Chemical Reactions and Equations [Class 10]

TYPE OF CHEMICAL REACTIONS CORROSION RANCIDITY

CHEMICAL REACTIONS AND EQUATIONS

CHEMICAL REACTIONS

Chemical reactions are the processes in which new substances with new properties are formed. During a chemical reaction, atoms of one element do not change into those of another element. Only a rearrangement of atoms takes place in a chemical reaction.

REACTANTS:- The substances which take part in a chemical reaction are called reactants.
PRODUCTS:- The new substances produced as a result of chemical reaction are called products.
Examples:- The burning of magnesium in air to form magnesium oxide is an example of a chemical reaction.
Magnesium(s) + Oxygen(g) → Magnesium oxide
Here, Magnesium and oxygen are reactants while Magnesium Oxide is a product.
Characteristics of a chemical reaction:-
(i)Evolution of a gas.
(ii) Formation of a precipitate.
(iii) Change in colour.
(iv) Change in temperature.
(v) Change in state.

CHEMICAL EQUATIONS

The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as a chemical equation.
Examples:-
Zn(s) + HCl(aq) → ZnCl₂(aq) + H₂(g)

In a chemical equation an arrow directed from left to right , is used.
Reactants are written in the form of symbols and formulae to the left of the arrow while products are used to the right of the arrow.
Physical states of substances are represented to the right of the symbol or formulae inside a parenthesis ( )
(s) →   solid
(l) →   liquid
(g) →   gas
(aq) →   aqueous solution

BALANCED CHEMICAL EQUATIONS

A chemical equation which contains an equal number of atoms of each element on both sides of the equation is called a balanced chemical equation.For example:-
H₂(g) + Cl₂(g) → 2HCl(g)
Why should the chemical equation be balanced ?:- According to the law of conservation of mass, matter can neither be created nor destroyed in a chemical reaction. During a chemical reaction , the total mass of reactants and products remain the same. Hence in a chemical reaction , the number of atoms of the various elements on both sides should be equal. Therefore, a chemical equation is to be balanced in accordance with the law of conservation of mass.

TYPES OF CHEMICAL REACTIONS

Combination Reactions

Decomposition Reactions

Displacement Reactions

Double Displacement Reactions

Exothermic Reactions

Endothermic Reactions

Precipitation Reactions

Oxidation Reactions

Reduction Reactions

Redox Reactions

1.COMBINATION REACTIONS

Those reactions in which two or more substances combine to form a single substance, are called combination reactions.
In a combination reaction :-

Two or more elements can combine.

Two or more compounds can combine.

An element or more compounds can combine.

Examples
1. 2Mg(s) + O₂(g) → 2MgO(s)
2. 2H₂(g) + O₂(g) →     2H₂O (l)
3. C(s) + O₂(g) →     CO₂
4. H₂(g) + Cl₂(g) →   2HCl(g)
5. CaO(s) + H₂O(l) →     Ca(OH)₂(aq) + heat
Ca(OH)₂(aq) + CO₂(g) →     CaCO₃(s) + H₂O(l)

2.DECOMPOSITION REACTIONS

Those reactions in which a compound splits up into two or more simpler substances are known as decomposition reactions.

A decomposition reaction is just the opposite of a combination reaction.

Types of decomposition reactions

Thermal Decomposition Reactions:-
It is carried out by heating it. Heat provides energy.
Examples :-
1. CaCO₃(s) → CaO(s) + CO₂(g)
2. FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g)
3.2PbNO₃(s) → 2PbO(s) + 4NO₂(g,brown) + O₂(g)

Electric Decomposition Reactions:-
These reactions are carried out by using electricity.
Examples:-
1. 2H₂(l) → 2H₂(g) + O₂(g)
2. 2Al₂O₃(l) → 4Al(l) + 3O₂(g)

Photo(Light)Decomposition Reactions:-
These reactions are carried out by using light energy.
Examples:-
1. AgCl(s) → 2Ag(s) + Cl₂(g)
2. AgBr(s) → 2Ag(s) + Br₂(g)

3.DISPLACEMENT REACTIONS

Those reactions in which a more reactive element displaces a less reactive element from its compounds are known as displacement reactions.

Examples:-
1. CuSO₄(aq) + Zn(s) →     Zn>SO₄(aq) + Cu(s)
[Here, Zn is more reactive than Cu ]
2. CuSO₄(aq) + Mg(s) →     MgSO₄(aq) + Cu(s)
[Here, Mg is more reactive than Cu ]
3. AgNO₃(aq) + NaCl(aq) →     AgCl(s) + NaNO₃(aq)
[Here, Cu is more reactive than Ag ]

4.DOUBLE DISPLACEMENT REACTIONS

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

Examples:-
1. CuSO₄(aq) + H₂S(g) →    CuS(s,black) + H₂SO₄(aq)
2. PbNO₃(aq) + 2KCl(aq) →     PbI₂(s,yellow) + Cu(s)
3. AgNO₃(aq) + NaCl(aq) →     AgCl(s,white) + NaNO₃(aq)

5. EXOTHERMIC REACTIONS

Those reactions in which heat is evolved (realeased) are called exothermic reactions.

Examples:-
1.C(s) + O₂(g) → CO₂(g) + heat
2.CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat

6. ENDOTHERMIC REACTIONS

Those reactions in which heat is absorbed are called endothermic reactions.

Examples:-
1. CaCO₃(s) → CaO(s) + CO₂(g)
2. FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g)
3.2PbNO₃(s) → 2PbO(s) + 4NO₂(g,brown) + O₂(g)

7. PRECIPITATION REACTIONS

Those reactions in which, an insoluble substance (called a precipitate) is formed as a reaction product along with another products are called precipitation reactions. When two clear aqueous solutions of ionic compounds are mixed then generally this type of reactions are observed.

Examples:-
1. AgNO₃(aq) + NaCl(aq) → AgCl(s, white ppt) + NaNO₃(aq)
2. Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s, white ppt.) + 2NaCl(aq)

8. OXIDATION REACTION

The addition of oxygen to a substance, or ,the removal of hydrogen from a substance is called oxidation.

Examples:-
1. Mg(s) + O₂(g) → MgO(s)
[Here, Mg has gained oxygen to form MgO.Hence, Mg has been oxidised to MgO.]
2. 2Cu(s) + O₂(g) → 2CuO(s)
[Here, Cu has gained oxygen to form CuO. Hence, Cu has been oxidised to CuO.]

9. REDUCTION REACTION

The addition of hydrogen to a substance, or ,the removal of oxygen from a substance is called reduction.

Examples:-
1. N₂(g) + H₂(g) → NH₃(g)
[Here,N₂(g) has gained hydrogen to form NH₃(g).Hence,N₂ has been reduced to NH₃(g).]
2. CuO(s) + H₂(g) → Cu(s) + H₂O(l)
[Here, CuO is losing oxygen to form Cu. Hence, CuO has been reduced to Cu.]

10. REDOX REACTION

The oxidation and reduction reactions together are called redox reaction.

Examples:-

OXIDISING AGENT [OXIDANT]:-(i)This substance gives oxygen for oxidation or removes hydrogen.
(ii) It gets reduced in a redox reaction.
REDUCING AGENT [REDUCTANT]:-(i)This substance gives hydrogen for reduction or removes oxygen.
(ii)It gets oxidised in a redox reaction.
NOTE:- Both oxidant and reductant are the reactants. So, these are located always towards left of a redox reaction.

EFFECTS OF OXIDATION REACTIONS IN EVERYDAY LIFE

CORROSION OF METALS
RANCIDITY OF FOOD

A. CORROSION

It is a process of slow and gradual conversion of a metal into its undesirable compounds (sulphides, carbonates, oxides, etc.) by the attack of air and moisture(water) or chemicals (like an acid) present in the atmosphere.

Examples:-

Reddish brown coating of iron (Rusting )- Iron metal is oxidised by the oxygen of air in the presence of water(moisture) to form hydrated iron (III) oxide called rust. Chemical formula of rust is given as Fe₂O₃.xH₂O
Black coating on silver
Green coating on copper

B. RANCIDITY

The condition produced by aerial oxidation of fats and oils in foods marked by obnoxious(unpleasant) smell and taste is called rancidity.

PREVENTION OR RETARDATION

Using anti-oxidants:- Anti-oxidant is a substance or chemical which prevents oxidation.Anti-oxidants are actually reducing agents.The two common anti-oxidants for prevention rancidity are BHA (Butylated Hydroxy-Anisole) and BHT(Butylated Hydroxy- Toluene).
By packaging fat and oil containing foods in nitrogen gas.
By keeping foods in a refrigerator.
By storing foods in air-tight containers.
By storing foods away from light.

SOME QUESTIONS WITH ANSWERS

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

Ans. A magnesium ribbon has a coating of a white layer of magnesium oxide on its surface.It is formed by the slow reaction of moist air on it .This hinders the burning of magnesium.So this layer is removed by rubbing with sandpaper before burning.

Q 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

Ans:-

H₂ + Cl₂ → 2HCl
3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃
2Na + 2H₂O → 2NaOH + H₂

Q 3. Write balanced chemical equations 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) react with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Ans :-

BaCl₂(aq) + Na₂SO₄ (aq) → BaSO₄(s) + 2NaCl(aq)
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Q 4. A solution of a substance 'X' is used for whitewashing.
(i)Name the substance 'X' and write its formula.
(ii)Write the reaction of the substance 'x' named in (i) above with water.
Ans :- (i) 'X' is quick lime ( calcium oxide ). Formula - CaO
(ii) CaO(s) + H₂O (l) → Ca(OH)₂(aq) + heat
Q 5. Why is the amount of gas collected in one of the test tubes double of the amount collected in the other in electrolysis of water experiment? Name this gas.

Ans :- The required gas is 'Hydrogen'. Hydrogen is double in amount than that of oxygen in the experiment.This is because water (H₂O) contains two parts of hydrogen element as compared to one part of oxygen element by volume.

Q 6. Why does the colour of copper sulphate solution change when an iron nail is dipped in it ?

Ans:- Iron is more reactive than copper. It displaces copper from copper sulphate solution according to the following reaction:
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Thus , as copper sulphate reacts to form iron(II) sulphate, the blue colour of copper sulphate solution fades and the solution turns green due to iron(II) sulphate.

Q 7. Identify the substances oxidised and the substances reduced in the following reactions:
(i) 4Na(s) + O₂(g) → 2Na₂O(s)
(ii) CuO(s) + H₂(g) → Cu(s) + H₂O (l)

Ans :-
(i) The substance gets oxidised is sodium (Na).
The substance gets reduced is Oxygen (O₂ ).
(ii) The substance gets oxidised is hydrogen(H₂ ).
The substance gets reduced is CuO.