Lesson: Chemical Reactions and Equations

Question 1

Which of the statements about the reaction below are incorrect?

$2 P b O (s) + C (s) \to 2 P b (s) + C O_{2} (g)$

(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

$F e_{2} O_{3} + 2 A l \to A l_{2} O_{3} + 2 F e$

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) $3 H_{2} (g) + N_{2} (g) \to 2 N H_{3} (g)$

(b) $2 H_{2} S (g) + 3 O_{2} (g) \to 2 H_{2} O (l) + 2 S O_{2} (g)$

(d) $2 K (s) + 2 H_{2} O (l) \to 2 K O H (a q) + H_{2} (g)$

Question 6

Balance the following chemical equations.

(a) $H N O_{3} + C a {(O H)}_{2} \to C a {(N O_{3})}_{2} + H_{2} O$

(b) $N a O H + H_{2} S O_{4} \to N a_{2} S O_{4} + H_{2} O$

(d) $B a C l_{2} + H_{2} S O_{4} \to B a S O_{4} + H C l$

Solution:

(i) $2 H N O_{3} + C a {(O H)}_{2} \to C a {(N O_{3})}_{2} + 2 H_{2} O$

(ii) $2 N a O H + H_{2} S O_{4} \to N a_{2} S O_{4} + 2 H_{2} O$

(iii) $N a C l + A g N O_{3} \to A g C l + N a N O_{3}$

(iv) $B a C l_{2} + H_{2} S O_{4} \to B a S O_{4} + 2 H C l$

Question 7

Write the balanced chemical equations for the following reactions.

(a) $C a l c i u m h y d r o x i d e + C a r b o n d i o x i d e \to C a l c i u m c a r b o n a t e + W a t e r$

(b) $Z i n c + S i l v e r n i t r a t e \to Z i n c n i t r a t e + S i l v e r$

(d) $B a r i u m c h l o r i d e + P o t a s s i u m s u l p h a t e$

$\to B a r i u m s u l p h a t e + P o t a s s i u m c h l o r i d e$

Solution:

(a) $C a {(O H)}_{2} + C O_{2} \to C a C O_{3} + H_{2} O$

(b) $Z n + 2 A g N O_{3} \to Z n {(N O_{3})}_{2} + 2 A g$

(d) $B a C l_{2} + K_{2} S O_{4} \to B a S O_{4} + 2 K C l$

Question 8

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

(a) $P o t a s s i u m b r o m i d e (a q) + B a r i u m i o d i d e (a q)$

$\to P o t a s s i u m i o d i d e (a q) + B a r i u m b r o m i d e (s)$

(b) $Z i n c c a r b o n a t e (s) \to Z i n c o x i d e (s) + C a r b o n d i o x i d e (g)$

(d) $M a g n e s i u m (s) + H y d r o c h l o r i c a c i d (a q)$

$\to M a g n e s i u m c h l o r i d e (a q) + H y d r o g e n (g)$

Solution:

(a) $2 K B r (a q) + B a I_{2} (a q) \to 2 K I (a q) + B a B r_{2} (s)$ ; Double displacement reaction

(b) $Z n C O_{3} (s) \to Z n O (s) + C O_{2} (g)$ ; Decomposition reaction

(d) $M g (s) + 2 H C l (a q) \to M g C l_{2} (a q) + H_{2} (g)$ ; 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:

$Na(x)+ \frac{1}{2} {Cl}_{2} (s) \to NaCl(s)+ 411kJ$ of energy

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

Example:

$\begin{array}{l} 6 {CO}_{2} (g) + 6 H_{2} O(l) \overset{Sunlight}{\to} C_{6} H_{12} O_{6} (aq) + 6 O_{2} (g) \\ Glu \cos e \end{array}$

Question 10

Why is respiration considered an exothermic reaction? Explain.

Solution:

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

$C_{6} H_{12} O_{6} (a q) + 6 O_{2} (g) \to 6 C O_{2} (g) + 6 H_{2} O (l) + E n e r g y$

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:

$A B + E n e r g y \to A + B$

$C a C O_{3} \overset{H e a t}{\to} C a O (s) + C O_{2}$

Combination reaction:

$\begin{array}{l} A B + E n e r g y \to A + B \\ C a O (s) + H_{2} O (l) \to C a {(O H)}_{2} (a q) + H e a t \end{array}$

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:

$C a C O_{3} \overset{H e a t}{\to} C a O (s) + C O_{2}$

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

$2 A g C l (s) \overset{L i g h t}{\to} 2 A g (s) + C l_{2} (g)$

$2 A l_{2} O_{3} (a q) \overset{E l e c t r i c i t y}{\to} 4 A l (s) + 3 O_{2} (g)$

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.

$A + B X \to A X + B$ ; where A is more reactive than B

Example:

$F e (s) + C u S O_{4} (a q) \to F e S O_{4} (a q) + C u (s)$

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

$A B + C D \to A D + C B$

Example:

$N a_{2} S O_{4} + B a C l_{2} \to B a S O_{4} + 2 N a C l$

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:

$\begin{array}{l} 2 A g N O_{3} (a q) + C u (s) \to C u {(N O_{3})}_{2} (a q) + 2 A g (s) \\ S i l v e r N i t r a t e + C o p p e r \to C o p p e r N i t r a t e + S i l v e r \end{array}$

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:

$\begin{array}{l} N a_{2} S O_{4} (a q) + B a C l_{2} (a q) \to B a S O_{4} (s) + 2 N a C l (a q) \\ S o d i u m s u l p h a t e B a r i u m c h l o r i d e B a r i u m s u l p h a t e S o d i u m c h l o r i d e \end{array}$

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

Example 2:

$\begin{array}{l} C u S O_{4} (a q) + H_{2} S \to C u S (s) + H_{2} S O_{4} (a q) \\ C o p p e r s u l p h a t e H y d r o g e n s u l p h i d e C o p p e r S u l p h i d e S u l p h u r i c a c i d \end{array}$

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): $H_{2} S + C l_{2} \to 2 H C l + S$

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

Example (ii): $Z n O + C \to Z n + C O$

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.

$2 C u + O_{2} \overset{H e a t}{\to} 2 C u O (s)$

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.