1.Answer: (1) a and b
2.Answer: (d) Displacement reaction
3.Answer: (a) Hydrogen gas and iron chloride are produced
4.Answer: When the number of atoms of reactants is equal to the number of atoms of the products,
the reaction is called a balanced chemical equation.
According to the Law of conservation of mass, total mass of the elements present in the reactants must be equal to the total mass of the elements present in products. That’s why a chemical equations should be balanced always.
(a) 3H2 + N2 ⇨ 2NH3
(b) 2H2S + 3O2 ⇨ 2H2O + 2SO2
(c) 3BaCl2 + Al2(SO4 )3 ⇨ 2AlCl3 + 3BaSO4
(d) 3K + 2H2O ⇨ 2KOH + H2
(a) 2KBr (aq) + BaI2 (aq) ⇨ 2KI (aq) + BaBr2 (s)
It is a double displacement reaction.
(b) ZnCO3 (s) ⇨ ZnO (s) + CO2 (g)
It is a decomposition reaction.
(c) H2 (g) + Cl2 (g) ⇨ 2HCl
It is a combination reaction.
(d) Mg (s) + 2HCl (aq) ⇨ MgCl2 (aq) + H2 (g)
It is a displacement reaction.
9.Answer: Exothermic reaction: A reaction in which heat is released is called the exothermic reaction.
Example: Burning of fuel is an example of exothermic reaction. When methane is burnt it gives heat along with carbon dioxide and water.
CH4 (g) + 2O2 (g)⇨ CO2 (g) + 2H2O
Endothermic reaction: A reaction in which heat is absorbed is called endothermic reaction.
Example: When silver chloride is left in the sunlight, it absorbs heat and turns grey because of formation of silver metal.
2AgCl (s) ⇨ 2Ag (s) + Cl2 (g)
10.Answer: In the course of respiration glucose is broken into carbon dioxide and energy is released. That’s why it is considered as an exothermic reaction.
C6H12O6 (aq) + 6O2 (g) ⇨ 6CO2 (aq) + 6H2O (l) + Energy
11.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.
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.
13.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)
2AgNO3 (aq) + Cu (s) ⇨ Cu(NO3 )2 (l) + 2Ag (s)
15.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.
16.Answer: Oxidation: 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.
Reduction: 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 called readuced.
17.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.
18.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, to prevent an iron article to come in contact with moisture present in air paint is applied. Paint prevents the iron to get rusted.
19.Answer: When fats and oils are oxidised, they become rancid and their smell and taste change. Hence, food items containing oil and fat are flushed with nitrogen which prevents them to get oxidized and becoming rancid.
20.Answer: Corrosion - Metals react with oxygen which is present in the atmospheric moisture. This leads to the formation of metal oxides. In due course of time, the 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, the iron articles weaken and finally wither away.
Rancidity: 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.