Chemistry 18.5

The Progress of Chemical Reactions 18.5 The Progress of Chemical Reactions During the course of the Tour de France, cyclists travel almost 4000 kilometers with steep mountains at 1600 meters or more. Crossing these high mountains requires extra energy. Chemical reactions can be thought of as processes involving peaks and valleys also. You will learn about the energy of a chemical reaction.

18.5 Rate Laws Rate Laws What is the general relationship between the value of the specific rate constant, k, and the speed of a chemical reaction?

18.5 Rate Laws The rate law is an expression for the rate of a reaction in terms of the concentration of reactants. The specific rate constant (k) for a reaction is a proportionality constant relating the concentrations of reactants to the rate of the reaction.

18.5 Rate Laws The value of the specific rate constant, k, is large if the products form quickly; the value is small if the products form slowly.

Observe the characteristics of a first-order reaction. Rate Laws Animation 24 Observe the characteristics of a first-order reaction.

18.5 Rate Laws In a first-order reaction, the reaction rate is directly proportional to the concentration of only one reactant. As the concentration of that reactant decreases, the rate of the reaction also decreases.

18.5 Rate Laws The graph shows how the concentration of a reactant changes as a reaction proceeds. INTERPRETING GRAPHS a. Navigate What is the dependent variable? b. Read What happens to the relative concentration of reactant A as time progresses? c. Interpret The short colored lines illustrate the reaction rates at two distinct points of time. Is the initial rate or the subsequent rate faster?

Conceptual Problem 18.2

Conceptual Problem 18.2

Conceptual Problem 18.2

for Conceptual Problem 18.2 Problem Solving 18.36 Solve Problem 36 with the help of an interactive guided tutorial.

18.5 Reaction Mechanisms Reaction Mechanisms What do the hills and valleys in a reaction progress curve represent?

18.5 Reaction Mechanisms An elementary reaction is a reaction in which reactants are converted to products in a single step. The series of elementary reactions or steps that take place during the course of a complex reaction is called a reaction mechanism. An intermediate is a product of one of the steps in the reaction mechanism.

18.5 Reaction Mechanisms A reaction progress curve shows an activation- energy peak for each elementary reaction. A reaction progress curve shows an activation-energy peak for each elementary reaction. Valleys indicate the formation of intermediates. INTERPRETING GRAPHS a. Navigate How many elementary reactions are part of this reaction? b. Read How many intermediates are formed? c. Interpret How would a catalyst affect the energy of the reactants and the energy of the products of this reaction?

18.5 Reaction Mechanisms The peaks correspond to the energies of the activated complexes. Each valley corresponds to the energy of an intermediate.

18.5 Section Quiz. 18.5.

18.5 Section Quiz. 1. Choose the correct words for the spaces. If the specific rate constant, k, is _________, then products form __________. large, slowly large, quickly small, quickly small, a first order reaction

18.5 Section Quiz. 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 ________. activated complexes, reactants intermediates, activated complexes reactants, products activated complexes, intermediates

18.5 Section Quiz. 3. Which of the following statements is true? Most reactions are elementary reactions. In most reactions, reactants are converted to products in a single step. Most reactions take place in a series of steps called intermediates. Most reactions take place in a series of steps called a reaction mechanism.

18.5 Section Quiz. 4. The reaction mechanism can best be determined from the specific rate constant. the overall chemical reaction. the rate law. experimentation.

END OF SHOW