Types of Chemical Reactions

We cover:

1. Types of Chemical Reactions

(i) Combination reactions
(ii) Decomposition reaction
(iii) Displacement reactions
(iv) Double displacement reactions
(v) Oxidation and Reduction Reactions (Redox)

2. Effects of Redox reactions in everyday life

TYPES OF CHEMICAL REACTIONS

During a chemical reaction atom of one element do not change into those of another element. Nor do atoms disappear from the mixture or appear from elsewhere. Chemical reactions involve the breaking and making of bonds between atoms to produce new substances. Here we look at five types of chemical reactions:

(i) Combination Reaction: A reaction in which a single product is formed from two or more reactants is known as a combination reaction.

CaO(s) + H2O(l) → Ca(OH)2(aq)

(Quick lime)                   (Slaked lime)

Calcium oxide and water combine to form a single product, calcium hydroxide. In simple language, we can say that when two or more substances (elements or compounds) combine to form a single product, the reactions are called combination reactions.

(ii) Decomposition Reaction: Those reactions in which a compound splits up into two or more simpler substances are known as decomposition reactions. The decomposition reactions are carried out either by (a) applying heat, (b) light or (c) electricity.

(a) Applying Heat: When calcium carbonate is heated, it decomposes to give calcium oxide and carbon dioxide:

CaCO3 (s)   —heat/decomposition–> CaO (s) + CO2 (g)

Calcium Carbonate                                      Calcium Oxide

When lead nitrate is heated strongly, it breaks down to form simpler substances like lead monoxide, nitrogen oxide, and oxygen.

2Pb(NO3)2(s) →2PbO(s) + 4NO2(g) + O2(g)

(Lead nitrate)        (Lead oxide) (Nitrogen Dioxide) (Oxygen)

(b) Light: When silver chloride is exposed to light, it decomposes to form silver metal and chlorine gas.

2AgCl(s) –Light/decomposition–>2 Ag(s)   + Cl2 (g)

Silver Chloride(White)                                Silver              Chlorine

(c) Electricity: When an electric current is passed through acidified water, it decomposes to give hydrogen gas and oxygen gas.

2H2O  —Electricity/decomposition-> 2H2(g) + O2 (g)

Water                                                             Hydrogen    Oxygen

This decomposition reaction takes place by the action of electricity. It is called electrolysis of water.

Uses of Decomposition reactions:
• The decomposition reactions carried out by electricity are used to extract several metals from their naturally occurring compounds like chlorides and oxides.

• For example, sodium metal is extracted by the electrolysis of molten aluminum oxide.

Decomposition reactions in our body:
• The digestion of food in the body is an example of decomposition reaction.

• When we eat foods like wheat, rice or potatoes, then the starch present in them decomposes to give simple sugar like glucose in the body; and protein decomposes to form amino acids.

(iii) Displacement Reaction: Those reactions, in which one element takes the place of another element in a compound, are known as displacement reactions. a more reactive element displaces a less reactive element from its compound.

Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

(Copper sulphate)       (Iron sulphate)

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

(Copper sulphate)       (Zinc sulphate)

Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s)

(Copper chloride)     (Lead chloride)

(iv) Double Displacement Reaction: reactions, in which two compounds react by an exchange of ions to form two new compounds, are called double displacement reactions. In this reaction, two compounds barium chloride and sodium sulphate react to form two new compounds, barium sulphate and sodium chloride. An exchange of ions takes place in this reaction.

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

(Sodium                  (Barium                 (Barium            (Sodium

sulphate)                  chloride)                sulphate)         chloride)

Formation of barium sulphate and sodium chloride

(V) Oxidation and Reduction(Redox): The addition of oxygen to a substance is called oxidation. The addition of hydrogen to a substance is called reduction. Or, the removal of hydrogen from a substance is called oxidation. The removal of oxygen from a substance is called reduction. The oxidation and reduction reactions are also called redox reactions.

If hydrogen gas is passed over this heated material (CuO), the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained.

CuO +H2—-> Cu+H2O

During this reaction, the copper(II) oxide is losing oxygen and is being reduced. The hydrogen is gaining oxygen and is being oxidised.

EXOTHERMIC REACTIONS

Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.

(a) Eg. Burning of natural gas

CH4(g) + 2O2 (g) → CO2 (g) + 2H2O (g)+Heat

(b) During digestion, food is broken down into simpler substances. 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 for this reaction is respiration.

C6H12O6(aq)+ 6O2(aq) → 6CO2(aq) +6H2O(l) + Heat

(Glucose)

(c) The decomposition of vegetable matter into compost is also an example of an exothermic reaction.

EFFECTS OF OXIDATION REACTIONS IN EVERYDAY LIFE

(a)Corrosion
• Iron articles are shiny when new, but get coated with a reddish-brown powder when left for some time. This process is commonly known as rusting of iron. Some other metals also get tarnished in this manner.

• Corrosion is caused mainly by the oxidation of metals by oxygen of air. Rusting of iron metal is the most common form of corrosion.

4Fe + 3O2 + xH2O—->2Fe2O3.XH2O

                                           Iron     Oxygen Water         Rust

• The black coating [Silver Sulphide] on silver and the green coating [Copper Carbonate] on copper are other examples of corrosion.

• Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, especially those of iron.

• Corrosion can be prevented by painting, applying grease or by electro-plating [coating a metal object by electrolytic deposition with chromium, silver, or another metal] or galvanization [the process of applying a protective zinc coating to steel or iron, to prevent rusting].

(b)Rancidity
• When fats and oils are oxidised, they become rancid and their smell and taste change.

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

• Usually, substances which prevent oxidation (antioxidants) are added to foods containing fats and oil. Keeping food in airtight containers helps to slow down oxidation.

• Chips manufacturers usually flush bags of chips with a gas such as nitrogen to prevent the chips from getting oxidised.

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