[shehri]

Hi all,

        Plz. explain me why a reversible reaction moves forward if Pressure increases in case no. of moles of reactants're more than no. of moles of products?Thanks.

[Borek]

Le Chatelier's principle.

[Yggdrasil]

Consider a reversible gas phase reaction:

A + B --> C

In this case there are more moles of reactants than products.  Now, the equilibrium constant (expressed in terms of partial pressures) is:

K = P_{c, eq}/(P_{a, eq}*P_{b, eq})

Where P_{a, eq}, P_{b, eq}, and P_{c, eq} are the partial pressures of each gas at equilibrium.  Now, lets say we have a system at equilibrium then compress the system so that all of the pressures increase by a factor of two (e.g. the new partial pressure of gas a is P'<sub>a[/sup] =  2Pa, eq).

Now, calculate the reaction quotient:

Q = P'c / P'aP'a = 2Pc, eq/(2Pa, eq*2Pb, eq)

After some rearranging:

Q = (1/2) * ( Pc, eq/(Pa, eqPb, eq) ) = (1/2)K

Since Q = (1/2)K < K, the reaction will proceed forward.</sub>