Thr Progress of Chemical Reaction Notes #9 Section 18.5

Rate Law The rate can be determined by how much the concentration of the reactants have changed per unit of time. A B Will the concentration of A increase o decrease as time passes?

Rate Law Is an expression for the rate of a reaction in terms of the concentration of the REACTANTS. Specific Rate Constant (k): is a CONSTANT relating the proportionality of the reactant concentration to the rate. If k is large, the products form quickly. If k is small, he products form slowly.

Two types of Ordered Reactions: First Order Reaction: the rate of reaction is proportional to the concentration of ONE reactant raised to the first power. A  C k = [A]1 = [A] Second Order Reaction: the rate of reaction is dependent on BOTH reactants. A +B  C + D k = [A]a[B]b *The exponents are determined experimentally! Add the exponents to get OVERALL order of reaction (a+b)

First Order Reaction A  C

Let’s do #37 on page 577 and #42 on page 579

Reaction Mechanisms Use graphs that show the progress of a reaction. It has many “peaks” and “valleys”.

Definitions An elementary reaction is a reaction in which reactants are converted to products in a single step. A B The series of elementary reactions or steps that take place during the course of a complex reaction is called a reaction mechanism. A  B  C An intermediate is a product of one of the steps in the reaction mechanism. A  B  C *B is the intermediate.

How many elementary reactions are part of this reaction? How many intermediates are formed?

Reaction Mechanisms Intermediates DO NOT appear in he overall chemical formula. They can ONLY be determined experimentally. Two elementary reactions: 1) N2O  N2 + O 2) N2O + O  N2 + O2 Overall Reaction: N2O  N2 + O2

ASSESSMENT: USE YOUR NOTES 1. Choose the correct words for the spaces. If the specific rate constant, k, is _________, then products form __________. a. large, slowly b. large, quickly c. small, quickly

2. Choose the correct words for the spaces 2. Choose the correct words for the spaces. In a graph that shows the progress of a reaction, the peaks in the curve correspond to the energies of the _______ and the valleys correspond to the energies of the ________. a. activated complexes, reactants b. reactants, products c. activated complexes, intermediates

3. For the following reaction rate law, determine the overall order of the reaction. Rate = k [MgSO4]1 [K2SO4]1

4. Sketch a reaction mechanism graph using the following mechanism 4. Sketch a reaction mechanism graph using the following mechanism. Write the balanced equation of the overall reaction. 2NO  N2O2 (fast, lower amount of energy needed) N2O2 + O2  2NO2 (slow, higher amount of energy needed)