Chapter 12 Chemical Kinetics

Section 12.2 Atomic MassesRate Laws: An Introduction Return to TOC Determining Rates Using The Initial Rate Method As a reaction proceeds, the amount of reactant decreases and the amount of product increases. The reverse reaction becomes important once there is appreciable product To reduce the difficulty, determine the reaction rate under conditions where the reverse reaction is negligible—when the reactants are first combined. This is referred to as the Initial Reaction Rate Copyright © Cengage Learning. All rights reserved 2

Section 12.2 Atomic MassesRate Laws: An Introduction Return to TOC Copyright © Cengage Learning. All rights reserved 3 Rate Law An expression that shows how the rate of a reaction depends on the concentrations of reactants. For the decomposition of nitrogen dioxide: 2NO 2 (g) → 2NO(g) + O 2 (g)  k = rate constant  n = order of the reactant Lowercase “k” Must be determined experimentally

Section 12.2 Atomic MassesRate Laws: An Introduction Return to TOC Copyright © Cengage Learning. All rights reserved 4 Rate Law Rate = k[NO 2 ] n The concentrations of the products do not appear in the rate law because the reaction rate is being studied under conditions where the reverse reaction does not contribute to the overall rate (initial conditions)

Section 12.2 Atomic MassesRate Laws: An Introduction Return to TOC Copyright © Cengage Learning. All rights reserved 5 Rate Law Rate = k[NO 2 ] n The value of the exponent n must be determined by experiment; it cannot be written from the balanced equation. The value of the exponent n is NOT handled the same way as it is in an equilibrium expression

Section 12.2 Atomic MassesRate Laws: An Introduction Return to TOC Copyright © Cengage Learning. All rights reserved 6 Types of Rate Laws Differential Rate Law (rate law) – shows how the rate of a reaction depends on concentrations. Integrated Rate Law – shows how the concentrations of species in the reaction depend on time. Depending on the available experimental data (convenience), either of the rate laws above can be calculated. Once this is achieved, then the other rate law is known as well.

Section 12.2 Atomic MassesRate Laws: An Introduction Return to TOC Differential Rate Law Example: A + B → Product Where.... k = rate law constant [A] = concentration of reactant A [B] = concentration of reactant B m = reaction order with respect to A n = reaction order with respect to B m + n = overall order of the reaction (which will have an effect on the units of the k ) Copyright © Cengage Learning. All rights reserved 7 The size of m and n refer to how the rate is affected by changes in concentration of each reactant

Section 12.2 Atomic MassesRate Laws: An Introduction Return to TOC Copyright © Cengage Learning. All rights reserved 8 Rate Laws: A Summary Because we typically consider reactions only under conditions where the reverse reaction is unimportant, our rate laws will involve only concentrations of reactants. Because the differential and integrated rate laws for a given reaction are related in a well– defined way, the experimental determination of either of the rate laws is sufficient.

Section 12.2 Atomic MassesRate Laws: An Introduction Return to TOC Copyright © Cengage Learning. All rights reserved 9 Rate Laws: A Summary Experimental convenience usually dictates which type of rate law is determined experimentally. Knowing the rate law for a reaction is important mainly because we can usually infer the individual steps involved in the reaction from the specific form of the rate law.