Dynamic Equilibrium and Reversible Reactions

Many chemical reactions do not simply run to completion, leaving no starting material behind. Instead, they reach a point where reactants and products coexist in unchanging amounts. This condition is called chemical equilibrium, and understanding it is central to explaining how industrial syntheses, biological systems, and even the acidity of the oceans behave.

A reversible reaction is one that can proceed in both the forward and reverse directions. We write reversible reactions with a double harpoon symbol, as in $N_2O_4(g) \rightleftharpoons 2NO_2(g)$. Colorless dinitrogen tetroxide decomposes into brown nitrogen dioxide, and nitrogen dioxide recombines into dinitrogen tetroxide. If we start with pure $N_2O_4$, the forward reaction is initially fast because the concentration of $N_2O_4$