1. Why should a magnesium ribbon be cleaned before burning in air?
Magnesium ribbon should be cleaned before burning in air to ensure a complete reaction with oxygen, leading to efficient combustion. Any oxide layer or impurities on its surface can interfere with the reaction, hindering the burning process. Cleaning removes these obstacles, allowing magnesium to react fully with oxygen and produce a vigorous combustion.
The reaction involved in burning magnesium in the air is:
2Mg(s)+O2 (g)→2MgO(s)
In this reaction, magnesium (Mg) reacts with oxygen (O2) from the air to form magnesium oxide (MgO).
2. Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
H₂ + Cl₂ → 2HCl
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
(iii) Sodium + Water → Sodium hydroxide + Hydrogen
2Na + 2H₂O → 2NaOH + H₂
3. Write a balanced chemical equation with state symbols for the following reactions.
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
Chemical Equation: BaCl₂(aq) + Na₂SO₄(aq) → 2NaCl(aq) + BaSO₄(s)
barium chloride (BaCl₂)
sodium sulphate (Na₂SO₄)
barium sulphate (NaCl)
sodium chloride. (BaSO₄)
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Chemical Equation: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Sodium hydroxide: NaOH
Hydrochloric acid: HCl
Sodium chloride: NaCl
Water: H₂O
4. A solution of a substance ‘X’ is used for whitewashing.
(i) Name the substance ‘X’ and write its formula.
The substance ‘X’ used for whitewashing is calcium oxide, commonly known as quicklime. Its chemical formula is CaO.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.
The reaction of calcium oxide (CaO) with water (H2O) produces calcium hydroxide [Ca(OH)2], also known as slaked lime. The equation for this reaction is:
► CaO + H2O → Ca(OH)2
5. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.
The gas collected in one of the test tubes is hydrogen gas (H2). The reason its amount is double compared to the other is because, in the electrolysis of water, the ratio of the volume of hydrogen gas produced to the volume of oxygen gas produced is 2:1, according to the balanced chemical equation:
2H2O → 2H2 + O2
6. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
The color change in the copper sulfate solution occurs because of a chemical reaction between the iron nail and the copper sulfate solution. The reaction is a type of single displacement reaction, where iron (Fe) displaces copper (Cu) from the copper sulfate solution.
Here’s what happens:
- Iron (Fe) from the nail reacts with copper sulfate (CuSO4) solution.
- The iron (Fe) loses electrons and gets oxidized, forming iron ions (Fe2+).
- Copper ions (Cu2+) from the copper sulfate solution gain electrons and get reduced, depositing solid copper (Cu) onto the surface of the iron nail.
- The blue color of the copper sulfate solution fades as copper ions are consumed and deposited as solid copper.
Reaction Involved: Fe(s)+CuSO4(aq)→Cu(s)+FeSO4(aq)
7. Give an example of a double displacement reaction other than the one given in Activity 1.10.
Ans: An example of a double displacement reaction is the reaction between silver nitrate (AgNO3) and sodium chloride (NaCl) to form silver chloride (AgCl) and sodium nitrate (NaNO3).
Reaction Involved: AgNO3(aq)+NaCl(aq)→AgCl(s)+NaNO3(aq)
8. Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) 4Na(s) + O2(g) → 2Na2O(s)
Oxidized: Sodium (Na)
Reduced: Oxygen (O2)
(ii) CuO(s) + H2(g) → Cu(s) + H2O(l)
Oxidized: Hydrogen (H2)
Reduced: Copper oxide (CuO)
Exercise Question and Answer
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
Ans: The correct answer is (i) (a) and (b)
Explanation:
In the reaction 2PbO(s) + C(s) → 2Pb(s) + CO2 (g), lead oxide is reduced to lead and carbon is oxidized to carbon dioxide.
Reduction: Gain of electrons or loss of oxygen.
Oxidation: Loss of electrons or gain of oxygen.
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.
The above reaction is an example of a (d) displacement reaction.
Explanation: In the given reaction, aluminum (Al) displaces iron (Fe) from iron(III) oxide (Fe2O3) to form aluminum oxide (Al2O3) and elemental iron (Fe). This is a displacement reaction because one element displaces another in a compound.
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.
The correct answer is (a) Hydrogen gas and iron chloride are produced.
Explanation: When dilute hydrochloric acid (HCl) is added to iron fillings (Fe), it undergoes a single displacement reaction. Iron reacts with hydrochloric acid to form iron chloride (FeCl2) and hydrogen gas (H2). So, option (a) is correct.
Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)
4. What is a balanced chemical equation? Why should chemical equations be balanced?
A balanced chemical equation is a way to show a chemical reaction using symbols and formulas. It’s like a recipe for how atoms rearrange during a reaction. In a balanced equation, the number of each type of atom is the same on both sides.
Important Because:
Balancing equations ensures the conservation of mass principle is followed.
It helps determine the exact amounts of substances involved and produced in a reaction.
5. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
3H₂(g) + N₂(g) → 2NH₃(g)
(b) Hydrogen sulphide gas burns in the air to give water and sulphur dioxide.
2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride
and a precipitate of barium sulphate.
3BaCl2(aq) + Al2(SO4)3(aq) → 3BaSO4(s) + 2AlCl3(aq)
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)
6. Balance the following chemical equations.
(a) HNO3 +Ca(OH)2 → Ca(NO3)2 + H2O
Balanced Equation: 2HNO3(aq) + Ca(OH)2(s) → Ca(NO3)2(aq) + 2H2O(l)
(b) NaOH + H2SO4 → Na2SO4 + H2O
Balanced Equation: 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)
(c) NaCl + AgNO3 → AgCl + NaNO3
Balanced Equation: NaCl(aq) + AgNO3(aq) → AgCl(s) + NaNO3(aq)
(d) BaCl2 + H2SO4 → BaSO4 + HCl
Balanced Equation: BaCl2(aq) + H2SO4(aq) → BaSO4(s) + 2HCl(aq)
7. Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
Balanced Equation: Ca(OH)2(s) + CO2(g) → CaCO3(s) + H2O(l)
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
Balanced Equation: Zn(s) + 2AgNO3(aq) → Zn(NO3)2(aq) + 2Ag(s)
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
Balanced Equation: 2Al(s) + 3CuCl2(aq) → 2AlCl3(aq) + 3Cu(s)
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Balanced Equation: 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)
Balanced Equation: 2KBr(aq) + BaI2(aq) → 2KI(aq) + BaBr2(s)
Reaction Type: Double displacement reaction
(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
Balanced Equation: ZnCO3(s) → ZnO(s) + CO2(g)
Reaction Type: Decomposition reaction
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
Balanced Equation: H2(g) + Cl2(g) → 2HCl(g)
Reaction Type: Combination Reaction
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Balanced Equation: Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Reaction Type: Displacement reaction
9. What does one mean by exothermic and endothermic reactions? Give examples.
Ans: Exothermic Reaction: A reaction that releases heat or energy to the surroundings is called an exothermic reaction. In such reactions, the products have lower energy than the reactants.
Example: Combustion reactions like burning wood, where heat and light are given off.
Endothermic Reaction: A reaction that absorbs heat or energy from the surroundings is called an endothermic reaction. In these reactions, the products have higher energy than the reactants.
Example: Dissolving ammonium nitrate in water, which absorbs heat and causes the surroundings to cool down.
10. Why is respiration considered an exothermic reaction? Explain.
Respiration is considered an exothermic reaction because it releases energy in the form of heat when glucose and oxygen react to produce carbon dioxide, water, and energy (in the form of ATP).
Equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Ans: These reactions involve the breakdown of a compound into simpler substances. They are called the opposite of combination reactions because combination reactions involve the formation of a compound from simpler substances.
Equation for Decomposition: AB → A + B
Equation for Combination: A+B→AB
12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
13. What is the difference between displacement and double displacement reactions?
Write equations for these reactions.
Displacement Reaction: In a displacement reaction, a more reactive element displaces a less reactive element from its compound.
Example: Zn(s)+CuSO4 (aq)→ZnSO4 (aq)+Cu(s)
Double Displacement Reaction: In a double displacement reaction, the positive and negative ions of two compounds exchange places to form new compounds.
Example: NaCl(aq)+AgNO3(aq)→AgCl(s)+NaNO3(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.
Ans: In the refining of silver, copper metal displaces silver from silver nitrate solution.
Reaction: Cu(s)+2AgNO3(aq)→2Ag(s)+Cu(NO3)2(aq)
15. What do you mean by a precipitation reaction? Explain by giving examples.
A precipitation reaction occurs when two soluble ionic compounds react and form an insoluble solid called a precipitate.
Example: AgNO3(aq)+NaCl(aq)→AgCl(s)+NaNO3 (aq)
In this reaction, silver chloride (AgCl) is insoluble and forms a white precipitate.
16. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction
Ans: Oxidation: Oxidation involves the gain of oxygen or loss of hydrogen or electrons.
Examples:
- Mg(s)+O2 (g)→MgO(s)
- CH4(g)+2O2(g)→CO2(g)+2H2O(g)
Reduction: Reduction involves the loss of oxygen or gain of hydrogen or electrons.
Examples:
- CuO(s)+H2 (g)→Cu(s)+H2O(l)
- Fe2O3 (s)+3CO(g)→2Fe(s)+3CO2 (g)
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.
Ans: Element ‘X’: The shiny brown colored element ‘X’ is most likely copper (Cu).
Black Colored Compound Formed: When copper is heated in air, it forms black copper oxide (CuO).
18. Why do we apply paint on iron articles?
We apply paint on iron articles to protect them from corrosion. Paint acts as a barrier between the iron surface and the surrounding environment, preventing moisture and oxygen from coming into contact with the iron, thus reducing the rate of corrosion.
19. Oil and fat containing food items are flushed with nitrogen. Why?
Oil and fat-containing food items are flushed with nitrogen to prevent them from getting oxidized and becoming rancid. Nitrogen displaces oxygen in the food packaging, creating an oxygen-free environment, which slows down the oxidation process and helps maintain the freshness of the food
20. Explain the following terms with one example each.
(a) Corrosion
Corrosion is when metals slowly break down because of a chemical reaction with their surroundings. It often makes them weaker or even destroys them.
Example: When iron gets rusty because it reacts with moisture and oxygen in the air.
(b) Rancidity
Rancidity is when fats or oils develop a bad smell and taste because they’re breaking down chemically. It usually happens because of exposure to air, light, or certain microbes.
Example: When cooking oils start smelling bad after being left out in the open for too long.