# Basic Concepts


Consider the following gas-phase chemical reaction:

$$H_2 + I_2 \rightleftharpoons 2HI$$

The apparatus shown belows contains 0.02 mol/L I2 gas in the left syringe and 0.01 mol/L H2 gas in the right syringe. When the two syringes are depressed, the two gases are mixed and the above reaction occurs. The graph at the right shows the variations of the H2, I2, and HI concentrations as a function of time. Run the experiment and examine the concentration-time plots.



As the reaction progresses, the concentrations of iodine and hydrogen decrease as they are consumed while the concentration of hydrogen iodide increases as it is formed. Eventually, however, the concentrations of all three species reach constant values. This behavior is a result of the reverse in which hydrogen iodide reacts to form iodine and hydrogen. Initially there is no hydrogen iodide, so the reverse reaction cannot occur. As hydrogen iodide accumulates, the reverse reaction can progress and becomes significant.

Eventually the rate at which iodine is being consumed by the forward reaction is perfectly balanced by the rate at which it is being produced by the back reaction. The same is true for hydrogen and hydrogen iodide. When the rates of forward and reverse reactions are perfectly balanced in this way, the reaction is said to be at equilibrium.