# What is the Keq equation?

Feb 5, 2017

For the reaction, $A + B r i g h t \le f t h a r p \infty n s C + D$ ;K_"eq"=([C][D])/([A][B]).

#### Explanation:

${K}_{e q} = \frac{\left[C\right] \left[D\right]}{\left[A\right] \left[B\right]}$

Given $A + B r i g h t \le f t h a r p \infty n s C + D$, there is a $\text{rate forward}$, ${k}_{f} \left[A\right] \left[B\right]$, and a $\text{rate backwards}$, ${k}_{r} \left[C\right] \left[D\right]$; ${k}_{f}$ and ${k}_{r}$ are some unspecified rate constants.

Equilibrium, by definition, explicitly specifies EQUALITY of FORWARD and REVERSE rates, and thus:

${k}_{f} \left[A\right] \left[B\right] = {k}_{r} \left[C\right] \left[D\right]$;

Under these conditions, chemical change has not ceased, but there is no net macroscopic change in the given concentrations.

Further we can write, ${k}_{f} / {k}_{r} = \frac{\left[C\right] \left[D\right]}{\left[A\right] \left[B\right]}$, and this is usually specified under standard conditions.

The quotient ${k}_{f} / {k}_{r}$ is better as known ${K}_{\text{eq}}$, the thermodynamic equilibrium constant, and it must be measured for a given reaction.