For the reaction H2(g) + I2(g) ⇋ 2HI(g), the rate of reaction is expressed as

The rate of reaction is the rate at which reactants are converted into products. It is expressed as the change in the concentration of the reactants or products with respect to time. In this case, the rate of the reaction is expressed as the change in the concentration of hydrogen (H2) and iodine (I2). This change can be observed by measuring the rate at which hydrogen and iodine molecules react and produce two molecules of hydrogen iodide gas (HI). This provides an accurate measurement of the rate of the reaction.