Question

Question: Explain the difference between K, K_p, and Q.

Short answer

In the equilibrium reaction, the Q value is calculated for both initial conditions and equilibrium conditions, while the K and K_p values are calculated for only equilibrium conditions.

Step-by-step solution

Write the expressions for K, Kp, and Q

Consider a general chemical reaction as shown below:

$\mathrm{mM}+\mathrm{nN}\to \mathrm{qQ}+\mathrm{rR}...\left(1\right)$

Here,

M and N are the reactant species.

Q and R are the product species.

m, n, q, and r are the coefficients of M, N, Q, and R, respectively.

According tothe law of mass action, the equilibrium constant expressions for the above reaction at equilibrium can be represented as follows:

For K in terms of concentrations:

$\mathrm{K}=\frac{{\left[\mathrm{Q}\right]}^{\mathrm{q}}{\left[\mathrm{R}\right]}^{\mathrm{r}}}{{\left[\mathrm{M}\right]}^{\mathrm{m}}{\left[\mathrm{N}\right]}^{\mathrm{n}}}$ ...(2)

For K_p in terms of partial pressures:

${\mathrm{K}}_{\mathrm{p}}=\frac{\left({{\mathrm{P}}_{\mathrm{Q}}}^{\mathrm{q}}\right)\left({{\mathrm{P}}_{\mathrm{R}}}^{\mathrm{r}}\right)}{\left({{\mathrm{P}}_{\mathrm{M}}}^{\mathrm{m}}\right)\left({{\mathrm{P}}_{\mathrm{N}}}^{\mathrm{n}}\right)}$ ...(3)

The expression for the reaction quotient, Q, for this reaction is shown below:

$\mathrm{Q}=\frac{{\left[\mathrm{Q}\right]}^{\mathrm{q}}{\left[\mathrm{R}\right]}^{\mathrm{r}}}{{\left[\mathrm{M}\right]}^{\mathrm{m}}{\left[\mathrm{N}\right]}^{\mathrm{n}}}$ ...(4)

The difference between K, Kp, and Q are described.

Q is the reaction quotient, whereas K and K_p are equilibrium constants. All are calculated by the law of mass action. Q is used for any set of conditions, while K and K_p are used for equilibrium conditions only. The reaction quotient becomes the equilibrium constant when the reaction is at equilibrium. The equilibrium constant value does not change for the equilibrium reaction at a specific temperature, but the reaction quotient value changes with changing the reaction conditions.