Key Factors Influencing Chemical Reaction Rates

Q#9.2: Explain factors affecting rate of reaction.

Ans.The following factors strongly influence the rate of a reaction

1.Nature Of Reactants. The rate of reaction between polar or ionic

molecule is very fast. While the rearrangement reactions are slower than ionic reactions. Similarly oxidation reduction reactions are also slower than ionic. Some metals like sodium reacts vigorously with oxygen at room temp while magnesium needs flame to react with oxygen.

2.Effect Of Concentration Of Reactants.

By increasing the amount of reactants, the rate of reaction also increases. Consider a reaction which is composed of a gaseous mixture or concentrated solution, contains many active particles per unit volume. Therefore the rate of such reactions is faster than dilute reactants. For example wood burns rapidly in the presence of pure oxygen (100% oxygen) than normal air (21% oxygen).

3.Particle size of solid reactants Or surface area.

The rate of heterogeneous reaction is directly proportional to the surface area of reactants, because by dividing the mass of reactants, the surface area increases, as a result rate of reaction also increases. For example marble chips on reaction with produces, CO2, CaCl2 and H2O. In this reaction, H+1 ions of the acid, attack, the carbonate ions at the surface of the chips. By grinding the large particles of chips into powder, the surface area in contact with the acid is greatly increased. Thus rate of reaction also increases. Similarly coal in powder form burns rapidly than lumps of soft coal. HCl

4.Effect of temp.

Generally increase in temp. Leads to increase the rate of reaction, ether a reaction is exothermic or endothermic. Normally 10C0 rise in temp. Doubles the reaction rate of a homogenous reaction. Temperature affect the rate of reaction in two ways.

i. Increase in temp. Leads to increase the K.E of molecules & thus molecular motion increases.

ii. Increase in temp. Leads to increase the energy by an amount greater than activation energy & thus reactants having higher energy thanactivation will have fast rate.

5.Catalyst.

A substance which changes the rate of a chemical reaction without itself being consumed is called catalyst. The rate of a reaction may be increased or in some cases decreased by using catalyst. Catalysts usually used in a small quantity. For example, the thermal decomposition

Of KClO₃ is catalyzed  MnO₂