Chapter 6.5 Rate Law

Why Rate Law? The aim of many industries is to maximize output while minimizing cost of production. Chemistry industry is no different. Chemists try to vary the concentrations of reactants in order to determine the minimum concentrations needed to produce maximum output. To do this, they employ the use of RATE LAW.

Rate Law A chemical rate law is an equation that connects the rate of reaction with the concentration of its reactants at a given temperature and pressure aA + bB  products In this chemical equation A and B are reactants, and a and b are the coefficients that balance the equation.

Rate Law Experimental evidence has shown that the rate of reaction is proportional to (α) the product of the initial concentrations where each concentration is raised to some exponential value. rate α [A]m[B]n

rate = k[A]m[B]n k is called the RATE CONSTANT and is unique for each equation (determined experimentally). The exponents, m and n: each one is called an ORDER OF REACTION and is used to describe the relationship between the initial concentration of a particular reactant and the rate of reaction. If m = 1, the reaction is first order with respect to [A] If m = 2, the reaction is second order with respect to [A]

The TOTAL ORDER OF REACTION is the sum of all the exponents in the rate law equation. 2A + 2B + 3C  products rate =k[A]1[B]2[C]0 This is a… first order reaction w.r.t [A] second order reaction w.r.t. [B] zero order reaction w.r.t. [C]

rate =k[A]1[B]2[C]0 Let’s look at A Let’s look at B Let’s look at C

Initial Rates Initial rate is the rate determined just after the reaction begins (just after t = 0s) Chemists commonly use the method of initial rates to determine the rate law equation for a reaction.

Steps in the Method of Initial Rates: Measure the instantaneous rate of reaction before there are any significant changes in concentration of the reactants. Carry out several runs using different initial concentrations, determining the initial rate resulting from each run. Compare these results to observe how the initial rate depends on the initial concentrations.

Example

HOMEWORK P. 380 #1, 2, 3, 4ab P. 382 #1, 3, 4abc