Science (Code 086)

Chapter 1:

Chemical Reactions & Equations

NCERT Solution & In-Text Questions

Class 10th CBSE

In-Text Questions Page-6

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

Magnesium is very reactive metal like (Na, Ca, etc.). When it expose to air it reacts with oxygen to form a layer magnesium oxide (MgO) on its surface.

`2Mg+O_{2} \Rightarrow 2MgO`

This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned by sand paper to remove this layer so that the underlying metal can be used for the reaction.

Concept: Always remember that magnesium being a reactive metal will react with oxygen if kept in open. So, it has to be cleaned before it is burned in air.

Question.2. Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride

`H_{2}+Cl_{2} \Rightarrow 2HCl`

Concept: First convert the word equation into skeletal equation and then balance the different atoms on both sides of the equation.

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

`3BaCl_{2}+Al_{2}(SO_{4})_{3}\Rightarrow BaSO_{4}+2AlCl_{3}`

(iii) Sodium + Water → Sodium hydroxide + Hydrogen

`2Na+2H_{2}O\Rightarrow 2NaOH+H_{2}(\uparrow)`

Question.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.

`BaCl_{2}(aq)+Na_{2}SO_{4}(aq)\Rightarrow BaSO_{4}(s)+2NaCl(aq)`

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

`NaOH (aq)+HCl (aq) \Rightarrow NaCl (aq)+H_{2}O (l)`

In-Text Questions Page-10

Question.1. A solution of a substance ‘X’ is used for white washing.
(i) Name the substance ‘X’ and write its formula.

The substance whose solution in water is used for white washing is calcium oxide (or quick lime). Its formula is CaO.

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

Calcium oxide (quick lime) reacts vigorously with water to form calcium hydroxide (slaked lime)

`CaO+H_{2}O\Rightarrow Ca(OH)_{2}`

Question.2. Why is the amount of gas collected in one of the test tubes in text book Activity 1.7 (i.e., electrolysis of water) double of the amount collected in the other? Name this gas.

In Activity 1.7, water is electrolysed to give H2 gas at one electrode and O2 gas at the other electrode.

`2H_{2}O (l)\Rightarrow H_{2} (g)+O_{2} (g)`

Thus two molecules of water on electrolysis give two molecules of hydrogen gas and one molecule of oxygen gas or in other words the amount of hydrogen gas collected would be double than that of oxygen gas.

In-Text Questions Page-13

Question.1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it ? OR
An iron nail is dipped in the solution of copper sulphate for about 30 minutes. State the change in colour observed. Give reason for the change.

When an iron nail is dipped in a copper sulphate solution, iron (which is more reactive than copper) displaces copper from copper sulphate solution forming iron sulphate, which is green in colour.

`CuSO_{4}+Fe\Rightarrow FeSO_{4}+Cu`

Therefore, the blue colour of copper sulphate solution fades and green colour appears.

Question.2. Give an example of a double displacement reaction other than the one given in Activity 1.10 (NCERT Text Book).

Sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium chloride.

`Na_{2}CO_{3}+CaCl_{2}\Rightarrow CaCO_{3}+2NaCl`

In this reaction, sodium carbonate and calcium chloride exchange ions to form two new compounds. Hence, it is a double displacement reaction.

Question.3. Identify the substances that are oxidised and the substances which are reduced in the following reactions.
(i) `4Na (s) + O_{2} (g) → 2Na_{2}O (s)`

Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.

(ii) `CuO (s) + H_{2} (g) → Cu (s) + H_{2}O (l)`

Copper oxide `(CuO)` is reduced to copper `(Cu)` while hydrogen `(H_{2})` gets oxidised to water `(H_{2}O)`.

Exercise Solution Page-14

Question.1. Which of the statements about the reaction below are incorrect ?
`2PbO(s)+C(s)\Rightarrow 2Pb(s)+CO_{2}(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

(i) (a) and (b)

Question.2.`Fe_{2}O_{3}+2Al\Rightarrow Al_{2}O_{3}+2Fe`
The above reaction is an example of a

(a) combination reaction
(b) double displacement reaction
(c) decomposition reaction
(d) displacement reaction

(d) Displacement reaction

Question.3. What happens when dilute hydrochloric acid is added to iron filings ? 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.

(a) Hydrogen gas and iron chloride are produced.
Explanation: Hydrogen gas and iron chloride are produced. The reaction is as follows:
`Fe(s)+2HCl(aq)\Rightarrow FeCl_{2}(aq)+H_{2}(\uparrow)`

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

A reaction which has an equal number of atoms of all the elements on both sides of the chemical equation is called a balanced chemical equation.
The law of conservation of mass states that mass can neither be created nor destroyed. Hence, in a chemical reaction, the total mass of reactants should be equal to the total mass of the products. It means that the total number of atoms of each element should be equal on both sides of a chemical equation. Hence, it is for this reason that chemical equations should be balanced.

Question.5. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.

`3H_{2}(g)+N_{2}(g)\Rightarrow 2aNH_{3}(g)`

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

`H_{2}S(g)+3O_{2}(g)\Rightarrow SO_{2}(g)+2H_{2}O(l)`

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

`3BaCl_{2}(aq)+Al_{2}(SO_{4})_{3}(aq)\Rightarrow 2AlCl_{3}(aq)+3BaSO_{4}(\downarrow)(s)`

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

`2K(s)+2H_{2}O(l)\Rightarrow 2KOH(aq)+H_{2}(g)`

Question.6. Balance the following chemical equations :
(a) `HNO_{3}+Ca(OH)_{2}\Rightarrow Ca(NO_{3})_{2}+H_{2}O`

`2HNO_{3}+Ca(OH)_{2}\Rightarrow Ca(NO_{3})_{2}+2H_{2}O`

(b) `NaOH+H_{2}SO_{4}\Rightarrow Na_{2}SO_{4}+H_{2}O`

`2NaOH+H_{2}SO_{4}\Rightarrow Na_{2}SO_{4}+2H_{2}O`

(c) `NaCl+AgNO_{3}\Rightarrow AgCl+NaNO_{3}`

`NaCl+AgNO_{3}\Rightarrow AgCl+NaNO_{3}`

(d) `BaCl_{2}+H_{2}SO_{4}\Rightarrow BaSO_{4}+HCl`

`BaCl_{2}+H_{2}SO_{4}\Rightarrow BaSO_{4}+2HCl`

Question.7. Write the balanced chemical equations for the following reactions :
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

`Ca(OH)_{2}+CO_{2}\Rightarrow CaCO_{3}+H_{2}O`

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

`Zn+2AgNO_{3}\Rightarrow Zn(NO_{3})_{2}+2Ag`

(c) Aluminium + Copper chloride → Aluminium chloride + Copper

`2Al+3CuCl_{2}\Rightarrow 2AlCl_{3}+3Cu`

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

`BaCl_{2}+K_{2}SO_{4}\Rightarrow BaSO_{4}+2KCl`

Question.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)

`2KBr(aq)+BaI_{2}(aq)\Rightarrow 2KI(aq)+BaBr_{2}(s)`
Type of reaction: Double displacement reaction

(b) Zinc carbonate(s) → Zinc oxide (s) + Carbon dioxide (g) bromide(s)

`ZnCO_{3}(s)\Rightarrow ZnO(s)+CO_{2}(g)`
Type of reaction: Decomposition reaction

(c) Hydrogen (g) + Chloride (g) → Hydrogen chloride (g)

`H_{2}(g)+Cl_{2}(g)\Rightarrow 2HCl(g)`
Type of reaction: Combination reaction

(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)

`Mg(s)+2HCl(aq)\Rightarrow MgCl_{2}(aq)+H_{2}(g)`
Type of reaction: Displacement reaction

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

Exothermic reactions : Those reactions in which heat is evolved are known as exothermic reactions. An exothermic reaction is indicated by writing “+ Heat”on the products side of an equation.
Example: Burning of natural gas: `CH_{4}(g)+2O_{2}(g)\Rightarrow CO_{2}(g)+2H_{2}O(g)+Heat`

Endothermic reactions : Those reactions in which heat is absorbed are known as endothermic reactions. An endothermic reaction is usually indicated by writing “- Heat” on the product side of a chemical equation.
Example: `C(s)+2S(s)\Rightarrow CS_{2}(l)-Heat`

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

Respiration is an exothermic process because during respiration glucose combines with oxygen in the cells of our body to form carbon dioxide and water along with the production of energy.
Glucose + oxygen → carbon dioxide + water + energy
`C_{6}H_{12}O_{6}(aq)+6O_{2}(g)\Rightarrow 6CO_{2}(g)+6H_{2}O(l)+Energy`
It is an exothermic reaction, because energy is produced during this process.

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

In a decomposition reaction, a single compound breaks down to produce two or more simpler substances. These reactions require a source of energy to proceed.
For example: `AB+Energy\rightarrow A+B`
Hydrolysis: `2H_{2}O(l)\Rightarrow 2H_{2}(g)+O_{2}(g)`
While, in a combination reaction, two or more substances simply combine to form a new substance with the release of energy.
For example: `A+B\rightarrow AB+Energy`
Combination Reaction: `2H_{2}(g)+O_{2}(g)\Rightarrow 2H_{2}O(l)+Energy`
Thus, decomposition reactions are the exact opposite of combination reactions.

Question.12. Write one equation each for the decomposition reactions where energy is supplied in the form of heat, light or electricity. OR
Decomposition reactions require energy either in the form of heat or light or electricity for breaking down the reactants. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light and electricity.

Decomposition reaction in which energy is supplied in the form of heat:
`CaCO_{3}(s)+Heat\Rightarrow CaO(s)+CO_{2}(g)`
Decomposition reaction in which energy is supplied in the form of light,i.e.sunlight:
`2AgCl(s)+Sunlight\Rightarrow 2Ag(s)+Cl_{2}(g)`
Decomposition reaction in which energy is supplied in the form of electricity i.e. Electrolysis:
`2H_{2}O(l)+Electricity\Rightarrow 2H_{2}(g)+O_{2}(g)`

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

In displacement reactions, a more reactive metal displaces a less reactive metal from its solution. `A+BX \Rightarrow AX+B` where A is more reactive than B
For example, `Fe(s)+CuSO_{4}(aq)\Rightarrow Cu(s)+FeSO_{4}(aq)`
This is a displacement reaction where iron displaces copper from its solution.
In double displacement reactions, two reactants in solution exchange their ions.
`AB+CD\Rightarrow AD+CB`
For example, `AgNO_{3}(aq)+NaCl(aq)\Rightarrow AgCl(s)+NaNO_{3}(aq)`
This is a double displacement reaction where silver nitrate and sodium chloride exchange `Cl^{-}` and `NO_3^-` ions between them.

Question.14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

`2AgNO_{3}(aq)+Cu(s)\Rightarrow Cu(NO_{3})_{2}(aq)+2Ag(s)`

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

A reaction in which an insoluble solid called precipitate is formed that separates from the solution is called a precipitation reaction. In other words, we can say that a precipitation reaction is a reaction in which soluble ions in separate solutions are mixed together to form an insoluble compound that settles out of solution as a solid. This insoluble compound is called a precipitate.
An example of a precipitation reaction
Aqueous silver nitrate `(AgNO_{3})`, when added to a solution containing potassium chloride `(KCl)`, precipitates a white solid, and silver chloride is observed.
`AgNO_{3}(aq)+KCl(aq)\Rightarrow AgCl(s)(\downarrow)+KNO_{3}(aq)`
The silver chloride `(AgCl)` has formed a solid, which is observed as a precipitate.

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

Oxidation: The addition of oxygen to a substance is called oxidation.
(i) `S(s)+O_{2}(g) \Rightarrow SO_{2}(g)` (Addition of oxygen to sulphur)
(ii) `2Mg(s)+O_{2}(g) \Rightarrow 2MgO(s)` (Addition of oxygen to magnesium)

(b) Reduction.

Reduction: The removal of oxygen from a substance is called reduction.
Example: (i) `CuO+H_{2}+Heat \Rightarrow Cu + H_{2}O`
Here, copper oxide is being reduced to copper because oxygen gets removed from copper oxide.
(ii) `ZnO+C\Rightarrow Zn+CO`
Here, zinc oxide is being reduced to zinc because oxygen gets removed from zinc oxide.

Question.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.

The shiny brown colored element X is copper metal (Cu). When copper metal is heated in air, it forms a black colored compound copper oxide. So, the black colored compound is the copper oxide or copper (II) oxide, CuO.
`2Cu+O_{2}+Heat\Rightarrow 2CuO`

Question.18. Why do we apply paint on iron articles ?

When articles made of iron comes in contact with the moisture present in air, it forms iron oxide, which is known as rust. Hence, paint is applied to prevent an iron article to come in contact with moisture present in air. Paint prevents rusting of iron.

Question.19. Oil and fat containing food items are flushed with nitrogen. Why ?

When fats and oils are oxidised, they become rancid and their smell and taste change. This phenomenon is called rancidity. Hence, food items containing oil and fat are flushed with nitrogen (unreactive gas) which prevents them to get oxidized and becoming rancid. Thus, to keep food items fresh and save from getting oxidised, food items are flushed with nitrogen.

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

Corrosion: It is the process in which metals are eaten up gradually by the action of air, moisture or a chemical (such as an acid) on their surface.
Example: When iron is exposed to moist air for a long period of time, its surface acquires a coating of a brown, flaky substance called rust. Rust is mainly hydrated iron (III) oxide `[Fe_{2}O_{3}\cdot xH_{2}O]`.

(b) Rancidity

Rancidity: The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity.
Rancidity spoils the food materials prepared in fats and oils which have been kept for a considerable time and makes them unfit for eating.
Rancidity can be prevented by adding anti-oxidants to foods containing fats and oils. It can also be prevented by flushing fat and oil containing foods with nitrogen before sealing.

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