1. Chemical Reactions and Equations

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Questions :

1. Why should a magnesium ribbon be cleaned before burning in air?

Answer: A magnesium ribbon should be cleaned before burning in air to remove the unwanted impurities present on the surface of the ribbon, especially the layer of magnesium oxide (MgO) which is formed due to reaction of magnesium with the oxygen present in the atmosphere. This unwanted impurities doesn't allow magnesium ribbon to show the chemical properties of pure magnesium and prevents or slows the burning of magnesium to avoid this the magnesium ribbon should be cleaned before burning in air.

2. Write a balanced equation for the following chemical reactions.

(i) Hydrogen + Chlorine → Hydrogen chloride.

Answer: H₂ + Cl₂→ 2HCl .

(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride.

Answer: 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄.

(iii) Sodium + Water → Sodium hydroxide + Hydrogen.

Answer: 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.

Answer: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq).

(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Answer: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l).

Questions :

1. A solution of a substance ‘X’ is used for whitewashing.

(i) Name the substance ‘X’ and write its formula.

Answer: The substance ‘X’ is quick lime or Calcium Oxide and its formula is CaO.

(ii) Write the reaction of the substance ‘X’ named in (i) above with water.

Answer: CaO + H₂O → Ca(OH)₂.

2. 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.

Answer: The activity is electrolysis of water. The gas collected in one of the test tubes in Activity 1.7 is double of the amount collected in the other because here, after electrolysis of 2 water molecule 2 molecules of hydrogen gas and 1 molecule of oxygen gas is released, by the formula V=nRT/P volume is directly proportional to the number of moles. As, 2 moles of hydrogen gas is collected and only 1 mole of oxygen gas is collected amount of volume of hydrogen gas collected would be double than that of oxygen. This gas is hydrogen gas.

Questions :

1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Answer: The colour of copper sulphate solution change when an iron nail is dipped in it because iron metal is more reactive than copper and hence displaces copper from the copper sulphate solution. After some time, the solution becomes iron sulphate and copper is precipitated which changes its colour from blue to green in colour. The displacement reaction is: Fe + CuSO₄ → FeSO₄ + Cu.

2. Give an example of a double displacement reaction other than the one given in Activity 1.10.

Answer: Reaction between sodium hydroxide (NaOH) and sulphuric acid (H₂SO₄) is an example of double displacement reaction. 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O.

3. Identify the substances that are oxidised and the substances that are reduced in the following reactions.

(i) 4Na(s) + O₂(g) → 2Na₂O(s)

Answer: In the following reaction sodium (Na) is oxidised by losing 2 electrons to oxygen atom and O₂ is reduced by gaining 2 electrons.

(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)

Answer: In the following reaction copper (Cu) is reduced by gaining 2 electrons and hydrogen (H₂) is oxidised by losing 2 electrons.

Exercises :

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) (a) and (b) of the statements about the reaction below are incorrect as in the reaction lead oxide is getting reduced not lead and also, in the reaction carbon is getting oxidised not carbon dioxide. They are products and even if the reverse reaction took place they will get opposite of what is stated in (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) displacement reaction.
Reason: As in the reaction aluminium metal which is more reactive than iron metal displaces iron from ferrous oxide and displacement reaction only means displacement of less reactive metal from its solution by more reactive metal hence the reaction is 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) Hydrogen gas and Iron chloride are produced. The hydrogen is displaced from hydrogen chloride by the iron in iron fillings to undergo the following reaction.
2HCl + Fe → FeCl₂ + H₂

4. What is a balanced chemical equation? Why should chemical equations be balanced?

Answer: A balanced equation is the one in total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants means the law of conservation of mass is followed. Balancing chemical equation is necessary for the reaction should obey The Law of Conservation of mass. Balancing the chemical equation has only defined method that number of specific atoms on L.H.S is equal to on R.H.S and is done by trial and error method.

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) Unbalanced equation: H₂ + N₂→ NH₃
Balanced equation: 3H₂ + N₂ → 2NH₃
(b) Unbalanced equation: H₂S + O₂ → H₂O + SO₂
Balanced equation: 2H₂S + 3O₂ → 2H₂O + 2SO₂
(c) Unbalanced equation: BaCl₂ + Al₂(SO₄)₃ → AlCl₃ + BaSO₄
Balanced equation: 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄
(d) Unbalanced equation: K + H₂O → KOH + H₂
Balanced equation: 2K + 2H₂O → 2KOH + H₂

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
(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) 2Ca(OH)₂ + 2CO₂ → 2CaCO₃ + 2H₂O
(b) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag
(c) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
(d) BaCl₂ + K₂SO₄→ BaSO₄ + 2KCl

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:
2KBr + BaI₂ → 2KI + BaBr₂
Type of reaction: Double Displacement Reaction
ZnCO₃ → ZnO + CO₂
Type of reaction: Decomposition Reaction
H₂ + Cl₂ → 2HCl
Type of reaction: Combination Reaction
Mg + 2HCl → MgCl₂ + H₂
Type of reaction: Displacement Reaction

9. What does one mean by exothermic and endothermic reactions? Give examples.

Answer: Endothermic reactions: Reactions in which energy is absorbed from the surroundings usually in the form of heat are known as endothermic reactions. Examples: Decomposition of quick lime CaCO₃(s) ^Δ_→ CaO(s) + CO₂(g), decomposition of silver chloride, silver bromide, etc.
Reactions in which heat is released in the surroundings along with the formation of products are called exothermic chemical reactions. Examples: Burning of coal:- C(s) + O₂(g) → CO₂(g) + Energy. Burning of natural gas:- CH₄(g) + 2O₂ (g) → CO₂ (g) + 2H₂ (g) + Energy, respiration, etc.

10. Why is respiration considered an exothermic reaction? Explain.

Answer: In this reaction glucose molecules broken down from carbohydrates combine with the oxygen present in our body's cells to form carbon dioxide and water, releasing energy and as exothermic reaction also means release of energy which same happens in respiration, respiration is considered an exothermic reaction. This energy is utilised for various purposes like, regulating body temperature. The reaction is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6HO + Energy.

11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer: Combination reaction is the reaction between two or more reactants to form a product whereas the decomposition reaction is defined as the splitting of a product into two or more reactants. This definitions clearly explains that the decomposition reaction is the opposite of the combination reaction. Also, combination reaction is usually exothermic as heat is released for the formation of bonds and decomposition reaction is usually endothermic as heat is absorbed for breaking of bonds.
Examples of Combination reaction: C + O₂ → CO₂; CaO + H₂O → Ca(OH)₂.
Examples of Decomposition reaction: ZnCO₃ → ZnO + CO₂; CaCO₃ → CaO + CO₂.

12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Answer: i. Thermal decomposition reaction : CaCO₃(s)_→^Δ CaO(s) + CO₂(g)
ii. Photolytic decomposition reaction : 2AgBr(s) _→^⛮ 2Ag(s) + Br₂(g)
iii. Electrolytic decomposition reaction : 2H₂O _→^⚡ 2H₂ + O₂

13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer:

Displacement Reaction	Double displacement Reaction
Displacement reaction happens when an element displaces another element from its compound.	Double displacement reaction happen when two different atoms or groups of atoms (ions) are exchanged.
It happens due to the difference in reactivity of metals.	It happens due to the mutual exchange of ions.
In it only a single displacement takes place.	In it two displacements take place.
Example: Fe + CuSO₄ → FeSO₄ + Cu	Example: 2KBr + BaI₂ → 2KI + BaBr₂

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: Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)

15. What do you mean by a precipitation reaction? Explain by giving examples.

Answer: The reaction of usually 2 aqueous salts which produces an insoluble substance in water or the liquid medium in which reaction is happening which generally sinks in it, called precipitate is known as precipitation reaction. For example, Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) in this reaction BaSO₄ is a precipitate formed, AgNO₃(aq) + KCl(aq) → AgCl(s) + KNO₃(aq) in this reaction AgCl is a precipitate formed.

16. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction

Answer: (a) Oxidation is the gain of oxygen. For example,
i. CuO(s) + H₂(g) → Cu(s) + H₂O(l) here hydrogen gets oxidised.
ii. 2ZnO + C → + 2Zn + CO₂ here carbon gets oxidised.
(b) Reduction is the loss of oxygen. For example,
i. 2PbO(s) + C(s) → 2Pb(s) + CO₂(g) here lead oxide gets reduced.
ii. CuO +H₂ → Cu + H₂O here copper gets reduced.

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 metal (Cu) and the black coloured compound formed is copper oxide (CuO). When the metal is heated in air,it reacts with atmospheric oxygen to form copper oxide. The reaction is 2Cu(s) + O₂(g) → 2CuO(s)

18. Why do we apply paint on iron articles?

Answer: Iron articles are painted to prevent them from rusting so that they should last long and its strength should not decrease. Because if they are not painted, the metal surface comes in contact with the atmospheric oxygen and in the presence of moisture it form Iron(III) oxide, ultimately leading to corrosion and weakness of metal. But if painted the surface does not come in contact with moisture and air as the layer of paint prevents direct contact thus preventing rusting.

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 them from becoming rancid. Because if they became rancid their smell and taste would become unpleasant, which can even lead to food poisoning, small shelf life and sales of company would also decrease, ultimately leading to less profit to avoid these a unreactive gas, nitrogen's enviroment in the packets is made which hampers the process of rancidity and thus increases profit with care of consumers.

20. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity

Answer: (a) Corrosion is a process in which a metal is attacked by substances around it such as moisture, acids, oxygen, etc., which ultimately leads to degradation of metal and its desired qualities. It is also a heavy load on economy, as for it lot of money is spent by government for huge iron structures like Eiffel Tower. The most common example is corrosion of iron called rusting, another example is of silver on which a black layer is formed of silver sulphide. Ag + H₂S → AgS + H₂.
(b) Rancidity is the condition produced in food materials due to atmospheric oxygen, by the oxidation of the oil and fat present in the food material and also due to rapid growth of microbes on food due to oxygen. Resulting in foul smell and unpleasant taste or change in it's taste and smell. It is also a heavy load on economy as for stopping it lots of money is required to be spent. It can be controlled by several methods like using air-tight containers, flushing packets with unreactive gas like nitrogen and using preservatives.

Best of luck and keep learning and be growing.