Equilibrium is reached when the forward reaction happens at exactly the same __________ as the backward reaction. What one word completes this sentence?

The word that completes the sentence is "rate."

Equilibrium in a chemical reaction is achieved when the rate of the forward reaction equals the rate of the backward (or reverse) reaction. This dynamic state implies that the concentrations of the reactants and products remain constant over time, although both reactions are still occurring. At this point, there is no net change in the concentration of the reactants and products, indicating a stable condition. This concept is essential in chemical kinetics and thermodynamics, as it helps in understanding how reactions progress and reach a state of balance between reactants and products.