By Sunil Bhardwaj

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1. Nature of Reactants:The reaction that contains the reactants with fewer bonds (i.e. fewer breakage) will be faster and have the higher rate. E. g. $$ { MnO }_{ 4 }^{ - }+8{ H }^{ + }+5{ Fe }^{ +2 }\overset { Fast }{ \longrightarrow } { Mn }^{ +2 }+5{ Fe }^{ +3 }+{ Mn }^{ +2 }+4{ H }_{ 2 }O $$ $$2{ MnO }_{ 4 }^{ - }+16{ H }^{ + }+5{ C }_{ 2 }{ O }_{ 4 }^{ -2 }\overset { Slow }{ \longrightarrow } 2{ Mn }^{ +2 }+10C{ O }_{ 2 }+8{ H }_{ 2 }O $$ The first reaction involves fewer bonds breakage (mainly electron transfer) than the second reaction, therefore the first reaction will have a higher rate.

2. Concentration of reactants:As a reaction proceeds, the reactants are consumed. Therefore, the concentration of the reactants decreases with the passage of time. It is observed experimentally that the rate of reaction also decreases as the time passes. This clearly indicates that the rate of reaction is directly proportional to the concentration of the reactants.

Rate of Reaction

3. Temperature:For a reaction to start a minimum amount of energy is needed this is threshold energy, When temperature is increased, the molecules of the reactants absorb heat energy and thus more molecules gain energy equal to or greater than the threshold energy and hence the reaction starts.Thus at higher temperature more and more molecules start reacting and rate of reaction increases.

4. Pressure:As the pressure of a gaseous reactants increases more of the reactants are compressed into a given volume. And there will be more collision between reacting molecules. Therefore as reactant pressure increases, the rate of reaction increases.

5. Surface area:Greater surface area means more available place for reaction, thus greater the rate of reaction.

Homogeneous reaction are reactions in which all the reactants are in the same phase. e.g reaction between two gases or reaction between two substances which are both dissolved in water.

Heterogeneous reactions are reactions in which the reactants are in different phases. e.g. reactions between a solid and a liquid or reaction between two liquids that don’t dissolve in each others (immiscible)

6. Phase: Reactions occurring in the solid phase are slow because the reactants cannot move freely. Reactions between gaseous species are much faster due to the speed of the gas molecules.

Aqueous Ions > Gaseous > Liquids > Solids

7. Catalyst and Inhibitors:Catalyst are chemicals added to a reaction to increase the rate. Please note that the catalyst does not get used up. In the end the amount of catalyst present is the same as what you started with. e.g. Pt catalyst is used in cars to convert CO and NO into less harmful gases in catalytic converters.

Inhibitors are chemicals which reduce the rate of a reaction.