissolved in water, it reacts vigorously to produce slaked lime (calcium hydroxide).

CaO (s) + H₂O (l) → Ca(OH)₂ (aq)

This solution of slaked lime is used for white washing walls. Calcium hydroxide reacts slowly with the carbon dioxide in the air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls.

Q15	How can rancidity be prevented?

A15

Rancidity refers to the spoilage of food due to decomposition of its oil-content by the process of oxidation. Rancidity can be easily detected by the alteration in taste and smell.

The substances used to prevent rancidity in fat-containing foods by oxidation are the antioxidants, such as ascorbic acid (vitamin C).

Rancidity can be prevented by:

1. Storing food in the cool atmosphere inside the refrigerators.

2. Stocking up food in air tight containers.

3. Adding antioxidants to foods.

4. Storing food in an environment of nitrogen.

Q16	Explain precipitation reaction.

A16

Some chemical reactions take place in aqueous solution which produce an insoluble solid product that settles at the bottom of the vessel. Such a chemical reaction is called a precipitation reaction and the solid product that settles at the bottom of the vessel is called the precipitate. Precipitation often occurs in double replacement reactions, and one of the most amazing of such reactions is the reaction between lead nitrate and potassium iodide:

2KI (aq) + Pb(NO₃)₂(aq) → 2KNO₃ (aq) + PbI (s)

^{Potassium iodide Lead nitrate Potassium nitrate Lead iodide}

Both the reactants, potassium iodide and lead nitrate are colourless solutions but when these solutions are poured together, the lead iodide product formed is a dramatic yellow solid which precipitates out of the solution.

Another example of precipitation reaction is when sodium sulphate reacts with barium chloride. In this reaction, barium sulphate is obtained as a precipitate. Hence, it is a precipitation reaction.

Na₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + 2 NaCl (aq)

^{Sodium sulphate Barium chloride Barium sulphate Sodium chloride}

The reaction of sodium carbonate with calcium chloride too is a precipitation reaction where, in the aqueous solution of sodium chloride, calcium carbonate is formed as a precipitate.

Na₂CO₃ (aq) + CaCl₂ (aq) → CaCO₃ (s) + 2NaCl (aq)

^{Sodium carbonate Calcium chloride Calcium carbonate Sodium chloride}

Q17	Explain combination reaction.

A17

In combination reaction, two or more reactants, which can be elements as well as compounds, combine to form a single new compound. The general equation used to represent a combination reaction is:

X + Y = XY

For example,

Fe + S → FeS

Here iron combines with sulphur to produce iron sulphide.

Combination reactions are usually exothermic because when the bond forms between the reactants, heat is released. For example, magnesium oxide reacts with carbon dioxide to combine and form magnesium carbonate.

MgO + CO₂ → MgCO₃

Q18	Explain redox reaction with an example.

A18

The word redox is a combination of the words reduction and oxidation. A chemical reaction in which oxidation (a reaction in which a reactant gains oxygen or loses hydrogen) as well as reduction (a reaction in which a reactant gains hydrogen or loses oxygen) takes place simultaneously; it is called an oxidation-reduction reaction or redox reaction.

Here is an example where copper undergoes oxidation because oxygen is added to copper.

2Cu + O₂ → 2CuO

Also, when hydrogen gas is passed over heated copper oxide, copper oxide loses oxygen and is thereby reduced to copper. The hydrogen, on the other hand, gains oxygen and is therefore oxidized to form water.

In other words, one reactant gets oxidized while the other gets reduced during a redox reaction.

Q19	Write a balanced chemical equation for the following chemical reactions. Also, identify and explain the type of reaction in each of the cases. a) H₂(g) + Cl₂(g) → HCl (g) b) ZnCO₃ (s) → ZnO (s) + CO₂ (g) c) KBr (aq) + BaI₂ (aq) → KI (aq) + BaBr₂ (s) d) Mg (s) + HCl (aq) → MgCl₂ (aq) + H₂ (g)

A19

a) The balanced chemical equation for the reaction would be:

H₂(g) + Cl₂(g) → 2HCl (g)

This is a combination reaction. Hydrogen reacts with chlorine to form hydrogen chloride.

b) This reaction is already balanced. This is a decomposition reaction. Zinc carbonate decomposes to form zinc oxide and carbon dioxide.

c) The balanced chemical equation for the reaction would be:

2KBr (aq) + BaI₂ (aq) → 2KI (aq) + BaBr₂ (s)

This is a double displacement reaction. Potassium bromide reacts with barium iodide to form potassium iodide and barium bromide.

d) The balanced chemical equation for the reaction would be:

Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)

This is a single displacement reaction. Magnesium reacts with hydrochloric acid to form magnesium chloride and hydrogen.

Q20	How are exothermic reactions different from endothermic reactions?

A20

Exothermic reactions	Endothermic reactions
Chemical reactions that release energy in the form of heat, light or sound are called exothermic reactions.	Chemical reactions that absorb energy or require energy, in the form of heat or light are called endothermic reactions.
Exothermic reactions are characterised by the giving away of heat or light while forming a chemical bond.	Endothermic reactions are characterised by taking away of heat while splitting a chemical bond.
For example, the burning of magnesium: 2Mg + O₂ → 2MgO + Heat energy	For example, in the thermal decomposition of limestone, i.e. calcium carbonate, lime (calcium oxide) and carbon dioxide are formed. This reaction happens at temperature above 900^°C because the reactants absorb heat from the surrounding. CaCO₃ + Heat energy → CaO + CO₂

Chemical Reaction and Equations 20 question F

Questions

Q1

Q2

Q3

Q4

Q5

Q6

Q7

Q8

Q9

Q10

Q11

Q12

Q13

Q14

Q15

Q16

Q17

Q18

Q19

Q20

Questions

Q1

A1

Q2

A2

Q3

A3

Q4

A4

Q5

A5

Q6

Q7

A7

Q8

Q9

A9

Q10

A10

Q11

A11

Q12

A12

Q13

A13

Q14

A14

Q15

A15

Q16

A16

Q17

A17

Q18

A18

Q19

A19

Q20

A20

Comments

Post a Comment

Popular posts from this blog

B ed 1st Year Class 2023

MATH CLASS

Saraswat odia grammar