Slow and fast rate of Chemical Reaction

RATE OF CHEMICAL REACTIONS


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FACTORS AFFECTING CHEMICAL EQUILIBRIUM



       Following are the factors which affect the equilibrium state of a reaction-

    1. Temperature
    2. Pressure
    3. Concentration of reactants and products
    4. Catalyst

       The effect of these factors on chemical equilibrium can be understood with the help of Le chatelier’s principle.

Le chatelier’s principle

       According to Le chatelier’s principle when the temperature, pressure or concentration of a reaction in equilibrium is changed then the reaction shifts in that direction in which the effect of these changes is reduced.

       We can understand the Le chatelier’s principle by taking example of reaction of nitrogen with hydrogen to produce ammonia as

N2 (g) +3H2 (g)           2NH3 (g)


Effect of change in temperature

       We already know that there are two types of reactions

    1. exothermic reactions and
    2. endothermic reactions.

       In a reversible reaction if forward reaction is of one type then the backward reaction will be of another type. Here we will discuss both the cases.

N2 (g) +3H2 (g)           2NH3 (g)


When the forward reaction is exothermic:

       According to Le chatelier’s principle if the temperature of the reaction is increased then the equilibrium will shift in that direction in which the effect of increased temperature is reduced i.e. the equilibrium will shift in backward direction, which means that on increasing the temperature the ammonia will decompose into nitrogen and hydrogen.

When the forward reaction is endothermic:

       According to Le chatelier’s principle if the temperature of the reaction is decreased then the equilibrium will shift in that direction in which the effect of decreased temperature is reduced i.e. the equilibrium will shift in forward direction, which means that on decreasing the temperature the nitrogen and hydrogen will combine together to form ammonia. Thus

According to Le chatelier’s principle when the temperature, pressure or concentration of a reaction in equilibrium is changed then the reaction shifts in that direction in which the effect of these changes is reduced

Effect of pressure

       According to Le chatelier’s principle , an increase in pressure will favour the reaction in that direction in which the volume of reactants is reduced and decrease in pressure will favour the reaction in that direction in which the volume of reactants is increased.

According to Le chatelier’s principle , an increase in pressure will favour the reaction in that direction in which the volume of reactants is reduced and decrease in pressure will favour the reaction in that direction in which the volume of reactants is increased

       In this reaction the volume of reactants is 4 units while the volume of products is 2 units. So according to Le chatelier’s principle an increase in pressure of this reaction will favour the forward reaction to form more ammonia while a decrease in pressure of the reaction will favour the backward reaction to form more nitrogen and hydrogen. Thus

According to Le chatelier’s principle when the temperature, pressure or concentration of a reaction in equilibrium is changed then the reaction shifts in that direction in which the effect of these changes is reduced

Effect of change in concentration

1. Effect of change in concentration of reactants:- according to Le chatelier’s principle if we increase the concentration of reactants then the reaction will shift in forward direction

N2 (g) +3H2 (g)           2NH3 (g)

An increase in concentration of nitrogen and hydrogen will result in shifting of reaction in forward direction in which more ammonia will be formed.

2. Effect of change in concentration of products:- if we increase the concentration of products (ammonia) then the reaction will shift in backward direction i.e. formation of nitrogen and hydrogen will take place.

According to Le chatelier’s principle when the temperature, pressure or concentration of a reaction in equilibrium is changed then the reaction shifts in that direction in which the effect of these changes is reduced

Effect of catalyst

       A catalyst has no effect on equilibrium state of a reaction. It is added into the reaction mixture only to achieve the equilibrium state quickly because addition of a catalyst increases the rate of both the forward reaction and backward reaction equally.




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