THE EFFECT OF TEMPERATUE ON THE POSITION OF EQUILIBRIUM, EQUILIBRIUM CONSTANT

 

THEEFFECT OF TEMPERATUE ON THE POSITION OF EQUILIBRIUM:

 

            Le’ Chatelier’s principle can be used to deduce the effect of temperature on the position of equilibrium of a reversible reaction.  An increase in temperature will favour the endothermic reaction i.e; the reaction which  absorbs the additional heat energy  conversely, decreasing the temperature will favour the raction that evolves heat energy i.e; the exothermic reaction. For example.

 

            N₂O    2NO   H = positive

 

            Increasing the temperature will favour the forward endothermic reaction. Thus, the position of the equilibrium will be shifted to the right. For the reaction,

            2SO  + 2SO    Negative

            Increasing the temperature will favour the reverse endothermic reaction. Thus, the position of the equilibrium will be shifted to the left or decreasing the temperature will favour the exothermic reaction.

 

INTHE EFFECT OF CATALYST ON EQUILIBRIUM:

 

            Catalyst decreases the activation energy of a reaction hence, it speeds up the reaction i.e; increases the rate of reaction but it does not shift the position of equilibrium catalyst help a reversible reaction to establish an equilibrium more quickly, but once equilibrium is achieved by catalyst further it cannot change the equilibrium concentration of reactants or products.

 

EQUILIBRIUMCONSTANT:

 

            Equilibrium constant is the ratio of the concentrations of products to the concentrations of reactants each concentration raised to the power coefficients of the substances in the balanced equation. The equilibrium constant is abbreviated as Kc. For a general reversible reaction.

 

            aA + bB  cC + dD

 

            At equilibrium mathematical expression of equilibrium constant is;

             

            Kc =    

 

            Where [  ] denotes concentration expressed in mole dm^-3. For reactions involving gases, the equilibrium constant is more conveniently expressed in terms of partial pressures rather then their concentrations. The equilibrium constant is then abbreviated as Kp. Mathematically it can be represented as;

 

            Kp =

 

            The units for Kp is (Atmosphere)^C+D-A-B Similarly, the units for K_c is;

            K_c = ( mol. Dm^-3)^C+D-A-B

 

EFFECT OF CONCENTRATION, PRESSURE AND PRESSURE OF CATALYST ON EQUILIBRIUM CONSTANT:

 

            Concentration, pressure and catalyst has no effect on equilibrium constant value at constant temperature. At equilibrium change in concentration will change position of equilibrium either by shifting position of equilibrium in the forwarded or reverse direction by keeping the value of K_c constant at constant temperature. Hence concentrations may change by K_c remains unchanged.

 

            K_c =

 

At equilibrium by changing the total pressure over a mixture of reacting and produced gases, position of equilibrium is shifted either in the forward or reverse disrection but ratio of partial pressures constant i.e; value of K_p is unchanged.

 

K_p =

 

Presence of catalyst has no effect on position of equilibrium an on equilibrium. Constant i.e; value of K_c or K_p remains constant. Presence of catalyst increases rate of reaction i.e; help a reversible reaction to achieve equilibrium quickly but it cannot change position as well as constant after reaching at equilibrium.

 

EFFECTOF TEMPERATURE ON EQUILIBRIUM CONSTANT:

 

            Change the concentrations and pressure is observed at constant temperature. Changing the temperature result in a new value of the equilibrium constant. This is because reversible reaction is exothermic in one direction but endothermic in the other direction.

 

                      -     

 

 

This is for general rule, for an exothermic reaction, increasing the temperature will move the equilibrium in the reverse direction (to the left) and for an endothermic reaction increasing the temperature will move the equilibrium in the reverse direction (to the left). Hence, value of K_c or K_p decreases.  On the other hand for an endothermic forward reaction.

 

                      –

 

                Increasing the temperature will move the equilibrium in the forward direction (to the right) and for an exothermic backward reaction, increasing the temperature will further move the  equilibrium in the forward direction (to the right). Hence, value of K_c or K_p increases. For example.

 

            N   + 3H    2NH    = - 92 K.J mole^-1

 

            Temperature (K)                                             Kp (Pa^-2)

                        298 K                                                  6.76 x 10⁵

                        500 K                                                  3.55 x 10^-2            Decrease

700 K                                                  7.76 x 10^-5

900 K                                                  1.00 x 10^-6

 

            H   + CO    H₂O_(g) + CO_(g)    = + 41 K.J mole^-1

 

            Temperature (K)                                        Kp (No units)

                        298 K                                                  1.00 x 10⁵

                        500 K                                                  7.76 x 10^-2      Decrease

700 K                                                  1.23 x 10^-5

900 K                                                  6.01 x 10^-6