Equilibrium Constant for Predicting the Extent of Reaction

Laws of chemical equilibrium and constant can be expressed in mathematical expressions giving a brief about the reaction in equilibrium. It can be expressed as the value of the reaction quotient, which cannot be changed further by any reaction.

A general reaction represents:

A + B  C + D

In which equilibrium is maintained between the reactants ( A & B ) and products ( C & D ), respectively. 

The chemical equilibrium constant K represents the relationship between the active reactants and the products. This article explains the equilibrium constant for predicting the extent of reaction. 

Law of mass action 

According to the equilibrium constant for predicting the extent of reaction, the law of mass action states the following:

At equilibrium, the rate of the two opposing reactions becomes equal i.e. 

Rate of forwarding reaction = Rate of backward reaction. 

K = k1 (forward) / k2 (backward)

At a particular temperature, K1 & K2  are constants. Therefore, the ratio K1 / K2 will be a constant. This is represented as K and is called the equilibrium constant.

Thus, 

K = K1 / K2     

K1 / K2 = (C) (D) / (A) (B)

The magnitude of the equilibrium constant K helps predict the extent to which a reaction proceeds.

Temperature is directly proportional to the rate of reaction between the reactants and the products formed.

An increase in temperature proportionally increases the rate of reaction.

A + B   C + D —— (1)

K = k1 (forward) / k2 (backward)

In the above reactions, where k1 and k2 are rates of constants for the forward and backward reactions, respectively, K is the equilibrium constant.

Here, (A) (B) (C) & (D) represent the active masses of concentration of A, B, C & D, respectively, at equilibrium. 

Equilibrium Constant For Predicting The Extent Of Reaction

When we deal with reversible reactions, it is important to figure out the reaction direction at any given point. We can predict the extent of the reaction by finding the reaction quotient and equilibrium constant. 

A high KC value indicates that the reaction has reached equilibrium with a high product yield, whereas a low KC value indicates that the reaction has reached equilibrium with a low product yield. 

 If KC > 103, the reaction is nearly complete. 

If KC <10-3, the reaction is rarely initiated. 

A significant amount of both reactants and products are present at equilibrium if the KC is in the range of 10-3 to 103.

The importance of chemical reaction prediction to help us:

• It can help us know about the substances we used, their basicity, acidity, and reaction rate.
• It can let us know the stoichiometric need for substances under specific conditions.
• It might tell us about the matter content repressive in the substance and knowing about the content’s chemical behaviour.
• It may help us predict and calculate the future outcome led by the reaction.

Some Solved Examples 

1) Consider the reaction  N2O4 (g) 2NO2

At equilibrium, rate (forward) = rate (backward)

K [N2O4] eq = k [NO2] eq 

Then, K (forward) / K (backward)

             = K [ NO2 ]² eq / [ N2O4 ]eq 

The ratio of constants gives a new constant, the equilibrium constant:

K = k (forward) / k (backward)

   = k [NO2]² eq / [N2O4] eq

2) N2 (g) + Br2 (g) 2 HBr (g)

    Keq = 1.9 × 1019 at 25° C

    [ product favoured ]

3) CO2 (g) + H2 (g) CO (g) + H2O (g)

      KC = [ CO ] [ H2O ] / [ CO2 ] [ H2]

[ CO2 ] = 0.1908 mol CO2 / 2.00 L = 0.0954 M [ H2 ] = 0.0454 M

[ CO ] = 0.0046 M

[ H2O ] = 0.0046 M

    KC= [ 0.0046 ] [ 0.0046 ] / [ 0.954 ] [ 0.454 ]

         = 0.0049 

4) Let us consider the reaction 

 Cu (s) + Zn²+ (aq) Cu²+ (aq) + Zn (s)

 K = [ Cu²+ (aq) ] / [ Zn²+ (aq) ]

    = 2.0 × 10–19 at 298 K

Here, the value of K is very small. 

Therefore, in this case, a backward reaction compared to the forward reaction proceeds to a greater extent before equilibrium is attained. As a result, in equilibrium, the concentrations of the reactants are much higher than those of the products formed in the reaction.

Note:

If K > 1, then equilibrium favours products.

If K < 1, then equilibrium favours the reactants.

The equilibrium constant cannot be zero because it implies zero at being in equilibrium.

Conclusion

The reaction rate depends on the magnitude, velocity, physical nature and temperature of the reactants placed. The magnitude of the equilibrium constant K indicates the rate of reaction to which the reaction is proceeding. An increase in K indicates an increase in products’ equilibrium concentration.

Reaction extent is the quantity that informs us how much the reaction has extended. The constant K measures the reactants infused to react and form the products. This article highlights the equilibrium constant for predicting the extent of reaction and the equilibrium constant for predicting the extent of reaction importance.