 Initially, the reaction vessel contains only the reactants, hydrogen gas and bromine gas. The forward reaction of hydrogen gas and bromine gas produces hydrogen bromide gas. As the forward reaction proceeds, the concentrations of hydrogen and bromine gases decrease, and the concentration of hydrogen bromide gas increases. Once hydrogen bromide gas is present in the vessel, the reverse reaction of hydrogen bromide gas producing hydrogen and bromine gas begins to take place as well. As the concentration of reactants decreases, the rate of the forward reaction decreases. As the concentration of products increases, the rate of the reverse reaction increases. Eventually, the system will reach equilibrium at which the rate of the forward reaction and the rate of the reverse reaction are equal. At equilibrium, there is no further net change in the concentration of reactants or products.