HL Chemistry, determining rate law

1. Rate expression and reaction mechanism (HL) A → B ; rate equation or rate expression rate = k[A]x k is called the rate constant. Xis called order of reaction with respect to reactant A . The rate equation is an experimentally determined equation that relates rate of reaction to the concentrations of substances in the reaction mixture.

2. aA+bB →cC+dD ; rate = k[A]X[B]Y • The rate constant is a constant of proportionality relating the concentrations in the experimentally determined rate equation to the rate of a chemical reaction. • The order of a reaction with respect to a particular reactant is the power of the reactant’s concentration in the experimentally determined rate equation. An overall order of reaction is the sum of the powers of the concentration terms in the experimentally determined rate equation.

3. Experimentaldetermination of a rate equation ; Rate expression/law cannot be predicted by looking at a balance chemical equation. A few experiments are conducted at different initial concentrations and the initial rates are determined (near t=0 ) The initial rate is taken because this is the only point at which we know the concentration of all the reactants and none has been used up.

4. For the reaction 2NO(g) + H2(g)̀N2O(g) + H2O(g), the following rate data were collected at a given temperature:Determine the rate law: rate = k[NO]X[H2]Y

8. In many cases we can determine the order of reactions by simple inspection and then we will justify our answers in words.

12. For the next class: Determine the rate expression and determine the value of the rate constant and its units using the data from exp. #3