Science Class- 10
Class 10 - Chapter 1: Chemical Reactions and Equations
1. Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s) → 2Pb(s) + CO2(g)
Answer: (i) (a) and (b)
Lead is not getting reduced; lead oxide (PbO) is getting reduced to lead.
Carbon is oxidised to carbon dioxide.
Therefore statements (a) and (b) are incorrect.
2. Fe2O3 + 2Al → Al2O3 + 2Fe
Answer: (d) Displacement reaction
Aluminium displaces iron from iron oxide to form aluminium oxide and iron.
3. What happens when dilute hydrochloric acid is added to iron filings?
Answer: (a) Hydrogen gas and iron chloride are produced.
Fe + 2HCl → FeCl2 + H2
4. What is a balanced chemical equation? Why should chemical equations be balanced?
A balanced chemical equation is an equation in which the number of atoms of each element is equal on both sides.
Chemical equations must be balanced to satisfy the Law of Conservation of Mass, which states that mass can neither be created nor destroyed.
5. Translate the following statements into chemical equations and balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
N2 + 3H2 → 2NH3
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
2H2S + 3O2 → 2H2O + 2SO2
(c) Barium chloride reacts with aluminium sulphate.
3BaCl2 + Al2(SO4)3 → 2AlCl3 + 3BaSO4
(d) Potassium reacts with water.
2K + 2H2O → 2KOH + H2
6: Balance the Following Chemical Equations
6(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
Step 1: Write the unbalanced equation.
HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
Step 2: Count atoms.
| Element | Reactants | Products |
|---|---|---|
| Ca | 1 | 1 |
| N | 1 | 2 |
| H | 3 | 2 |
| O | 5 | 7 |
Step 3: Balance nitrate ions.
2HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
Step 4: Balance hydrogen by placing 2 before water.
2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
Step 5: Verify atoms.
| Element | Reactants | Products |
|---|---|---|
| Ca | 1 | 1 |
| N | 2 | 2 |
| H | 4 | 4 |
| O | 8 | 8 |
Balanced Equation:
2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
6(b) NaOH + H2SO4 → Na2SO4 + H2O
Step 1: Write the equation.
NaOH + H2SO4 → Na2SO4 + H2O
Step 2: Sodium atoms are not balanced.
2NaOH + H2SO4 → Na2SO4 + H2O
Step 3: Hydrogen atoms become 4 on the left side.
2NaOH + H2SO4 → Na2SO4 + 2H2O
Step 4: Verify atoms.
| Element | Reactants | Products |
|---|---|---|
| Na | 2 | 2 |
| S | 1 | 1 |
| H | 4 | 4 |
| O | 6 | 6 |
Balanced Equation:
2NaOH + H2SO4 → Na2SO4 + 2H2O
6(c) NaCl + AgNO3 → AgCl + NaNO3
Step 1: Write the equation.
NaCl + AgNO3 → AgCl + NaNO3
Step 2: Count atoms.
| Element | Reactants | Products |
|---|---|---|
| Na | 1 | 1 |
| Cl | 1 | 1 |
| Ag | 1 | 1 |
| N | 1 | 1 |
| O | 3 | 3 |
All atoms are already balanced.
Balanced Equation:
NaCl + AgNO3 → AgCl + NaNO3
6(d) BaCl2 + H2SO4 → BaSO4 + HCl
Step 1: Write the equation.
BaCl2 + H2SO4 → BaSO4 + HCl
Step 2: Chlorine atoms are 2 on the left but only 1 on the right.
BaCl2 + H2SO4 → BaSO4 + 2HCl
Step 3: Verify atoms.
| Element | Reactants | Products |
|---|---|---|
| Ba | 1 | 1 |
| Cl | 2 | 2 |
| H | 2 | 2 |
| S | 1 | 1 |
| O | 4 | 4 |
Balanced Equation:
BaCl2 + H2SO4 → BaSO4 + 2HCl
7. Write the balanced chemical equations.
(a)
Ca(OH)2 + CO2 → CaCO3 + H2O
(b)
Zn + 2AgNO3 → Zn(NO3)2 + 2Ag
(c)
2Al + 3CuCl2 → 2AlCl3 + 3Cu
(d)
BaCl2 + K2SO4 → BaSO4 + 2KCl
8. Write the balanced chemical equation and identify the type of reaction.
(a)
2KI + BaI2 → 2KI + BaBr2
Type: Double displacement reaction
(b)
ZnCO3 → ZnO + CO2
Type: Decomposition reaction
(c)
H2 + Cl2 → 2HCl
Type: Combination reaction
(d)
Mg + 2HCl → MgCl2 + H2
Type: Displacement reaction
9. What do you mean by exothermic and endothermic reactions? Give examples.
Exothermic Reaction: A reaction that releases heat energy.
CH4 + 2O2 → CO2 + 2H2O + Heat
Endothermic Reaction: A reaction that absorbs heat energy.
CaCO3 + Heat → CaO + CO2
10. Why is respiration considered an exothermic reaction? Explain.
Respiration releases energy in the form of heat when glucose reacts with oxygen.
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
Therefore respiration is an exothermic reaction.
11. Why are decomposition reactions called the opposite of combination reactions?
In a combination reaction, two or more substances combine to form a single product.
A + B → AB
In a decomposition reaction, one compound breaks into two or more simpler substances.
AB → A + B
Hence decomposition reactions are the opposite of combination reactions.
12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
(a) Heat (Thermal decomposition)
CaCO3 → CaO + CO2
(b) Light (Photochemical decomposition)
2AgCl → 2Ag + Cl2
(c) Electricity (Electrolytic decomposition)
2H2O → 2H2 + O2
13. What is the difference between displacement and double displacement reactions?
Displacement Reaction:
A more reactive element displaces a less reactive element from its compound.
Zn + CuSO4 → ZnSO4 + Cu
Double Displacement Reaction:
Exchange of ions occurs between two compounds.
BaCl2 + Na2SO4 → BaSO4 + 2NaCl
14. Write the reaction involved in the recovery of silver from silver nitrate solution using copper.
Cu + 2AgNO3 → Cu(NO3)2 + 2Ag
15. What do you mean by a precipitation reaction? Explain with examples.
A precipitation reaction is a reaction in which an insoluble solid (precipitate) is formed.
BaCl2 + Na2SO4 → BaSO4↓ + 2NaCl
The white insoluble BaSO4 formed is called a precipitate.
16. Explain oxidation and reduction in terms of gain or loss of oxygen.
(a) Oxidation
Addition of oxygen is called oxidation.
2Mg + O2 → 2MgO
C + O2 → CO2
(b) Reduction
Removal of oxygen is called reduction.
CuO + H2 → Cu + H2O
PbO + C → Pb + CO
17. A shiny brown coloured element 'X' on heating in air becomes black in colour.
Element X: Copper (Cu)
Black compound formed: Copper oxide (CuO)
2Cu + O2 → 2CuO
18. Why do we apply paint on iron articles?
Paint prevents iron from coming in contact with air and moisture. This prevents rusting and protects the iron article from corrosion.
19. Oil and fat containing food items are flushed with nitrogen. Why?
Nitrogen prevents oxidation of oils and fats. This helps prevent rancidity and increases the shelf life of food products.
20. Explain the following terms with one example each.
(a) Corrosion
Corrosion is the gradual destruction of metals due to reaction with air, moisture or chemicals.
Example: Rusting of iron.
4Fe + 3O2 + xH2O → 2Fe2O3·xH2O
(b) Rancidity
Rancidity is the oxidation of fats and oils, causing unpleasant smell and taste.
Example: Chips or fried food becoming stale when left exposed to air for a long time.