9.1 Chemical reaction rates  (Page 21/8)

A rate is a measure of how some property varies with time. Speed is a familiar rate that expresses the distance traveled by an object in a given amount of time. Wage is a rate that represents the amount of money earned by a person working for a given amount of time. Likewise, the rate of a chemical reaction is a measure of how much reactant is consumed, or how much product is produced, by the reaction in a given amount of time.

The rate of reaction    is the change in the amount of a reactant or product per unit time. Reaction rates are therefore determined by measuring the time dependence of some property that can be related to reactant or product amounts. Rates of reactions that consume or produce gaseous substances, for example, are conveniently determined by measuring changes in volume or pressure. For reactions involving one or more colored substances, rates may be monitored via measurements of light absorption. For reactions involving aqueous electrolytes, rates may be measured via changes in a solution’s conductivity.

For reactants and products in solution, their relative amounts (concentrations) are conveniently used for purposes of expressing reaction rates. If we measure the concentration of hydrogen peroxide, H ₂ O ₂ , in an aqueous solution, we find that it changes slowly over time as the H ₂ O ₂ decomposes, according to the equation:

The rate at which the hydrogen peroxide decomposes can be expressed in terms of the rate of change of its concentration, as shown here:

This mathematical representation of the change in species concentration over time is the rate expression    for the reaction. The brackets indicate molar concentrations, and the symbol delta (Δ) indicates “change in.” Thus, ${[{\text{H}}_2{\text{O}}_2]}_{t_1}$ represents the molar concentration of hydrogen peroxide at some time t ₁ ; likewise, ${[{\text{H}}_2{\text{O}}_2]}_{t_2}$ represents the molar concentration of hydrogen peroxide at a later time t ₂ ; and Δ[H ₂ O ₂ ] represents the change in molar concentration of hydrogen peroxide during the time interval Δ t (that is, t ₂ − t ₁ ). Since the reactant concentration decreases as the reaction proceeds, Δ[H ₂ O ₂ ] is a negative quantity; we place a negative sign in front of the expression because reaction rates are, by convention, positive quantities. [link] provides an example of data collected during the decomposition of H ₂ O ₂ .

To obtain the tabulated results for this decomposition, the concentration of hydrogen peroxide was measured every 6 hours over the course of a day at a constant temperature of 40 °C. Reaction rates were computed for each time interval by dividing the change in concentration by the corresponding time increment, as shown here for the first 6-hour period: