Chemical Reactions and Equations

Chapter 1 Chemical Reactions and Equations

Physical Change: If a change involves change in colour or state but no new substance is formed, then it is a physical change.

Change in physical properties.

– Melting

– Boiling

– Condensation

– [Note- No change occurs in the identity of the substance].

Chemical Change: If a change involves formation of new substances, it is a chemical change.

– Atoms in the reactants are rearranged to form one or more different substances.

– Old bonds are broken, new bonds are formed.

– Reactants lose their properties to form product of different properties.

4 Fe(s) + 3O₂ →2Fe₂O₃ (rust). Iron Oxygen Ferric oxide

In a chemical change some bonds are broken and some bonds are formed. The products are quite different from the reactants. Therefore, it normally can’t be reversed.

A chemical reaction involves a chemical change in which substances react to form new substances with entirely new properties. Substances that react or take part in the reaction are known as reactants and the substances formed are known as products.

During a chemical reaction, there is a breaking of bonds between atoms of the reacting molecules to give products.

A chemical reaction can be observed with the help of any of the following observations:

Evolution of a gas

Change in temperature

Formation of a precipitate

Change in colour

Change of state

Chemical equation:

The symbolic representation of a chemical reaction is called a chemical equation.

Features of a chemical equation:

The reactants are written on the left-hand side with a plus sign between them.

The products are written on the right-hand side with a plus sign between them.

An arrow separates the reactants from the products. The arrow head points towards the products and indicates the direction of the reaction.

Skeletal chemical equation: A chemical equation which simply represents the symbols and formulas of reactants and products taking part in the reaction is known as skeletal chemical equation for a reaction.

For example: For the burning of Magnesium in the air, Mg + O₂ → MgO is the skeletal equation.

Balanced chemical equation: The process of equating the number of atoms on both the sides of a chemical equation is known as balancing of a chemical equation.

A balanced equation is a chemical equation in which number of atoms of each element is equal on both sides of the equation i.e number of atoms of an element on reactant side = number of atoms of that element on the product side.

The first step in balancing a chemical equation is to write the number of atoms of each element present on the left hand side and right hand side.

We should always start balancing with the compound that contains maximum number of atoms.

It can be reactant or a product. Then in that compound select the element which has the maximum number of atoms.

While balancing a chemical equation, the molecular formulas of the reactants and products

should not change. The molecular formulas are simply multiplied by suitable coefficients.

To make a chemical equation more informative, the reaction conditions such as temperature,

pressure or catalyst are written on the arrow separating the reactants and products.

The evolution of gas is indicated by an upward arrow.

The formation of precipitate is indicated by a downward arrow.

Heat evolved during the reaction is written as + Heat on the product side.

Heat absorbed during the reaction is written as + Heat on the reactant side.

Identification:

Combustion : AB + O₂ → Oxide of A & B.

Combination : Combination reaction is a reaction in which 2 or more substances combine to give a single product. Combination reaction can be between two elements, between an element and a compound or between two compounds.

A + B → C

Decomposition : In a decomposition reaction, a single reactant decomposes to give two or more products. Decomposition reactions require energy in the form of heat, light or electricity

Types of decomposition reactions:

• Decomposition reactions which require heat are known as thermolytic.

• Decomposition reactions which require light are known as photolytic.

• Decomposition reactions which require electricity are known as electrolytic.

AB → A + B

Displacement : A + BC (aq) → AC (aq) + B

Double Displacement : AB (aq) + CD (aq) → AD (aq) + CB

Definitions with examples:

1. Combination Reaction :Two or more reactant combine to form a single product.

2 Mg (s) + O₂ (g) → 2 MgO

Magnesium + Oxygen → Magnesium oxide (White ash) (basic) turns Red litmus blue

2. Decomposition Reaction: A single compound decomposes or break down to give two or more simpler substances.

2 FeSO₄ (s) → Fe₂O₃(s) + SO₂(g) + SO₃(g)

Ferrous Sulphate → Ferric oxide + Sulphur dioxide + Sulphur trioxide

3. Displacement Reaction: A more reactive element [metal] displaces less reactive element [metal] from its aqueous salt solution.

The reactivity series is a list of metals arranged in the order of decreasing reactivity.

The most reactive metal is placed at the top and the least reactive metal is placed at the bottom.

Fe (s) + ZnSO₄ (aq) → FeSO₄ (aq) + Zn (s)

4. Double Displacement Reaction: Aqueous solution of two ionic compounds react by exchange of their ions is called double displacement Reaction

BaCl₂ (aq) + Na₂SO₄ (aq) → BaSO₄ (s) + 2 NaCl(aq)

Pb(NO₃)₂ + 2 KI (aq) → PbI₂ ( ↓ ) + 2 KNO₃ (aq)

Precipitation reaction: An insoluble solid known as precipitate is formed during a double displacement reaction. Such reactions are also known as precipitation reactions.

5. Oxidation Reaction: In oxidation reaction, addition of oxygen or removal of hydrogen or loss of electron takes place.

2 Mg(s) + O₂ (g) → 2MgO (s)

2 Cu + O₂ → 2 CuO (Black) (Copper II Oxide)

6. Reduction Reaction: In reduction Reaction addition of hydrogen or removal of oxygen or gain of electron takes place.

CuO(s) + H₂ (g) →Cu(s) + H₂O (l)

7. Redox Reaction: A reaction in which oxidation and reduction take place simultaneously in a reaction, is known as a redox reaction.

Oxidation is a chemical process in which a substance gains oxygen or loses hydrogen.

Reduction is a chemical process in which a substance gains hydrogen or loses oxygen.

If a substance gains oxygen or loses hydrogen during a reaction, it is said to be oxidised.

If a substance gains hydrogen or loses oxygen during a reaction, it is said to be reduced.

A substance that loses oxygen or gains hydrogen is known as an oxidising agent.

A substance that loses hydrogen or gains oxygen is known as a reducing agent.

An oxidising agent gets reduced whereas a reducing agent gets oxidised.

In terms of electronic concept, Oxidation is defined as a loss of electrons while reduction is defined as a gain of electrons.

CuO (s)+ H₂ → Cu(s) + H₂O (l).

8. Exothermic and endothermic reactions: If heat is evolved during a reaction, then such a reaction is known as Exothermic reaction. If heat is absorbed from the surroundings, then such a reaction is known as endothermic reaction.

Exothermic Reaction: Reaction in which heat is evolved.

C (s) + O₂ (g) → CO₂ (g) + Heat

10. Neutralisation Reaction : When an acid and a base react together to form salt and water.

HCl (aq) + NaOH (aq) → H₂O (l) + NaCl (aq)

(acid) (base) (Water) (Salt)

Law of Conservation of Mass

As per the law of conservation of mass, the total mass of the elements present in the products of a chemical reaction is equal to the total mass of the elements present in the reactants.

In a chemical reaction matter is conserved. Total no. of atoms = Total no. of atoms Total mass = Total mass. [While Balancing a Chemical Equation Formula of reactants and products should not be changed]. Balancing:

Corrosion: Corrosion is the slow eating up of metals by the action of air and moisture on their surfaces.

Corrosion in case of Iron is known as Rusting.

Chemically, rust is hydrated ferric oxide (Fe₂O₃.xH₂O)

Advantages of corrosion: Though corrosion is undesirable, it can be advantageous in case of aluminium which on exposure to air, gets coated with a protective layer of aluminium oxide. This protects the metal underneath from further corrosion and damage.

Prevention: Painting, Galvanization, oiling greasing.

Corrosion of Aluminium has advantage , since Al₂O₃ formed as a result of corrosion act as protective layer.

Rancidity: When oils and fats or foods containing oils and fats are exposed to air, they get oxidised due to which the food becomes stale and gives a bad smell. This is called Rancidity.

Oxidation of oils or fats in a flood, resulting into a bad smell and taste.

Rancidity can be prevented by: Adding anti-oxidants.

Vacuum Packing

Replacing air by Nitrogen

Refrigeration of food stuff

QUESTION BANK

Very Short answer type questions:

1. What happens when magnesium ribbon burns in air?

Ans. When magnesium ribbon burns in air, it combines with the oxygen to form magnesium oxide.

2Mg(s) + O₂(g) → 2MgO(s)

2. Name the gas evolved when zinc reacts with dil. HCl.

Ans. Hydrogen gas is evolved.

3. What is a chemical equation?

Ans. A chemical equation is a symbolic notation that uses formulae instead of words to represent a chemical equation.

4. On what chemical law, balancing of chemical equation is based?

Ans. Balancing of a chemical equation is based on the law of conservation of mass.

5. Represent decomposition of ferrous sulphate with the help of balanced chemical equation.

Ans. 2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g)

6. When carbon dioxide is passed through lime water, it turns milky, why?

Ans. Lime water (calcium hydroxide) combines with carbon dioxide to form a suspension

of calcium carbonate which makes lime water milky.

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

7. A zinc rod is left for nearly 20 minutes in a copper sulphate solution. What change would you observe in zinc rod?

Ans. Zinc rod will change into zinc sulphate.

8. What type of reaction is this: Na₂SO₄ + BaCl₂ → BaSO₄ +2NaCl

Ans. It is a double displacement reaction.

9. Identify the compound oxidized in the following reaction. H₂S (g) + Cl₂ → S(s) + 2HCl (g)

Ans. H₂S is oxidized.

Short answer type questions:

1. An iron knife kept dipped in a blue copper sulphate solution turns the blue solution light green. Why?

Ans. Iron is more reactive than copper. So, it displaces Cu from CuSO₄ solution and forms ferrous sulphate which is of Light Green Colour.

CuSO₄ (aq) + Fe (s)→ FeSO₄ (aq) + Cu(s)

Blue colour light green colour

2. A copper coin is kept in a solution of silver nitrate for some time. What will happen to the coin and the colour of the solution?

Ans: Copper is more reactive than silver, so it will displace silver from its salt solution:

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

So the solution will turn blue due to the formation of copper nitrate.

3. What do you understand by precipitation reaction? Explain with suitable examples.

Ans. Precipitation reaction: An insoluble solid known as precipitate is formed during a double displacement reaction. Such reactions are also known as precipitation reactions.

For example,

(1) When aqueous solution of sodium sulphate is mixed with the aqueous solution or barium chloride, barium sulphate comes in the form of white precipitate

Na₂SO₄ (aq)+BaCl₂(aq) → BaSO₄(↓)+2NaCl(aq)

(2) When aqueous solution of sodium chloride is mixed with the aqueous solution of silver nitrate, silver chloride comes in the form of white precipitate.

NaCl (aq)+AgNO₃(aq) → AgCl(↓)+NaNO₃(aq)

4. What is lime-water test for the detection of carbon dioxide?

Ans. When carbon dioxide gas is passed through lime water, it turns milky due to the formation of milky suspension (precipitate) of calcium carbonate.

Carbon dioxide is produced by the action of dilute HCl on sodium carbonate.

Na₂CO₃(s)+2HCl(aq)→2NaCl+H₂O(l)+CO₂

Carbon dioxide gas produced in this reaction is passed through lime water it changes to milky colour due to the formation of calcium carbonate.

Long answer type questions:

1. What is corrosion? State the conditions necessary for rusting of iron. How rusting is harmful?

Ans: Corrosion: The process of eating away of the metal by the action of atmospheric reagents changing the metal into its compound is called corrosion.

Corrosion in case of Iron is known as Rusting.

Chemically, rust is hydrated ferric oxide (Fe₂O₃.xH₂O)

Rusting of Iron: When iron and iron objects are exposed to atmosphere, they are attacked by air and moisture (water) of the atmosphere and a brown and orange coloured layer is formed on the surface. It is called rust which is mainly hydrated iron (III) oxide Fe₂O₃xH₂O.

Harmful Effect of Rusting: Hydrated iron (III) oxide is brittle substance and moves away from the surface thus the object is damaged. The objects get holes, cavities and rough surface. Conditions necessary for rusting:

(1) Open surfaces of the metal.

(2) Open surfaces of the metal.

(3) Presence of air (Oxygen).

(4) Presence of moisture (water).

2. What is rancidity? Write the common method to prevent it.

Ans. Rancidity: When oils and fats or foods containing oils and fats are exposed to air, they get oxidised due to which the food becomes stale and gives a bad smell. This is called Rancidity.

Rancidity can be prevented by:

Adding anti-oxidants.

Vacuum Packing

Replacing air by Nitrogen

Refrigeration of food stuff

3. a) Why cannot a chemical change be normally reversed?

b) Why is it always essential to balance a chemical equation?

c) What happens when CO₂ gas is passed through lime water and why does it disappear on passing excess CO₂?

d) Can rusting of iron takes place in distilled water?

Ans: a. In a chemical change some bonds are broken and some bonds are formed. The products are quite different from the reactants. Therefore, it normally can’t be reversed.

b. A chemical equation has to be balanced to satisfy the law of conservation of mass.

c. On passing CO₂ gas through lime water, it turns milky due to formation of insoluble calcium carbonate which dissolves on passing excess CO₂ due to formation of soluble calcium bicarbonate.

Ca(OH)₂ + CO₂(g) → CaCO₃(s) + H₂O(l)

CaCO₃(s) + H₂O(l) + CO₂ (g) → Ca(HCO₃)₂(soluble)

d. No, rusting of iron cannot take place in distilled water because it neither contains dissolved oxygen nor CO₂ both are essential for rusting of iron.

4. A, B and C are three elements which undergo chemical reactions in the following way:

A₂O₃ + 2B→B₂O + 2A

3 CSO₄ + 2B → B₂(SO₄)₃ + 3C

3 CO+ 2A→ A₂O₃ +3C

Answer the following

a) Which element is most reactive?

b) Which element is least reactive?

Ans: a) The most reactive element is ‘B’. It has displaced both ‘A’ and ‘C’ from their compounds.

b) The least reactive element is ‘C’ as it has been displaced by both ‘A’ and ‘B’.

5. Write balanced equations for the following word equations.

1. Potassium chloride + Silver nitrate→Potassium nitrate + Silver chloride

2. Aluminium hydroxide + Sodium nitrate→ Aluminium nitrate + Sodium hydroxide

3. Iron metal + Copper(II) sulphate → Iron(ii) sulphate + copper metal

4. Aluminium metal + Copper(II) chloride →Aluminium chloride + copper metal

5. Potassium bromide→Potassium metal + Bromine

6. Calcium carbonate →Calcium oxide + Carbon dioxide gas

7. Zinc metal + Oxygen gas → Zinc oxide

8. Chlorine gas + Sodium metal →Sodium chloride

9. Aluminium sulphate + Barium chloride→Aluminium chloride + Barium sulphate

10. Sodium hydrogen carbonate → Sodium carbonate + Carbon dioxide + Water

Question 1:

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

Answer:

Magnesium is very reactive metal. When stored it reacts with oxygen to form a layer magnesium oxide on its surface. 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 exposed into air.

Question 2:

Write the balanced equation for the following chemical reactions.

Hydrogen + Chlorine → Hydrogen chloride
Barium chloride + Aluminium sulphate → Barium sulphate +

Aluminium chloride

Sodium + Water → Sodium hydroxide + Hydrogen

Answer

(i) $\displaystyle {{H}_{{2\left( g \right)}}}+C{{l}_{2}}_{{\left( g \right)}}\to 2HC{{l}_{{\left( g \right)}}}$

(ii) $\displaystyle 3BaCl{{H}_{{2\left( s \right)}}}+A{{l}_{2}}{{\left( {S{{O}_{4}}} \right)}_{3}}_{{\left( g \right)}}\to 3BaS{{O}_{4}}_{{\left( s \right)}}+2AlC{{l}_{3}}_{{\left( s \right)}}$

(iii) $\displaystyle 2N{{a}_{{\left( s \right)}}}+2{{H}_{2}}O\to 2NaO{{H}_{{\left( {aq} \right)}}}+{{H}_{2}}_{{\left( s \right)}}$

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.

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

Answer

(i) $\displaystyle BaC{{l}_{2}}_{{\left( {aq} \right)}}+N{{a}_{2}}S{{O}_{{4\left( {aq} \right)}}}\to BaS{{O}_{{4\left( s \right)}}}+2NaC{{l}_{{\left( {aq} \right)}}}$

(ii) $\displaystyle NaO{{H}_{{\left( {aq} \right)}}}+HC{{l}_{{\left( {aq} \right)}}}\to NaC{{l}_{{\left( {aq} \right)}}}+{{H}_{2}}{{O}_{{\left( l \right)}}}$

Question 1:

A solution of a substance ‘X’ is used for white washing.

Name the substance ‘X’ and write its formula.
Write the reaction of the substance ‘X’ named in (i) above with water. Answer
The substance ‘X’ is calcium oxide. Its chemical formula is CaO.
Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).

Question 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

Water (H₂O) contains two parts hydrogen and one part oxygen. Therefore, the amount of hydrogen and oxygen produced during electrolysis of water is in a 2:1 ratio. During electrolysis, since hydrogen goes to one test tube and oxygen goes to another, the amount of gas collected in one of the test tubes is double of the amount collected in the other.

Question 1:

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

Answer

When an iron nail is placed in a copper sulphate solution, iron displaces copper from copper sulphate solution forming iron sulphate, which is green in colour.

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.

Answer

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

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 that are reduced in the following reactions.

(i) $\displaystyle 4N{{a}_{{\left( s \right)}}}+{{O}_{{2\left( g \right)}}}\to 2N{{a}_{2}}{{O}_{{\left( s \right)}}}$

(ii) $\displaystyle Cu{{O}_{{\left( s \right)}}}+{{H}_{{2\left( g \right)}}}\to C{{u}_{{\left( s \right)}}}+{{H}_{2}}{{O}_{{\left( l \right)}}}$

Answer

Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.
Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H₂) gets oxidised to water (H₂O).

Exercise solution Question 1:

Which of the statements about the reaction below are incorrect?

$\displaystyle 2Pb{{O}_{{\left( s \right)}}}+{{C}_{{\left( s \right)}}}\to P{{b}_{{\left( s \right)}}}+C{{O}_{2}}_{{\left( g \right)}}$

Lead is getting reduced.
Carbon dioxide is getting oxidised.
Carbon is getting oxidised.
Lead oxide is getting reduced.
(a) and (b)
(a) and (c)
(a), (b) and (c)
all

Answer

(i)(a) and (b)

Question 2:

$\displaystyle F{{e}_{2}}{{O}_{3}}+2Al\to A{{l}_{2}}{{O}_{3}}+2Fe$

The above reaction is an example of a (a) combination reaction.

double displacement reaction.
decomposition reaction.
displacement reaction.

Answer

(d) The given reaction is an example of a displacement reaction.

Question 3:

What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.

Hydrogen gas and iron chloride are produced.
Chlorine gas and iron hydroxide are produced.
No reaction takes place.
Iron salt and water are produced.

Answer

(a) Hydrogen gas and iron chloride are produced. The reaction is as follows:

$\displaystyle F{{e}_{{\left( s \right)}}}+2HC{{l}_{{\left( {aq} \right)}}}\to FeC{{l}_{2}}_{{\left( {aq} \right)}}+{{H}_{2}}\uparrow$

Question 4:

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

Answer

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.

Hydrogen gas combines with nitrogen to form ammonia.
Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer

(a) $\displaystyle 3{{H}_{{2\left( g \right)}}}+{{N}_{{2\left( g \right)}}}\to 2N{{H}_{{3\left( g \right)}}}$

(b) $\displaystyle 3{{H}_{2}}{{S}_{{\left( g \right)}}}+3{{O}_{{2\left( g \right)}}}\to 2{{H}_{2}}{{O}_{{\left( l \right)}}}+2S{{O}_{{2\left( g \right)}}}$

(c) $\displaystyle 3BaC{{l}_{{2\left( {aq} \right)}}}+A{{l}_{2}}{{\left( {S{{O}_{4}}} \right)}_{{3\left( {aq} \right)}}}\to 2AlC{{l}_{{3\left( {aq} \right)}}}+3BaS{{O}_{{4\left( s \right)}}}$

(d) $\displaystyle 2{{K}_{{\left( s \right)}}}+2{{H}_{2}}{{O}_{{\left( l \right)}}}\to 2KO{{H}_{{\left( {aq} \right)}}}+{{H}_{{2\left( g \right)}}}$

Question 6:

Balance the following chemical equations.

(a) $\displaystyle HN{{O}_{3}}+Ca{{\left( {OH} \right)}_{2}}\to Ca{{\left( {N{{O}_{3}}} \right)}_{2}}+{{H}_{2}}O$

(b) $\displaystyle NaOH+{{H}_{2}}S{{O}_{4}}\to N{{a}_{2}}S{{O}_{4}}+{{H}_{2}}O$

(d) $\displaystyle BaC{{l}_{2}}+{{H}_{2}}S{{O}_{4}}\to BaS{{O}_{4}}+HCl$

Answer

(a) $\displaystyle 2HN{{O}_{3}}+Ca{{\left( {OH} \right)}_{2}}\to Ca{{\left( {N{{O}_{3}}} \right)}_{2}}+2{{H}_{2}}O$

(b) $\displaystyle 2NaOH+{{H}_{2}}S{{O}_{4}}\to N{{a}_{2}}S{{O}_{4}}+2{{H}_{2}}O$

(d) $\displaystyle BaC{{l}_{2}}+{{H}_{2}}S{{O}_{4}}\to BaS{{O}_{4}}+2HCl$

Question 7:

Write the balanced chemical equations for the following reactions.

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
Zinc + Silver nitrate → Zinc nitrate + Silver
Aluminium + Copper chloride → Aluminium chloride + Copper
Barium chloride + Potassium sulphate → Barium sulphate +

Potassium chloride Answer

(a) $\displaystyle Ca{{\left( {OH} \right)}_{2}}+C{{O}_{2}}\to CaC{{O}_{3}}+{{H}_{2}}O$

(b) $\displaystyle Zn+2AgN{{O}_{3}}\to Zn{{\left( {N{{O}_{3}}} \right)}_{2}}+2Ag$

(d) $\displaystyle BaC{{l}_{2}}+{{K}_{2}}S{{O}_{4}}\to BaS{{O}_{4}}+2KCl$

Question 8:

Write the balanced chemical equation for the following and identify the type of reaction in each case.

(a)Potassium bromide _(a_q₎ + Barium iodide _(aq) → Potassium iodide _(aq) +

Barium bromide_(s)

Zinc carbonate ₍_s₎ → Zinc oxide ₍_s₎ + Carbon dioxide ₍_g₎
Hydrogen ₍_g₎+ Chlorine ₍_g₎ → Hydrogen chloride ₍_g₎
Magnesium ₍_s₎+ Hydrochloric acid ₍_aq₎→ Magnesium chloride ₍_aq₎+ Hydrogen ₍_g₎

Answer

(a) $\displaystyle 2KB{{r}_{{\left( {aq} \right)}}}+Ba{{I}_{{2\left( {aq} \right)}}}\to 2K{{I}_{{\left( {aq} \right)}}}+BaB{{r}_{{2\left( s \right)}}}$ Double displacement reaction

(b) $\displaystyle ZnC{{O}_{{3\left( s \right)}}}\to Zn{{O}_{{\left( s \right)}}}+C{{O}_{{2\left( s \right)}}}$ Decomposition reaction

(c) $\displaystyle {{H}_{{2\left( g \right)}}}+C{{l}_{{2\left( g \right)}}}\to 2HC{{l}_{{\left( g \right)}}}$ Combination reaction

(d) $\displaystyle M{{g}_{{\left( s \right)}}}+2HC{{l}_{{\left( {aq} \right)}}}\to MgC{{l}_{{2\left( {aq} \right)}}}+{{H}_{{2\left( g \right)}}}$ Displacement reaction

Question 9:

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

Answer

Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions.

Example: Mixture of sodium and chlorine to yield table salt

$\displaystyle N{{a}_{{\left( s \right)}}}+\frac{1}{2}C{{l}_{{2\left( g \right)}}}\to NaC{{l}_{{\left( {aq} \right)}}}+411\text{ kj of energy}$

In other words, combination reactions are exothermic.

Reactions that absorb energy or require energy in order to proceed are called endothermic reactions.

For example: In the process of photosynthesis, plants use the energy from the sun to convert carbon dioxide and water to glucose and oxygen.

$\displaystyle 6C{{O}_{{2\left( g \right)}}}+6{{H}_{2}}{{O}_{{\left( l \right)}}}\xrightarrow{{sunlight}}{{C}_{6}}{{H}_{{12}}}{{O}_{{6\left( {aq} \right)}}}+6{{O}_{{2\left( g \right)}}}$

Question 10:

Why is respiration considered an exothermic reaction? Explain.

Answer

Energy is required to support life. Energy in our body is obtained from the food we eat. During digestion, large molecules of food are broken down into simpler substances such as glucose. Glucose combines with oxygen in the cells and provides energy. The special name of this combustion reaction is respiration. Since energy is released in the whole process, it is an exothermic process.

$\displaystyle {{C}_{6}}{{H}_{{12}}}{{O}_{{6\left( {aq} \right)}}}+6{{O}_{{2\left( g \right)}}}\to 6C{{O}_{{2\left( g \right)}}}+6{{H}_{2}}{{O}_{{\left( l \right)}}}$

Question 11:

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

Answer

Decomposition reactions are those in which a compound breaks down to form two or more substances. These reactions require a source of energy to proceed. Thus, they are the exact opposite of combination reactions in which two or more substances combine to give a new substance with the release of energy.

Decomposition reaction:

$\displaystyle AB\xrightarrow{{Energy}}A+B$

$\displaystyle 2{{H}_{2}}{{O}_{{\left( l \right)}}}\xrightarrow{{Electrolysis}}2{{H}_{{2\left( g \right)}}}+{{O}_{{2\left( g \right)}}}$

Combination reaction:

$\displaystyle 2{{H}_{{2\left( g \right)}}}+{{O}_{{2\left( g \right)}}}\to 2{{H}_{2}}{{O}_{{\left( l \right)}}}+Energy$

Question 12:

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

Answer

Thermal decomposition:

$\displaystyle FeS{{O}_{4}}\xrightarrow{\Delta }F{{e}_{2}}{{O}_{3}}+S{{O}_{2}}+S{{O}_{3}}$

Decomposition by light:

$\displaystyle 2AgCl\xrightarrow{{Light}}2Ag+C{{l}_{2}}$

Decomposition by electricity:

$\displaystyle 2{{H}_{2}}O\xrightarrow{{Electricity}}2{{H}_{2}}+{{O}_{2}}$

Question 13:

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

Answer

In a displacement reaction, a more reactive element replaces a less reactive element from a compound.

$\displaystyle A+BX\to AX+B$ where A is more reactive than B

In a double displacement reaction, two atoms or a group of atoms switch places to form new compounds.

$\displaystyle AB+CD\to AD+CB$

For example:

Displacement reaction:

$\displaystyle CuS{{O}_{4}}+Zn\to ZnS{{O}_{4}}+Cu$

Double displacement reaction:

$\displaystyle N{{a}_{2}}S{{O}_{4}}+BaC{{l}_{2}}\to BaS{{O}_{4}}+2NaCl$

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.

Answer

$\displaystyle 2AgN{{O}_{3}}+Cu\to Cu{{\left( {N{{O}_{3}}} \right)}_{2}}+2Ag$

Question 15:

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

Answer

A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction.

For example:

$\displaystyle N{{a}_{2}}C{{O}_{3}}+CaC{{l}_{2}}\to CaC{{O}_{3}}+2NaCl$

In this reaction, calcium carbonate is obtained as a precipitate. Hence, it is a precipitation reaction.

Another example of precipitation reaction is:

$\displaystyle N{{a}_{2}}S{{O}_{4}}+BaC{{l}_{2}}\to BaS{{O}_{4}}+2NaCl$

In this reaction, barium sulphate is obtained as a precipitate.

Question 16:

Explain the following in terms of gain or loss of oxygen with two examples each.

Oxidation
Reduction

Answer

Oxidation is the gain of oxygen.

For example:

(i)

(ii)

In equation (i), H₂ is oxidized to H₂O and in equation (ii), Cu is oxidised to CuO.

Reduction is the loss of oxygen.

For example:

(i)

(ii)

In equation (i), CO₂ is reduced to CO and in equation (ii), CuO is reduced to Cu.

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.

Answer

‘X’ is copper (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the reaction involved on heating copper is given below.

Question 18:

Why do we apply paint on iron articles?

Answer

Iron articles are painted because it prevents them from rusting. When painted, the contact of iron articles from moisture and air is cut off. Hence, rusting is prevented their presence is essential for rusting to take place.

Question 19:

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

Answer

Nitrogen is an inert gas and does not easily react with these substances. On the other hand, oxygen reacts with food substances and makes them rancid. Thus, bags used in packing food items are flushed with nitrogen gas to remove oxygen inside the pack. When oxygen is not present inside the pack, rancidity of oil and fat containing food items is avoided.

Question 20:

Explain the following terms with one example each.

Corrosion
Rancidity

Answer

Corrosion:

Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc.

For example, iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide.

This hydrated iron oxide is rust.

Rancidity:

The process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.

For example, the taste and smell of butter changes when kept for long.

Rancidity can be avoided by:

Storing food in air tight containers
Storing food in refrigerators
Adding antioxidants
Storing food in an environment of nitrogen

Chemical Reactions and Equations

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