Chemical Reaction

When one or more substances undergo a chemical change, with the absorption or release of energy, so as to form one or more new products, then the change taking place collectively is called chemical reaction.

In a chemical reaction, the substance(s) which take part in bringing about a chemical change are called reactants.

The substance(s) which are produced as a result of chemical change are called products.

Conditions for a chemical reaction

1. There must be one or more substances to take part in a chemical reaction.
2. There must be absorption or release of energy during a chemical reaction.
3. At least one substance should be formed during a chemical reaction.

Evolution of a gas

Some chemical reactions are characterised by the evolution of a gas. For eg. when baking soda( Sodium Hydrogen Carbonate - NaHCO₃) is treated with vinegar( Acetic Acid - CH₃COOH), it gives off colourless Carbon dioxide( CO₂ ) gas, with a lot of effervescence( bubbly). It can be represented as :
NaHCO₃ + CH₃COOH CH₃COONa + H₂O + CO₂

CH₃COONa is Sodium Acetate.

Change of colour

Some chemical reactions are characterised by the change of colour of reactants. For eg. when copper carbonate( green colour - CuCO3) is heated strongly, it leaves behind a black residue of copper copper oxide( CuO) and gives off Carbon dioxide( CO₂ ). It can be represented as

CuCO3CuO + CO2

Formation of precipitate (ppt)

Some chemical reactions are characterised by the formation of a precipitate( an insoluble substance), when the solutions of two chemical compounds are mixed together. For eg. a white precipitate of silver chloride is formed when a colourless solution of silver nitrate is mixed with a colourless solution of sodium chloride. It can be represented as

AgNO₃(Solution) + NaCl(Solution) NaNO₃(Solution) + AgCl(White Precipitate)

Energy Changes

Some chemical reactions are characterised by the release or absorption of energy. For eg. when a small piece of quick lime(calcium oxide - CaO) is placed in about 100 cc of water, the water becomes very hot and sometimes start boiling. It is because, the calcium oxide(CaO) reacts with water to form calcium hydroxide with the release of heat energy. It can be represented as

CaO + H₂O Ca(OH)₂ + Heat Energy

Change of State

Some chemical reactions are characterised by the change of state. For eg. Ammonia gas(NH3 - Hydrogen Nitride) reacts with Hydrochloric acid(HCl - Hydrogen Chloride) gas to form dense white fumes of ammonium chloride(NH₄Cl), which is in the solid state. It can be represented as

NH3 + HCl Solid NH₄Cl

Types of Chemical Reactions


Chemical Combination/Composition - When two or more elements or compounds react chemically to form only one new product, then the reaction which takes place is called chemical combination.

Examples

When a mixture of iron fillings and sulphur is heated strongly, it reacts to form iron sulphide(FeS). It can be represented as

Fe + S FeS

When quicklime(CaO) is mixed with water, it reacts to form a single new product, calcium hydroxide(Ca(OH)_{2). It can be represented as}

CaO + H₂O Ca(OH)<sub>2

Note : Heat energy is also released during this reaction. Heat is an energy and so is not considered as a product.

Chemical Decomposition/Decombination - When a single chemical compound decomposes on heating or by some other kind of energy, so as to form two or more new substances, then the chemical reaction which takes place is called chemical decomposition.

Examples

Mercury oxide(HgO) on strong heating decomposes to form mercury(Hg) and oxygen(O). It can be represented as

HgO</sub> Hg + O

When marble chips(Calcium Carbonate - CaCO3) are heated strongly, they decompose to form Calcium Oxide(CaO) and Carbon Dioxide(CO_{2). It can be represented as}

CaCO3CaO + CO2

Chemical Displacement/Replacement - When a more reactive element displaces a less reactive element from its aqueous salt solution(from its water solution), the reaction which takes place is known as chemical displacement.

There is a list of reactivity of common elements. It is given below in the order of decreasing reactivity.

Potassium > Sodium > Calcium > Magnesium > Aluminium > Zinc > Iron > Lead > Hydrogen > Copper > Mercury > Silver > Gold

In the above list Hydrogen a non metal is included. because the metals above hydrogen donot react with water or dilute sulphuric acid or dilute hydrochloric acid and hence do not displace hydrogen.

Examples

Iron(Fe), reacts with copper sulphate(CuSO4) solution to displace Copper(Cu - Because its less reactive than Iron) and forms a solution of iron sulphate(FeSO4).It can be represented as

Fe + CuSO4(Solution) FeSO4 + Cu.

Zinc(Zn) reacts with dilute Sulphuric Acid(H₂SO₄ - Hydrogen Sulphate) to displace Hydrgen(H - Less Reactive) and form a solution of Zinc Sulphate(ZnSO<sub>4).

Zn +</sub> H₂SO4(Dilute) ZnSO4 + H