When a chemical reaction occurs how are new substances produced? Do atoms of one element transform themselves into atoms of another element? Do atoms withdraw from the mixture or turn up from elsewhere? Neither of these happens; what takes place is the breaking of old bonds between atoms and making of new bonds to form new substances. This can happen in a number of ways.
- Combination or Addition or Synthesis Reaction
- Decomposition Reaction
- Displacement or Substitution Reaction
- Double Displacement Reaction
- Oxidation - Reduction ReactionsCombination Reaction:
Combination or addition or synthesis is a chemical change in which atoms or molecules of two or more substance combine to form molecules of a single new substance. For e.g.,
- Burning of hydrogen in air or oxygen to produce water.
- Combination of ammonia and hydrogen chloride to produce ammonium chloride.
- Burning of magnesium in air gives magnesium oxide.
Chemical reactions are often accompanied by evolution or absorption of energy.Exothermic-Reactions">
This is only possible when the energy required to break the bonds of the reactants is less than the energy released in the formation of new bonds of the products.
The chemical reactions, which proceed with evolution of heat energy i.e. when heat is released, are called exothermic reactions.
The amount of heat (energy) produced is written along with the products. This indicates that heat is given out.Endothermic-Reactions">
The chemical reactions, which proceed with the absorption of heat energy, are called endothermic reactions.
The amount of heat absorbed is written along with the reactants since it is absorbed.Decomposition Reaction:
A chemical reaction in which molecules of a substance break down to form simpler molecules of two or more new substances is recognized as a decomposition reaction. When a substance, decomposes due to heat it is called thermal decomposition, while decomposition due to electricity, is called electrolytic decomposition.
- Mercuric oxide, when heated, undergoes thermal decomposition, to give mercury and oxygen.
- Similarly, if blue crystals of copper nitrate are heated, they undergo thermal decomposition to give black coloured copper oxide, reddish brown fumes of nitrogen dioxide, and a colourless gas of oxygen.
- When water taken in an electrolytic cell, is acidified with a small quantity of sulphuric acid and a direct current passed through; it undergoes electrolytic decomposition to yield hydrogen and oxygen.
- If an electric current is passed through molten lead bromide, it decomposes to give lead and bromine.
A substitution or displacement reaction is a chemical change in which atoms of one element replace the atoms of another element from the molecules of a compound. Elements which are higher in the Activity Series displace those elements which are placed below them. More electro positive elements displace lesser electro positive elements. Conversely, higher electro negative elements will displace lesser electro negative elements. For e.g.,
- When an iron knife is dipped in an aqueous solution of copper sulphate, a layer of reddish brown precipitate is formed on the iron knife. The iron from the knife being more reactive than copper, displaces it from the copper sulphate solution. The copper so separated gets collected on the surface of iron knife.
- When small clean pieces of magnesium are added into dilute sulphuric acid, magnesium sulphate and hydrogen are formed. Atoms of magnesium displace the atoms of hydrogen from sulphuric acid.
- When a colourless solution of potassium iodide is passed through a colourless solution of chloroform, the chloroform layer becomes purple. Chlorine being more electro-negative than iodine, displaces iodine from potassium iodide.
The iodine so liberated, dissolves in the chloroform, giving it a purple colour.Double Displacement Reaction:
This is a reaction in which the positive and negative ions of two substances in a solution are mutually interchanged.
In all the above reactions a white substance, which is insoluble in water, is formed. This insoluble substance formed is known as a precipitate. A reaction that produces a precipitate is called a precipitation reaction.
Neutralisation is a type of double displacement reaction, in which, the reactants are a base and an acid, and the products are salt and water. The positive charge of the hydrogen ion of the acid, and the negative charge of hydroxyl ions or oxide ions of the base, lose their electrical charge, and become covalent molecule of water.
A neutralisation reaction is basically a reaction between H+ and OH- ions i.e.,
Oxidation Reduction Reaction:
"Oxidation is a reaction in which oxygen is added or hydrogen is removed from a substance."
- Addition of Oxygen
Oxygen adds on to magnesium to become magnesium oxide.
- Removal of Hydrogen
Hydrogen is removed from hydroiodic acid to liberate free iodine.
"Reduction is a reaction in which oxygen is removed from a substance or hydrogen is added to a substance."
- Removal of Oxygen
Oxygen is removed from copper oxide to form copper metal.
Addition of Hydrogen
Hydrogen adds to chlorine to form hydrogen chloride gas.
Nowadays, one can explain oxidation- reduction in terms of 'electrons transfer', which one will learn later on. Also, it is necessary to point out that oxidation and reduction reactions occur simultaneously. In brief, we call these reactions as "redox" reactions. Many reactions discussed earlier in this chapter can be broadly classified as redox and non-redox reactions.
Example of Redox Reaction
Example of Non-redox Reaction
Usually chemical reactions proceed only in one direction. But in some cases the reaction reverses itself. Example, when steam is passed over red hot iron, magnetic oxide of iron and hydrogen gas are formed. If hydrogen gas is passed over heated magnetic oxide of iron, the reverse reaction takes place, producing iron and steam.
Reversibility of a reaction is depicted by double arrow as
This is also a type of reversible reaction, in which heat decomposes a substance. But if the products are not allowed to escape, then on cooling they recombine to form the original substance.
Nitrogen dioxide decomposes above 50oC to form nitric oxide and oxygen. But, if the products are cooled to below 50oC, then the freshly formed nitric oxide and oxygen recombine to form nitrogen dioxide.
When ammonium chloride is heated, it decomposes to form ammonia and hydrogen chloride. But when the products are cooled, they recombine to form ammonium chloride.
It is to be remembered here that since NH3 and HCl are gases, they should not be allowed to escape from the container after their formation.