# Chemical Reactions and Equations

## NCERT Solution

### Part 2

Question 11: Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer: In a combination reaction two or more substances are combined to form new substance while in the decomposition reaction one substance decomposes into two or more substances. Hence, the decomposition reactions are called opposite of combination reactions.

Decomposition reaction CaCO3 (s) ⇨ CaO (s) + CO2 (g)

Combination reaction H2 (g) + Cl2 ⇨ 2HCl (g)

Question 12: Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Answer: CaCO3 (s) ⇨ CaO (s) + CO2 (g)

In this reaction energy is supplied in the form of heat.

2AgCl (s) + sunlight ⇨ 2Ag (s) + Cl2 (g)

In this reaction energy is supplied in the form of light,i.e.sunlight

2H2O (l) + electricity ⇨ 2H2 + O2

In this reaction energy is supplied in the form of electricity.

## Chapter List

Question 13: What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer: When a more reactive element displaces a less reactive element from its salt solution, then it is called displacement reaction. While there is an exchange of ions between the reactants to produce new substances, it is called double displacement reaction.

In displacement reaction one displacement takes place. While in double displacement reaction two displacements takes place. Following are examples of displacement reaction and double displacement reaction.

Mg (s) + 2HCl (aq) ⇨ MgCl2 + H2 (g)

2KBr (aq) + BaCl2 ⇨ 2KCl (aq) + BaBr2 (s)

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.

Answer: 2AgNO3 (aq) + Cu (s) ⇨ Cu(NO3 )2 (l) + 2Ag (s)

Question 15: What do you mean by a precipitation reaction? Explain by giving examples.

Answer: Reactions in which precipitate is formed are called precipitation reactions. Generally in a precipitation reaction insoluble salts are settled down as precipitate.

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

In the above reaction, when sodium sulphate reacts with barium chloride, the white precipitate of barium sulphate is formed along with the sodium chloride.

Question 16: Explain the following in terms of gain or loss of oxygen with two examples each:

(a) Oxidation

Answer: When a substance gains oxygen or loses hydrogen during a reaction, it is called that the substance is oxidized. This phenomenon is called oxidation. In the following reaction, copper gets oxygen and hence is oxidised.

2Cu + O2 + Heat ⇨ 2CuO

CuO + H2 + Hear ⇨ Cu + H2O

In the above reaction hydrogen gains oxygen, and hence oxidized.

(b) Reduction

Answer: If a substance loses oxygen or gains hydrogen during a reaction, it is that the substance is reduced. This phenomenon is called reduction. In the following reaction zinc loses oxygen, and hence called is reduced.

ZnO + C ⇨ ZnCO

MnO2 + 4HCl ⇨ MnCl2 + 2H2O + Cl2

In this reaction Mangnese dioxide loses oxygen and hence is manganese dioxide is said to be reduced.

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.

Answer: Copper is a brown coloured shiny element. When heated in air, it becomes black in colour.

2Cu + O2 + Heat ⇨ 2CuO

Hence, the element ‘X’ is copper and black coloured compound formed is copper oxide.

Question 18: Why do we apply paint on iron articles?

Answer: When articles made of iron come in contact with the moisture present in air, it forms iron oxide, which is known as rust. Hence, paint is applied to prevent an iron article to come in contact with moisture present in air. Paint prevents rusting of iron.

Question 19: Oil and fat containing food items are flushed with nitrogen. Why?

Answer: When fats and oils are oxidised, they become rancid and their smell and taste change. This phenomenon is called rancidity. Hence, food items containing oil and fat are flushed with nitrogen which prevents them to get oxidized and becoming rancid.

Question 20: Explain the following terms with one example each:

(a) Corrosion

Answer: Corrosion: Metals react with oxygen which is present in the atmospheric moisture. This leads to formation of metal oxides. In due course of time, metal keeps on changing into its oxide and finally the whole metal is lost due to oxidation. This process is called corrosion.

Example: Iron articles; like iron gates or bridges tend to rust because of oxidation by atmospheric moisture. We know that rust is nothing but iron oxide. Conversion of iron into rust leads to corrosion of the iron articles. Due to this, iron articles weaken and finally wither away.

(b) Rancidity

Answer: When fats and oils are oxidised, their smell and taste change. This process is called rancidity. Oily food often become rancid and start giving out obnoxious smell. The taste also becomes bad. Such oily food is not fit for eating.

Example: When packets of potato chips or other oily snacks are kept open for a long time; their taste and smell become bad. The oily food is no longer safe to eat.