Equilibrium is reached in a reversible reaction when the rate of the forward reaction equals the rate of the reverse reaction. The graph shows the changes in concentration during the formation of HI(g) from a mixture of H₂(g) and I₂(g). If the measurement is taken at equilibrium, how many moles of HI(g) are present in a 750 mL sample of the reaction mixture?

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11ecba2d_1216_2b79_a3bf_71021a8c87cf_MD0008_00 Equilibrium is achieved in a reversible reaction when the forward reaction and the reverse reaction occur simultaneously at the same rate. Once equilibrium is achieved, the forward reaction generates products as fast as the reverse reaction converts those products back into the original reactants, and this causes the concentrations of the reactants and the products to become constant. Although constant, the equilibrium concentrations are not necessarily equal because equilibrium refers to a state of equal reaction rates (changes in concentration over time) but not to a state of equal concentrations. In the graph of concentration vs. time for the reaction between H₂(g) and I₂(g), the constant (horizontal) regions of the curves (after approximately 5 hours) indicate a state of equilibrium in which the concentrations of the chemical species no longer change. From this region of the graph, the equilibrium concentration of HI(g) is seen to be 6.0 M. Therefore, if a 750 mL sample of the reaction mixture were analyzed at equilibrium, the number of moles of HI(g) present in the sample would be determined as follows: 11ecba2d_1216_2b7a_a3bf_17de4ba3adda_MD0008_00 (Choice A) At equilibrium, a 750 mL sample of the reaction mixture contains 0.8 mol each of H₂(g) and I₂(g), but the question asks for the moles of HI(g). (Choice B) A 750 mL sample measured at the intersection point of the lines on the graph (after approximately 1 hour) contains 1.9 moles of HI(g), but this point does not indicate the equilibrium of the reaction. (Choice D) At equilibrium, the HI(g) has a molar concentration of 6.0 mol/L. This is the number of moles of HI(g) per liter rather than the number of moles in a 750 mL sample of the reaction mixture. Educational objective: Equilibrium is achieved when two opposing chemical reactions occur simultaneously at the same rate such that the concentrations of the chemical species become constant. Equilibrium reaction rates are equal, but the equilibrium concentrations of chemical species may be unequal.

Equilibrium Is Reached in a Reversible Reaction When the Rate