Chapter:- 1. chemical reactions and equations
1. Which of the statements about the reaction below are incorrect?(1 mark)
2PbO(s) + C(s) →2Pb(s) + CO2(g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all
Explanation : The statements (a) and (b) are incorrect
2. Fe 2O 3 + 2Al → Al2O 3 + 2Fe (1 mark)
The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.
Explanation : This is an example of displacement reaction. Al is displayed by Fe in Fe 2O 3.
3. What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.(1
mark)
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.
Explanation :- Hydrogen gas and iron chloride are produced. The chemical equation for the reaction is:
Fe(s) + 2HCl(aq) ⟶ FeCl 2(aq) + H2↑
4.What is a balanced chemical equation? Why should chemical equations be balanced? (2 mark)
According to law of conservation of mass the number of atoms of each element remains the same, before and after a chemical reaction. Hence, we need to balance a skeletal chemical equation.
5.Translate the following statements into chemical equations and then balance them. (four marks)
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
(i) step 1 :- let us write a skeletal equation and draw boxes around each formula
H2+ N2 ⟶ NH 3
step 2 :- list the Number atoms of different elements on both sides| S.No | No. of atoms on reactants side | No. of atoms on products side |
| Nitrogen | 2 | 1 |
| Hydrogen | 2 | 3 |
step 3 :-let us start balancing the equation.
| S.No | No. of atoms on reactants side | No. of atoms on products side |
| Nitrogen | 2 | 2x1 |
| Hydrogen | 3x2 | 2x3 |
step 4 :- check the correctness of the number of atoms on both sides.
3 H2 + N2 ⟶ 2 NH 3
N2 (g) + 3H2 (g) → 2NH3(g).
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(ii) 2 H 2S + 3 O 2 ⟶ 2 H 2O + 2 SO 2
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(iii) 3 BaCl 2 + Al 2(SO 4) 3 → 2 AlCl3 + 3 BaSO 4
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(iv) 2 K + 2 H 2O → 2 KOH + H 2
6. Balance the following chemical equations. (four marks)
(a) HNO 3 + Ca(OH) 2 → Ca(NO 3) 2 + H 2O
(b) NaOH + H 2SO 4 → Na 2SO 4 + H 2O
(c) NaCl+AgNO 3 → AgCl + NaNO 3
(d) BaCl 2 + H 2SO 4 → BaSO 4 + HCl
(a) 2HNO3 + Ca(OH)2 ⟶ Ca(NO3)2 + 2H2O
(b) 2 NaOH + H 2SO 4 ⟶ Na 2SO 4 + 2 H 2O
(c) NaCl + AgNO3 ⟶AgCl + NaNO3
(d) BaCl 2 + H 2SO 4 ⟶ BaSO 4 + 2 HCl
7. Write the balanced chemical equations for the following reactions. (4m)
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
(a) Ca(OH) 2 + CO 2 ⟶ CaCO 3 + H 2O
(b) Zn + 2 AgNO 3 ⟶ Zn(NO 3) 2 + 2 Ag
(c) 2 Al + 3CuCl 2 ⟶ 2 AlCl 3 + 3Cu
(d) BaCl 2 + K 2SO 4 ⟶ BaSO 4 + 2KCl
8. write the balanced chemical equation for the following and identify the type of reaction in each case.
(8 marks)
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
(a) balanced chemical equation:-
2 KBr (aq) + BaCl 2(aq) → 2 Kl(aq) + BaBr 2(s)
Type : Double displacement reaction
(b)balanced chemical equation:-
ZnCO 3 (s) → ZnO (s) + CO 2 (g)
Type : Decomposition reaction
(c) balanced chemical equation:-
H 2 (g) + Cl 2 (g) → 2HCl(g)
Type : Combination reaction
(d) balanced chemical equation:-
Mg (s) + 2 HCl (aq) → MgCl 2 (aq) + H 2 (g)
Type : Displacement reaction
9.What does one mean by exothermic and endothermic reactions? Give examples? (8M)
Example :
(i) C (s) + O 2 (g) → CO 2 (g) + Heat
(ii) N 2 (g) + 3 H 2 (g) → 2 NH 3 (g) + Heat
Endothermic reactions :
Reactions in which energy is absorbed are known as endothermic reactions.
An endothermic reaction is usually indicated by writing “ - Heat” on the product side of a chemical equation.
Examples :
(i) C (s) + 2 S (s) → CS2 (l) - Heat
(ii) N 2 (g) + O 2 (g) → 2NO(g) - Heat
10. Why is respiration considered an exothermic reaction? Explain? (four marks )
Energy in our body is obtained from the food we eat. During digestion, large molecules of food are broken down into simpler substances such as glucose. Glucose reacts with oxygen in cells and produces energy. This reaction is called respiration. Respiration is considered an exothermic reaction because heat is given out along with the products.
C 6H 12O 6(aq) + 6 O 2(g) → 6 CO 2(g) + 6H 2O(l)
11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions? (eight marks )
In a combination reaction two or more substances combine to form a new single substance.
In a decomposition reaction, a single substance decomposes to give two or more substances.
Examples of combination reactions :-
CaO(s) + H 2O(l) → Ca(OH) 2(aq) + Heat
C(s) + O 2(g) → CO 2(g) + Heat
2H 2(g) + O 2(g) → 2H 2O(l) + Heat
In the above examples, we have seen that the heat evolved in combination reactions. In many cases the combination reaction is exothermic
chemical reactions.
Examples of decomposition reactions:-
2AgCl(s)+Sunlight → 2Ag(s) + Cl 2(g)
CaCO 3(s) + Heat → CaO(s) + CO 2(g)
2HgO + Electricity → 2Hg + O 2
In the above examples we have seen that the decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants. In many cases the decomposition reaction is endothermic chemical reactions.
So, Decomposition reactions are opposite to combination reactions.
12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity. (eight marks)
(1) Thermal decomposition:-
When a decomposition reaction is carried out by heating, it is called thermal decomposition.
CaCO3(s) + Heat → CaO(s) + CO2(g)
(2) Photo decomposition:- When a decomposition reaction is carried out by light, it is called Photo decomposition
2AgCl(s) + Sunlight → 2Ag(s) + Cl 2(g)
(3) Electrolytic decomposition:- When a decomposition reaction is carried out by electricity , it is called Electrolytic decomposition
2H 2O(l) + electricity ⟶ 2H 2(g) + O 2(g)
13. What is the difference between displacement and double displacement reactions? Write equations for these reactions? (eight marks)
When an element displaces another element from its compound, a displacement reaction occurs.
Whereas, Two different atoms or groups of atoms (ions) are exchanged in double displacement reactions.
In displacement reaction a more reactive substance displaces a less reactive one from its salt solution whereas in double displacement reaction a mutual exchange of ions happens between two compounds.
In a displacement reaction, only a single displacement takes place whereas in the double displacement reaction, as the name suggests two displacement takes place between the molecules.
Example:
Displacement reaction
Fe(s) + CuSO 4(aq) → FeSO 4(aq) + Cu(s)
Double displacement reaction Na 2SO 4(aq) + BaCl 2(aq) → BaSO4(s) + 2NaCl(aq)
14 In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved? (Two marks)
The metallic copper reacts with silver nitrate solution to form the copper nitrate as precipitate and the silver as metallic silver in solution.
Copper + Silver nitrate → Copper nitrate + Silver
Cu(s) + 2AgNO 3(aq) → Cu(NO 3) 2(aq) + 2Ag(s)
15. What do you mean by a precipitation reaction? Explain by giving examples.(4 marks)
A precipitation reaction is a chemical reaction in which a precipitate is formed when two solutions containing soluble salts are reacted with each other.
The precipitate is a insoluble solid substance that settles down at the bottom of the container.
The precipitation reaction is typically a double displacement reaction, in which the ions are exchanged between the compounds.
Few examples of precipitation reactions are:
Na 2SO 4(aq) + BaCl 2(aq) → BaSO 4(s) + 2NaCl(aq)
Na 2CO 3(aq)+CaCl 2(aq)→CaCO 3(s)↓+2NaCl(aq)
16. Explain the following in terms of gain or loss of oxygen with two examples each. (four marks)
(a) Oxidation
(b) Reduction
(a) Oxidation:-
Oxidation is the gain of oxygen or loss of hydrogen.
For example:
(i) 2Cu + O 2 ⟶ 2CuO
In this reaction Cu is oxidised to CuO.
(ii) C(s) + O 2(g) → CO 2(g)
In this reaction C is oxidised to CO 2.
(b) Reduction : Reduction is the loss of oxygen
or gain of hydrogen.
For example
CuO + H 2 → Cu + H 2O
In this reaction CuO is reduced to Cu.
ZnO + C → Zn + CO
In this reaction ZnO is reduced to Zn.
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. (Two marks)
The brown coloured element ‘X ‘ is copper. On heating in air it forms copper oxide, which is black in colour.
2 Cu(s) + O 2 → 2 CuO
Brown (from air) Black
18. Why do we apply paint on iron articles? (Two marks)
iron articles are shiny when new, but get coated with a reddish brown powder when left for some time. This process is commonly known as rusting of iron.
Rusting causes damage to iron articles. So, To prevent iron articles from rusting we apply paint on them. Paint on the surface of Iron articles stops them to getting contact with atmospheric oxygen and moisture and thus the rusting is prevented.
19.Oil and fat containing food items are flushed with nitrogen. Why? (four marks)
Oxidisation spoils the potato chips and other food items containing oil and fat when they are left for a long time.
In order to remove oxygen from the bag to prevent the chips from getting oxidised (Rancid), the chips manufacturers usually flush bags of chips with gas such as nitrogen.
As the nitrogen is an inert gas, it does not react with these substances and it also it creates a oxygen free surroundings in side the bag
20. Explain the following terms with one example each. (Eight marks)
(a) Corrosion
(b) Rancidity
(a) Corrosion :
When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion.
The black coating on silver and the green coating on copper are other examples of corrosion.
Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, specially those of iron.
(b) Rancidity :
When food materials containing fat/oil are left for a long time, the oxidation may take place which results in a change in their smell and taste. This is called rancidity.
Usually substances which prevent oxidation (antioxidants) are added to foods containing fats and oil.
Keeping food in air tight containers helps to slow down oxidation
The chips manufacturers usually flush bags of chips with gas such as nitrogen to prevent the chips from getting oxidised.
1.Why should a magnesium ribbon be cleaned before burning in air?
Magnesium is a highly reactive metal. When exposed to air it forms a layer of magnesium oxide on its surface. This magnesium oxide layer is quite stable and can interrupt the burning of magnesium in oxygen.
So it is necessary to remove the oxide layer by ribbon with sandpaper before burning.
2. Write the balanced equation for the following chemical reactions. (four marks)
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
(i) H 2+Cl 2 ⟶2HCl
(ii) 3 BaCl 2 + Al 2(SO 4) 3 → 3 BaSO 4 + 2 A∣Cl 3
(iii) 2 Na +2 H 2O ⟶ 2 NaOH + H 2 ↑
3. Write a balanced chemical equation with state symbols for the following reactions. (four marks)
(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.
(i) BaCl2 (aq) + Na2SO4 (aq) → BaSO4 (s) + 2 NaCl (aq)
(ii) NaOH (aq) + HCl (aq) → NaCl (aq) + H2O(l)
1. A solution of a substance ‘X’ is used for whitewashing.(Two marks)
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.
(i) The substance 'X' is calcium oxide. Its chemical formula is CaO.
(ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).
CaO(s)+H2O(l) ⟶ Ca(OH)2
2. Why is the amount of gas collected in one of the test tubes in Activity Electrolysis of water(1.7)double the amount collected in the other? Name this gas.(Two marks)
The amount of gas collected in one of the test tubes in Activity Electrolysis of water is double the amount collected in the other is “hydrogen” and the other gas is “oxygen”.
Two molecules of water decomposes to give two molecules of hydrogen gas and one molecule of oxygen gas In electrolysis
2 H2O(l) → 2 H2(g) + O2(g)
1 Why does the colour of copper sulphate solution change when an iron nail is dipped in it? .(Two)
When an iron nail is dipped in a copper sulphate solution, Due to Displacement reaction, the iron nail becomes brownish as the copper deposited on it and the blue coloured copper sulphate solution turns to greenish Ferrous sulphate solution.
● Fe(s) + CuSO 4 (aq) ⟶ FeSO 4 (aq) + Cu(s)
2. Give an example of a double displacement reaction other than the one given in Activity 1.10. .(2 m)
When a solution of lead nitrate Pb(NO3)2 (aq) is mixed with a solution of potassium iodide KI(aq), a double displacement reaction takes place and forms a yellow precipitate lead iodide PbI2 (s) and potassium nitrate KNO3 (aq)
Pb(NO3)2 (aq) + 2 KI(aq) ⟶ PbI 2 (s) + 2KNO3 (aq)
Or
Sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium chloride.
Na2CO3(aq) + CaCl2(aq) ⟶ CaCO3(s) + 2 NaCl(aq)
3. Identify the substances that are oxidised and the substances that are reduced in the following
reactions. .(Two)
(i) 4 Na + O2 → 2 Na2O
(ii) CuO + H2 → Cu(s) + H2O
In a chemical reaction, if a substance gains oxygen or loses hydrogen(electrons) , it is oxidised. If a substance loses oxygen or gains hydrogen(electrons) , it is reduced.
(i) Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.
(ii) As it gains oxygen, Hydrogen (H2) is oxidised and turns into water(H 2O) and Copper oxide (CuO) is reduced to copper (Cu) by losing oxygen
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