Class 10 Science Notes | Chemical Reactions and Equations Class 10

Chemical Reactions and Equations

Introduction

Chemical changes result from chemical reactions taking place between substances.

Chemical reactions: The processes in which a substance or substances undergo change to produce new substances with new properties are known as chemical reactions. For example, when calcium carbonate is heated, calcium oxide and carbon dioxide are formed.

$CaCO_3 \overset{\triangle}{\rightarrow} CaO + CO_2$

(Calcium carbonate

$\overset{\triangle}{\rightarrow}$ Calcium oxide + Carbon dioxide)

The breaking up of calcium carbonate into calcium oxide and carbon dioxide is, thus, a chemical reaction because calcium carbonate changes into new substances, calcium oxide, and carbon dioxide.

Burning of a magnesium ribbon in air and collection of magnesium oxide in a watch-glass

You must have observed that magnesium ribbon burns with a dazzling white flame and changes into a white powder. This powder is magnesium oxide. It is formed due to the reaction between magnesium and oxygen present in the air.

The reaction between lead nitrate and potassium iodide 👉 yellow precipitate of lead iodide is formed.

$Pb(NO_3)_2 + 2KI rightarrow PbI_2 + 2KNO_3$

Reactant: The substance which takes part in a chemical reaction is called a reactant.

$Na + H_2O \rightarrow NaOH + H_2$

Here,

$Na$ and

$H_2O$ are reactants.

Product: A product is a new substance formed in a chemical reaction.

$Na + H_2O \rightarrow NaOH + H_2$

Here,

$NaOH$ and

$H_2$ are the products.

Characteristics of chemical reactions

There are some characteristics by which we can identify the occurrence of a chemical reaction. These are described below.

1. Evolution of gas Some of the chemical reactions occur with the evolution of gas or a mixture of gases. Metals like zinc, magnesium etc react with dilute hydrochloric acid with the evolution of hydrogen gas.

$Zn + 2HCl \rightarrow ZnCl_2 + H_2 \uparrow$

$Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2 \uparrow$

2. Formation of precipitate Sometimes, when two solutions are mixed together, a solid get separated from the solution. The solid thus separated is called the precipitate. When an aqueous solution of sodium sulphate is mixed with a solution of barium chloride, a white precipitate of barium sulphate is produced.

$Na_2SO_4 + BaCl_2 \rightarrow BaSO_4 + 2NaCl$

3. Change in temperature There are several reactions that occur with a change in temperature. When quick lime (calcium oxide) is treated with water in a beaker, a large quantity of heat is produced. Such reactions in which heat is produced are called exothermic reactions.

$CaO + H_2O \rightarrow Ca(OH)_2 + \triangle$

(Calcium oxide + Water

$\rightarrow$ Calcium hydroxide)

4. Change in colour In some chemical reactions a change in color is observed.

The reaction between lead nitrate and potassium iodide 👉 yellow precipitate of lead iodide is formed.

$Pb(NO_3)_2 + 2KI rightarrow PbI_2 + 2KNO_3$

Chemical equation: A chemical equation is a representation of the chemical reaction using symbols and formulae.

$Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2$

Unbalanced chemical equation (Skeletal equation): When the number of atoms of the elements on the two sides of the equation are not the same.

$Mg + O_2 \rightarrow MgO$

Balanced chemical equation: When the number of atoms of the elements on the two sides of the equation is the same.

$2Mg + O_2 \rightarrow 2MgO$

Balancing chemical equation

Balance the equation $Mg + H_2O \rightarrow Mg(OH)_2 + H_2$

Count each type of atom

H and O are unbalanced.

To balance O, 2 is set just before $H_2O$

$Mg + 2H_2O \rightarrow Mg(OH)_2 + H_2$

Now check the atoms on both the side. The equation is balanced now.

Balance the equation

$AlCl_3 + Ca(OH)_2 \rightarrow Al(OH)_3 + CaCl_2$

Balanced

$2AlCl_3 + 3Ca(OH)_2 \rightarrow 2Al(OH)_3 + 3CaCl_2$

Balance the equation

$Fe + H_2O \rightarrow Fe_3O_4 + H_2$

Balanced

$3Fe + 4H_2O \rightarrow Fe_3O_4 + 4H_2$

Types of chemical reactions

1. Combination reaction

2. Decomposition reaction

3. Displacement reaction

4. Double displacement reaction

5. Oxidation and reduction reaction

1. Combination reaction

The reactions in which two or more substances combine to form a single substance is known as a combination reaction.

X + Y $\rightarrow$ XY

(a) When two elements combine to form a compound.

$2H_2 + O_2 \rightarrow 2H_2O$

(Hydrogen + Oxygen → Water)

(b) When two or more compounds combine together.

$NH_3 + HCl \rightarrow NH_4Cl$

(Ammonia + Hydrogen chloride → Ammonium chloride)

$CaO + CO_2 \rightarrow CaCO_3$

(Calcium oxide + Carbon dioxide

$\rightarrow$ Calcium carbonate)

$CaO + H_2O \rightarrow Ca(OH)_2 + \triangle$

(Calcium oxide + Water

$\rightarrow$ Calcium hydroxide)

(c) When one element and one compound combines to give a new compound.

$2CO + O_2 \rightarrow 2CO_2$

(Carbon monoxide + Oxygen

$\rightarrow$ Carbon dioxide)

$2SO_2 + O_2 \rightarrow 2SO_3$

(Sulphur dioxide + Oxygen

$\rightarrow$ Sulphur trioxide)

2. Decomposition reaction

The reaction in which the compound is broken down into two or more simpler substances are known as decomposition reaction.

XY $\rightarrow$ X + Y

a) Thermal decomposition

$CaCO_3 (s)\overset{\triangle}{\rightarrow} CaO (s)+ CO_2(g)$

(Calcium carbonate

$\overset{\triangle}{\rightarrow}$ Calcium oxide + Carbon dioxide)

$2Pb(NO_3)_2 (s) \overset{\triangle}{\rightarrow} 2PbO (s)+ 4NO_2(g) + O_2(g)$

(Lead nitrate

$\overset{\triangle}{\rightarrow}$ Lead monoxide + Nitrogen dioxide + Oxygen)

👉Emission of brown fumes. These fumes are of nitrogen dioxide (

$NO_2$ ).

$2FeSO_4 (s) \overset{\triangle}{\rightarrow} Fe_2O_3 (s) + SO_2 (g) + SO_3 (g)$

(Ferrous sulphate

$\overset{\triangle}{\rightarrow}$ (Ferric oxide + Sulphur dioxide + Sulphur dixoxide )

Ferrous sulphate crystals (

$FeSO_4 .7H_2O$ ) lose water when heated and the colour of the crystals changes. It then decomposes to ferric oxide (

$Fe_2O_3$ ), sulphur dioxide (

$SO_2$ ) and sulphur trioxide (

$SO_3$ ).

👉Ferrous sulphate crystals (

$FeSO_4 .7H_2O$ ) green colour.

👉 Ferric oxide (

$Fe_2O_3$ ) brown colour.

b) Electrolytic decomposition

Some compounds are decomposed by passing electricity.

c) Photochemical decomposition

There are some reactions which take place in the presence of light. These are called photochemical reaction.

3. Displacement reaction
A reaction, in which an atom or a group of atoms present in a molecule is displaced by another atom, is known as displacement reaction.

a) Displacement of less reactive metal by a more reactive metal

When a piece of iron is added to a solution of copper sulphate, iron displaces copper from copper sulphate.

$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$

(Iron + Copper sulphate

$\rightarrow$ Iron sulphate + Copper)

Displacement occurs because iron is more reactive than copper.

Iron displaces copper from copper sulphate

The blue color of copper sulphate solution disappears due to the formation of

$FeSO_4$ , which is light green. A reddish coating of copper metal is deposited on the surface of iron metal.

When a strip of copper is added to a solution of silver nitrate, silver is displaced by copper.

$Cu + 2AgNO_3 \rightarrow Cu(NO_3)_2 + 2Ag$

(Copper + Silver nitrate

$\rightarrow$ Copper nitrate + silver)

Silver displaced by copper

A shining deposit of silver is seen on the copper strip. This reaction occurs because copper is more reactive than silver.

When a piece of zinc is added to a solution of copper sulphate, zinc suplhate and copper are formed.

$Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$

(Zinc + Copper sulphate

$rightarrow$ Zinc sulphate + Copper )

In this reaction, copper is displaced from copper suphate by zinc because zinc is more reactive than copper. A reddish deposit of copper is observed on the zin strip. The blue color of copper sulphate disappears due to the formation of colorless zinc sulphate.

b) Displacement of less reactive nonmetal from its compound.

When $Cl_2$ gas is passed through a solution of sodium bromide $(NaBr)$ , sodium chloride and bromine are formed. The solution becomes brown due to the liberation of bromine in the solution.

$2NaBr + Cl_2 \rightarrow 2NaCl + Br_2$

(Sodium bromide + Chlorine

$\rightarrow$ Sodium chloride + Bromine)

When $Cl_2$ gas is passed through a solution of potassium iodide, potassium chloride and iodine are produced.

$2KI + Cl_2 \rightarrow 2KCl + I_2$

(Potassium ioditde + chlorine

$\rightarrow$ potassim chloride + Iodine)

This reaction proves that chlorine is more reactive than bromine and iodine.

c) Displacement of hydrogen from acids by active metals.

When a piece of zinc is added to a dilute solution of sulphuric acid $(H_2SO_4)$ , hydrogen gas is evolved and zinc sulphate is formed.

$Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2 \uparrow$

Zinc can displace hydrogen from other acids, such as hydrochloric acid

$(HCl)$ , phosphoric acid

$(H_3PO_4)$ , and acetic acid

$(CH_3COOH)$ . This can be demonstrated by the following experiments.

$Zn + 2HCl \rightarrow ZnCl_2 + H_2 \uparrow$

$3Zn + 2H_3PO_4 \rightarrow Zn_3(PO_4)_2 + 3H_2 \uparrow$

(Zinc + Phosphoric acid

$\rightarrow$ Zinc phosphate + Hydrogen )

$Zn + 2CH_3COOH \rightarrow (CH_3COO)_2Zn + H_2 \uparrow$

(Zinc + Acetic acid

$\rightarrow$ Zinc acetate + Hydrogen )

It is hydrogen gas coming out from each test tubes. Thus hydrogen gas has been displaced from acids by zinc.

4. Double displacement reaction

The reactions in which two compounds react by an exchange of ions to form two new compounds are called double displacement reactions.

XY + AB $\rightarrow$ XB + AY

Here, X and A are the positive ions and Y and B are the negative ions.

A double displacement reaction usually occurs in solution and one of the products, being insoluble precipitate out (separate as a solid).

Examples

When a solution of silver nitrate is added to a solution of sodium chloride, a white precipitate of silver chloride is formed along with sodium nitrate.

$NaCl + AgNO_3 \rightarrow AgCl + NaNO_3$

(Sodium chloride + silver nitrate

$\rightarrow$ Silver chloride (white ppt) + Sodium nitrarte)

Sodium chloride and Silver nitrate

When a solution of barium chloride is added to a solution of sodium sulphate, a white precipitate of barium sulphate is obtained, sodium chloride is formed is left behind the solution.

$BaCl_2 + Na_2SO_4 \rightarrow BaSO_4 + 2NaCl$

(Barium chloride + Sodium sulphate

$\rightarrow$ Barium sulphate (White ppt) + Sodium chloride)

When ferrous sulphide reacts with sulphuric acid, a double displacement reaction takes place with the evolution of hydrogen sulphide gas, ferrous sulphate remains in solution.

$FeS + H_2SO_4 \rightarrow FeSO_4 + H_2S \uparrow$

(Ferrous sulphide + Suphuric acid

$\rightarrow$ Ferrous sulphate + Hydrogen sulphide

When potassium iodide solution is added to lead nitrate solution, then a yellow precipitate of lead iodide is produced along with potassium nitrate solution.

$Pb(NO_3)_2 + 2KI \rightarrow PbI_2 + 2KNO_3$

(Lead nitrate + Potassium iodide

$\rightarrow$ Lead iodide (yellow ppt) + Potassium nitrate)

When hydrogen sulphide gas is passed through copper sulphate solution, the black precipitate of copper sulphide is formed along with sulphuric acid.

$CuSO_4 + H_2S \rightarrow CuS + H_2SO_4$

(Copper suphate + Hydrogen sulphide

$\rightarrow$ Copper sulphide (black ppt) + sulphuric acid)

When ammonium hydroxide solution is added to aluminium chloride solution, the white precipitate of aluminium hydroxide is formed along with ammonium chloride solution.

$AlCl_3 + 3NH_4OH \rightarrow Al(OH)_3 + 3NH_4Cl$

(Aluminium chloride + Ammonium hydrogen

$\rightarrow$ Aluminium hydroide (white ppt) + Ammonium chloride)

5. Oxidation and reduction reactions

Oxidation is defined as (i) the addition of oxygen to a substance (ii) the removal of hydrogen from a substance

Reduction is defined as (i) the addition of hydrogen to a substance (ii) the removal of oxygen from a substance.

When phosphorus burns in the air, oxygen is added to it to form phosphorus pentoxide (

$P_2O_5$ ), so phosphorus gets oxidised.

The process of reduction is just the opposite of oxidation.

Oxidising agent

The substance which gives oxygen for oxidation is called oxidising agent.
The substance which removes hydrogen is also called an oxidising agent

Reducing agent

The substance which gives hydrogen for reduction is called a reducing agent.
The substance which removes oxygen is called a reducing agent.

The oxidation and reduction reactions are also called redox reactions. In the name 'redox', the term 'red' stands for 'reduction' and 'ox' stands for 'oxidation'.

Example 1

When copper oxide is heated with hydrogen, the copper metal and water are formed.

$CuO + H_2 \overset {\triangle}{\rightarrow} Cu + H_2O$

In this reaction, $CuO$ is changing into $Cu$ . That is, oxygen is being removed from copper oxide. So, we can say that copper oxide is reduced to copper.
$H_2$ is changing into $H_2O$ . That is oxygen is being added to hydrogen, so we can say that hydrogen is being oxidised to water.

We find that hydrogen is being oxidised to water and at the same time copper oxide is being reduced to copper. This shows that oxidation and reduction occur together.

Copper oxide is giving the oxygen required for the oxidation of hydrogen, therefore copper oxide is the oxidising agent. Hydrogen is responsible for removing oxygen from copper oxide, therefore hydrogen is the reducing agent.

Substance oxidised : $H_2$
Substance reduced : $CuO$
Oxidising agent : $CuO$
Reducing agent : $H_2$

Example 2

When hydrogen sulphide reacts with chlorine, then sulphur and hydrogen chloride is formed.

$H_2S + Cl_2 \rightarrow 2HCl + S$

(Hydrogen sulphide + Chlorine

$\rightarrow$ Hydrogen chloride + Sulphur )

In this reaction, $H_2S$ is changing to $S$ . Hydrogen is being removed from hydrogen sulphide, so we can say that hydrogen sulphide is being oxidised.
In this reaction, $Cl_2$ is changing to $HCl$ . Hydrogen is being added to chlorine. So we can say that chlorine is being reduced to hydrogen chloride.

Redox reaction

Chlorine is oxidising agent and hydrogen sulphide the reducing agent.

Substance oxidised: $H_2S$
Substance reduced: $Cl_2$
Oxidising agent: $Cl_2$
Reducing agent: $H_2S$

Example 3

When Zinc oxide is heated with carbon, then zinc metal and carbon monoxide are formed.

$ZnO + C \overset{\triangle}{\rightarrow} Zn + CO$

Substance oxidised: $C$
Substance reduced: $ZnO$
Oxidising agent: $ZnO$
Reducing agent: $C$

Example 4

When copper is heated in the air, it reacts with oxygen to form a black compound copper oxide.

$2Cu + O_2 \overset{\triangle}{\rightarrow} 2CuO$

Substance oxidised: $Cu$
Oxidising agent: $O_2$

Effects of oxidation reactions in everyday life.

Oxidation has a damaging effect on metals as well as on food. The damaging effect of oxidation on metals is studied as corrosion and that on food is studied as rancidity. There are two common effects of oxidation reactions:

Corrosion
Rancidity

1. Corrosion: Corrosion is the process in which metals are eaten up gradually by the action of air, moisture. Corrosion is caused mainly by the oxidation of metals. Rusting of iron is the most common form of corrosion.

During the corrosion of iron, iron metal is oxidised by the oxygen of air in the presence of water (moisture) to form hydrated iron (III) oxide (

$Fe_2O_3. xH_2O$ ) called rust.

$4Fe + 3O_2 + 2xH_2O \rightarrow 2Fe_2O_3. xH_2O$

The number of water molecules (

$x$ ) in the rust varies, it is not fixed. The rusting of iron is a redox reaction. Corrosion weakens the iron objects and structure such as railing, car bodies, bridges, and ships, etc., and cuts short of their life. A lot of money has to be spent every year to prevent the corrosion of iron and to replace the damaged part.

2. Rancidity: When fats and oils are oxidized, they become rancid. When the food materials prepared in fats and oils are kept for a long time, they start giving an unpleasant smell and taste. The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity.

Prevention of rancidity

Rancidity can be prevented by adding antioxidants to foods containing fats and oils. An antioxidant is a substance that prevents oxidation. Antioxidants are reducing agents. When antioxidants are added to food, then the fats and oils present in them do not get oxidised easily and hence do not turn rancid. BHA (Butylated Hydroxy- Anisole ) and BHI (Butylated Hydroxy - Toluene) are the two common antioxidants used in food to prevent the development of rancidity.
Rancidity can be prevented by packaging fat and oil-containing foods in nitrogen gas. When the packed food is surrounded by unreactive nitrogen. there is no oxygen to cause its oxidation and make it rancid. The manufacturers of potato chips fill the plastic bags containing chips with nitrogen gas to prevent the chips from being oxidised and turn rancid.
Rancidity can be retarded by keeping food in a refrigerator. When the food is kept in a refrigerator, the oxidation of fats and oils in it is slowed down due to low temperature.
Rancidity can be retarded by storing food in air-tight containers. When food is stored in air-tight containers, then there is little exposure to the oxygen of the air. Due to reduced exposure to oxygen, the oxidation of fats and oils present in food is slowed down and hence the development of rancidity is retarded.
Rancidity can be regarded by storing food away from light. In the absence of light, the oxidation of fats and oils present in food is slowed down.

Few other reactions

Exothermic reaction: Reactions in which heat is produced is called an exothermic reaction.

$CaO + H_2O \rightarrow Ca(OH)_2 + \triangle$

(Calcium oxide + Water

$\rightarrow$ Calcium hydroxide)

Do you know that respiration is an exothermic reaction?

For example, rice, potatoes, and bread contain carbohydrates. These carbohydrates are broken down to form glucose. This glucose combines with oxygen in the cells of our body and provides energy. The special name of this reaction is respiration.

$C_6H_12O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$ + energy

(Glucose + Oxygen

$\rightarrow$ Carbon dioxide + water)

Endothermic Reaction: Reactions in which heat is absorbed is known as an endothermic reaction.

The decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants are the examples of endothermic reactions.

Precipitation

You will observe that a white substance, which is insoluble in water, is formed. This insoluble substance formed is known as a precipitate. Any reaction that produces a precipitate can be called a precipitation reaction.

$Na_2SO_4 + BaCl_2 \rightarrow BaSO_4 + 2NaCl$

(Sodium sulphate + Barium chloride

$\rightarrow$ Barium sulphate + Sodium chloride)

White precipitate of $BaSO_4$ is formed by the reaction of $SO_4^{2-}$ and $Ba^{2+}$ . The other product formed is sodium chloride which remains in the solution.

Any reaction in which an insoluble solid (called precipitate) is formed that separates from the solution are called a precipitation reaction.

$Mg + O_2 \rightarrow 2MgO$

$Pb(NO_3)_2 + 2KI rightarrow PbI_2 + 2KNO_3$

$Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2 \uparrow$

References

NCERT Science Class X
S. Chand Chemistry Class X
Foundation Science (Chemistry) Class X

Class 10 Science Notes | Chemical Reactions and Equations Class 10 | CBSE

Chemical Reactions and Equations

Introduction

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