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Chemical Reactions and Equations - Q&A

1. Which of the statements about the reaction below are incorrect?
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
Answer: (i)
Explanation:
(a) is incorrect because Lead Oxide (PbO) is getting reduced to Lead (Pb), not Lead metal itself.
(b) is incorrect because Carbon (C) is getting oxidised to Carbon Dioxide (CO₂), not CO₂ itself.
(c) is correct because Carbon gains oxygen.
(d) is correct because Lead oxide loses oxygen.
Thus, the incorrect statements are (a) and (b).

2. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe
The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.
Answer: (d)
Explanation: In this reaction, Aluminium (Al) is more reactive than Iron (Fe) and displaces Iron from Iron(III) oxide (Fe₂O₃). Therefore, it is a displacement reaction.

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.
(c) No reaction takes place.
(d) Iron salt and water are produced.
Answer: (a)
Explanation: Metals react with dilute acids to form salt and hydrogen gas. The chemical equation is:
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

4. What is a balanced chemical equation? Why should chemical equations be balanced?
Answer:
A balanced chemical equation is an equation in which the number of atoms of each element remains the same on both the reactant and product sides.

Chemical equations should be balanced to satisfy the Law of Conservation of Mass. This law states that mass can neither be created nor destroyed in a chemical reaction. Therefore, the total mass (and total number of atoms) must be equal on both sides of the equation.

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 sulpur 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.
Answer:
(a) 3H₂(g) + N₂(g) → 2NH₃(g)
(b) 2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)
(c) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 2AlCl₃(aq) + 3BaSO₄(s)
(d) 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)

6. Balance the following chemical equations.
(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl
Answer:
(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃ (Already balanced)
(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

7. Write the balanced chemical equations for the following reactions.
(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
Answer:
(a) Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
(b) Zn(s) + 2AgNO₃(aq) → Zn(NO₃)₂(aq) + 2Ag(s)
(c) 2Al(s) + 3CuCl₂(aq) → 2AlCl₃(aq) + 3Cu(s)
(d) BaCl₂(aq) + K₂SO₄(aq) → BaSO₄(s) + 2KCl(aq)

8. Write the balanced chemical equation for the following and identify the type of reaction in each case.
(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)
Answer:
(a) 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s)
Type: Double Displacement Reaction

(b) ZnCO₃(s) → ZnO(s) + CO₂(g)
Type: Decomposition Reaction

(c) H₂(g) + Cl₂(g) → 2HCl(g)
Type: Combination Reaction

(d) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Type: Displacement Reaction

9. What does one mean by exothermic and endothermic reactions? Give examples.
Answer:
Exothermic reactions: Reactions in which heat is released along with the formation of products are called exothermic reactions.
Example: Burning of natural gas.
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + Heat

Endothermic reactions: Reactions in which energy is absorbed from the surroundings (in the form of heat, light, or electricity) are known as endothermic reactions.
Example: Decomposition of Calcium Carbonate.
CaCO₃(s) + Heat → CaO(s) + CO₂(g)

10. Why is respiration considered an exothermic reaction? Explain.
Answer:
Respiration is considered an exothermic reaction because energy is released during the process. In our body cells, glucose (from digested food) combines with oxygen to produce carbon dioxide and water. This reaction releases a significant amount of energy which our body uses to maintain life processes.
Reaction:
C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + Energy

11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer:
Decomposition reactions are called the opposite of combination reactions because:
In a combination reaction, two or more reactants combine to form a single product.
General Form: A + B → AB
Example: 2H₂(g) + O₂(g) → 2H₂O(l)

In a decomposition reaction, a single reactant breaks down to form two or more simpler products.
General Form: AB → A + B
Example: 2H₂O(l) → 2H₂(g) + O₂(g) (Electrolysis of water)

12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Answer:
(i) By Heat (Thermal Decomposition):
CaCO₃(s) + Heat → CaO(s) + CO₂(g)

(ii) By Light (Photolytic Decomposition):
2AgCl(s) + Sunlight → 2Ag(s) + Cl₂(g)

(iii) By Electricity (Electrolytic Decomposition):
2H₂O(l) + Electricity → 2H₂(g) + O₂(g)

13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer:
Displacement Reaction: A chemical reaction in which a more reactive element displaces a less reactive element from its compound.
Equation: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) (Iron displaces Copper)

Double Displacement Reaction: A chemical reaction in which there is an exchange of ions between two reactants to form new compounds.
Equation: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) (Exchange of Sulphate and Chloride ions)

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:
Copper is more reactive than silver, so it displaces silver from the silver nitrate solution.
Reaction:
Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)
(Copper) + (Silver Nitrate) → (Copper Nitrate) + (Silver)

15. What do you mean by a precipitation reaction? Explain by giving examples.
Answer:
A reaction in which an insoluble solid, known as a precipitate, is formed and separates from the solution is called a precipitation reaction.
Example: Mixing aqueous solutions of Sodium Sulphate and Barium Chloride forms a white precipitate of Barium Sulphate.
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) (White Precipitate) + 2NaCl(aq)

16. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction
Answer:
(a) Oxidation: It is defined as the gain of oxygen by a substance.
Examples:
1. 2Cu + O₂ → 2CuO (Copper gains oxygen)
2. C + O₂ → CO₂ (Carbon gains oxygen)

(b) Reduction: It is defined as the loss of oxygen from a substance.
Examples:
1. CuO + H₂ → Cu + H₂O (Copper Oxide loses oxygen)
2. ZnO + C → Zn + CO (Zinc Oxide loses oxygen)

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:
The shiny brown coloured element 'X' is Copper (Cu).
The black coloured compound formed is Copper(II) Oxide (CuO).
Reaction: 2Cu(s) + O₂(g) + Heat → 2CuO(s) (Black)

18. Why do we apply paint on iron articles?
Answer:
We apply paint on iron articles to prevent rusting (corrosion).
Rusting occurs when iron comes into contact with moist air (oxygen and water vapour). Paint forms a protective coating on the surface of the iron, cutting off its contact with the atmosphere. This prevents the oxidation of iron and protects it from damage.

19. Oil and fat containing food items are flushed with nitrogen. Why?
Answer:
Oil and fat-containing food items are flushed with nitrogen to prevent rancidity.
When fats and oils are exposed to oxygen in the air, they get oxidised and become rancid (spoiled), producing a bad smell and taste. Nitrogen is an inert (unreactive) gas. Flushing the food packets with nitrogen replaces the oxygen, thereby preventing oxidation and keeping the food fresh for a longer time.

20. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity
Answer:
(a) Corrosion: It is the process where metals are slowly eaten away by the reaction with air, moisture, or chemicals (acids) in the environment.
Example: Rusting of Iron (formation of reddish-brown Hydrated Iron Oxide on iron surfaces).

(b) Rancidity: It is the condition produced by the aerial oxidation of fats and oils in foods, characterized by an unpleasant smell and taste.
Example: The bad taste and smell developed in fried snacks (like chips) when kept open for a long time.

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Quick Review Flashcards - Click to flip and test your knowledge!

Question

What event is said to have occurred whenever a chemical change takes place?

Answer

A chemical reaction.

Question

List two examples of chemical reactions occurring in daily life related to food.

Answer

Food being cooked and food getting digested in the body.

Question

What happens to the identity of the initial substance during a chemical reaction?

Answer

The nature and identity of the substance change.

Question

Which four observations can help determine if a chemical reaction has occurred?

Answer

Change in state, change in colour, evolution of a gas, and change in temperature.

Question

Why must a magnesium ribbon be cleaned with sandpaper before burning?

Answer

To remove the protective layer and ensure it reacts effectively with oxygen.

Question

Describe the flame produced when a magnesium ribbon is burnt in air.

Answer

It burns with a dazzling white flame.

Question

What is the white powder formed after burning a magnesium ribbon?

Answer

Magnesium oxide ($MgO$).

Question

Magnesium reacts with which component of the air to form magnesium oxide?

Answer

Oxygen ($O_{2}$).

Question

In a chemical equation, what is the term for the substances written on the left-hand side (LHS)?

Answer

Reactants.

Question

In a chemical equation, what is the term for the substances written on the right-hand side (RHS)?

Answer

Products.

Question

What does the arrowhead in a chemical equation represent?

Answer

The direction of the reaction.

Question

What is a 'skeletal' chemical equation?

Answer

An unbalanced equation where the number of atoms is not the same on both sides.

Question

Which law states that mass can neither be created nor destroyed in a chemical reaction?

Answer

The Law of Conservation of Mass.

Question

Why is it necessary to balance a skeletal chemical equation?

Answer

To ensure the number of atoms of each element remains the same before and after the reaction.

Question

What gas is evolved when zinc granules react with dilute sulphuric acid?

Answer

Hydrogen gas ($H_{2}$).

Question

Identify the balanced equation for the reaction: $Zn + H_{2}SO_{4} \rightarrow$ ____.

Answer

$ZnSO_{4} + H_{2}$.

Question

What is the 'hit-and-trial' method in chemistry?

Answer

A method of balancing equations by using the smallest whole number coefficients.

Question

When balancing a chemical equation, why must you never change the subscripts within a formula?

Answer

Changing subscripts would alter the identity of the chemical compound.

Question

What notation represents the 'aqueous' state in a chemical equation?

Answer

$(aq)$.

Question

When is the symbol $(aq)$ used for a reactant or product?

Answer

When the substance is present as a solution in water.

Question

In the reaction $3Fe(s) + 4H_{2}O(g) \rightarrow Fe_{3}O_{4}(s) + 4H_{2}(g)$, what physical state of water does $(g)$ represent?

Answer

Steam.

Question

Where are specific reaction conditions, such as pressure or catalysts, usually written in an equation?

Answer

Above or below the reaction arrow.

Question

What is the required pressure for the reaction of $CO(g)$ and $2H_{2}(g)$ to form $CH_{3}OH(l)$?

Answer

$340 \text{ atm}$.

Question

What is a combination reaction?

Answer

A reaction where two or more reactants combine to form a single product.

Question

What is the common name for calcium oxide ($CaO$)?

Answer

Quick lime.

Question

What is the common name for calcium hydroxide ($Ca(OH)_{2}$)?

Answer

Slaked lime.

Question

Write the chemical equation for the reaction of quick lime with water.

Answer

$CaO(s) + H_{2}O(l) \rightarrow Ca(OH)_{2}(aq) + \text{Heat}$.

Question

Which substance is used for whitewashing walls?

Answer

A solution of slaked lime (calcium hydroxide).

Question

Whitewashed walls develop a shiny finish due to the formation of which substance?

Answer

Calcium carbonate ($CaCO_{3}$).

Question

How many days does it typically take for calcium carbonate to form on a whitewashed wall?

Answer

Two to three days.

Question

What is the chemical formula for marble?

Answer

$CaCO_{3}$.

Question

What defines an exothermic chemical reaction?

Answer

A reaction in which heat is released along with the formation of products.

Question

Why is respiration classified as an exothermic process?

Answer

Because energy is released when glucose combines with oxygen in body cells.

Question

Write the balanced equation for respiration.

Answer

$C_{6}H_{12}O_{6}(aq) + 6O_{2}(aq) \rightarrow 6CO_{2}(aq) + 6H_{2}O(l) + \text{energy}$.

Question

What is a decomposition reaction?

Answer

A reaction where a single reactant breaks down to give simpler products.

Question

What is thermal decomposition?

Answer

A decomposition reaction that is carried out by heating.

Question

What colour change occurs when ferrous sulphate crystals are heated?

Answer

The green colour of the crystals changes (as they lose water and decompose).

Question

Identify the solid product formed when ferrous sulphate ($FeSO_{4}$) decomposes.

Answer

Ferric oxide ($Fe_{2}O_{3}$).

Question

Which two gases are released during the thermal decomposition of ferrous sulphate?

Answer

Sulphur dioxide ($SO_{2}$) and sulphur trioxide ($SO_{3}$).

Question

What are the products of the thermal decomposition of limestone ($CaCO_{3}$)?

Answer

Calcium oxide ($CaO$) and carbon dioxide ($CO_{2}$).

Question

Which decomposition product is vital for the manufacture of cement?

Answer

Calcium oxide (Quick lime).

Question

What observation indicates the decomposition of lead nitrate when heated?

Answer

The emission of brown fumes of nitrogen dioxide ($NO_{2}$).

Question

Write the chemical equation for the thermal decomposition of lead nitrate.

Answer

$2Pb(NO_{3})_{2}(s) \xrightarrow{\text{Heat}} 2PbO(s) + 4NO_{2}(g) + O_{2}(g)$.

Question

In the electrolysis of water, why is the volume of gas collected at one electrode double the other?

Answer

Water ($H_{2}O$) contains two atoms of hydrogen for every one atom of oxygen.

Question

Which gas is collected in the greater volume during the electrolysis of water?

Answer

Hydrogen gas.

Question

What happens to the colour of silver chloride when it is exposed to sunlight?

Answer

It turns from white to grey.

Question

Silver chloride decomposes in sunlight into which two elements?

Answer

Silver and chlorine.

Question

Which silver salt, besides silver chloride, is used in black and white photography?

Answer

Silver bromide ($AgBr$).

Question

What defines an endothermic reaction?

Answer

A reaction in which energy (heat, light, or electricity) is absorbed.

Question

What is a displacement reaction?

Answer

A reaction in which a more reactive element removes another element from its compound.

Question

What happens to the blue colour of copper sulphate solution when an iron nail is dipped in it?

Answer

The blue colour fades as iron displaces the copper.

Question

Why does an iron nail become brownish when placed in copper sulphate solution?

Answer

Because copper metal is deposited on the surface of the nail.

Question

Between zinc and copper, which element is more reactive?

Answer

Zinc.

Question

What is a double displacement reaction?

Answer

A reaction involving an exchange of ions between the reactants to form new compounds.

Question

What is a precipitate?

Answer

An insoluble substance formed during a chemical reaction in solution.

Question

What is the colour of the precipitate formed when sodium sulphate reacts with barium chloride?

Answer

White (Barium sulphate).

Question

Write the chemical equation for the reaction between sodium sulphate and barium chloride.

Answer

$Na_{2}SO_{4}(aq) + BaCl_{2}(aq) \rightarrow BaSO_{4}(s) + 2NaCl(aq)$.

Question

Define oxidation in terms of oxygen gain/loss.

Answer

Oxidation is the gain of oxygen during a reaction.

Question

Define reduction in terms of oxygen gain/loss.

Answer

Reduction is the loss of oxygen during a reaction.

Question

What is a redox (oxidation-reduction) reaction?

Answer

A reaction where one reactant is oxidised while the other is reduced simultaneously.