# How do you calculate the Keq of a reaction?

Jun 17, 2017

These are not $\text{calculated}$, but $\text{MEASURED}$.

#### Explanation:

For the reaction........

$A + B r i g h t \le f t h a r p \infty n s C + D$

There is a $\text{rate forward}$, ${k}_{f} \left[A\right] \left[B\right]$, where ${k}_{f}$ is some constant....

And a $\text{rate backwards}$, ${k}_{r} \left[C\right] \left[D\right]$, where ${k}_{r}$ is some other constant....

Now, by definition, a condition of $\text{chemical equilibrium}$ occurs not when chemical change ceases, but when there is equality of forward and reverse rates, and thus.........

$\text{rate backwards}$, ${k}_{r} \left[C\right] \left[D\right] \equiv \text{rate forwards}$, ${k}_{f} \left[A\right] \left[B\right]$. And so we take the quotient......

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

We more commonly call the quotient ${k}_{r} / {k}_{f}$, ${K}_{c}$, the thermodynamic equilibrium constant.