# How can one tell a reaction has reached equilibrium?

Mar 9, 2017

When the rate of forward reaction is equal to the rate of reverse reaction.

#### Explanation:

For the generalized equilibrium we have:

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

Now we have a rate forward,

${k}_{f} \left[A\right] \left[B\right]$, and a rate backwards, ${k}_{r} \left[C\right] \left[D\right]$.

${k}_{f}$ and ${k}_{r}$ are unspecified rate constants, whose ratio must be determined by measurement.

The condition of chemical equilibrium is reached when,

$\text{rate forward "-=" rate backwards}$,

i.e. $\frac{\left[C\right] \left[D\right]}{\left[A\right] \left[B\right]} = {k}_{f} / {k}_{r} = {K}_{\text{eq}}$.

${K}_{\text{eq}}$ is the thermodynamic equilibrium constant, WHICH must be measured for a given reaction, and for a given temperature. Because chemical change is conceived to continue even after equilibrium is reached, even tho $\text{net chemical change}$ is nil, this is labelled as $\text{dynamic chemical equilibrium.}$ After equilibrium is reached, NO NET CHEMICAL CHANGE OCCURS.