# When does a chemical reaction reach equilibrium?

Jul 30, 2018

This requires a specific definition....

#### Explanation:

...namely, that the condition of equilibrium is achieved when the forward rate of rate of reaction is equal to the reverse rate of reaction, and there is no NET change to the concentrations of reactants and products....

And so for the hypothetical reaction...

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

...there is a forward rate, ${k}_{f} \left[A\right]$, and a reverse rate, ${k}_{r} \left[B\right]$, ${k}_{f}$, and ${k}_{r}$ are some constants...here with units of $m o l \cdot {L}^{-} 1 \cdot {s}^{-} 1$

And given that we specified equality of rates at equilibrium...

${k}_{f} \left[A\right] \equiv {k}_{r} \left[B\right]$...thus ${k}_{f} / {k}_{r} \equiv \frac{\left[B\right]}{\left[A\right]}$

The quotient, ${k}_{f} / {k}_{r}$ is otherwise known as ${K}_{\text{eq}}$, $\text{the thermodynamic equilibrium constant}$, a constant that must be measured for a given scenario. It can normally be related to the free energy of the reaction where...$\ln {K}_{\text{eq}} = - \frac{\Delta {G}^{\circ}}{R T}$..