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Chapter 18 Reaction Rates and Equilibrium

18.1 Rates of Reaction 18.2 The Progress of Chemical Reactions 18.3 Reversible Reactions and Equilibrium 18.4 Solubility Equilibrium 18.5 Free Energy and Entropy

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How is a bicycle race like a chemical reaction?

CHEMISTRY & YOU

Riders in the Tour de France bicycle race must cross steep mountains with heights of 1900 meters or more.

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Rate Laws

The rate of a reaction depends in part on the concentrations of the reactants.

•  Suppose there were a reaction with only one reactant and one product.

A à B •  The rate at which A forms B can be

expressed as the change in A (ΔA) with time.

rate = ΔA Δt

Rate Laws

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Rate Laws

•  This equation is a rate law, an expression for the rate of a reaction in terms of the concentration of the reactants.

•  The specific rate constant (k) for a reaction is a constant that relates the concentrations of reactants to the rate of the reaction.

rate = = k × [A] ΔA Δt

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Rate Laws

The value of the specific rate constant, k, in a rate law is: •  large if the products form quickly; •  small if the products form slowly.

rate = = k × [A] ΔA Δt

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Rate Laws

First-Order Reactions The order of a reaction is the power to which the concentration of a reactant must be raised to match the experimental data on concentration and rate. •  In a first-order reaction, the rate is directly

proportional to the concentration of only one reactant.

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Rate Laws

Higher-Order Reactions

For the reaction of A with B, the rate of reaction is dependent on the concentrations of both A and B.

aA + bB à cC + dD

rate = k[A]x[B]y

In some reactions, two substances react to produce products.

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Rate Laws

Higher-Order Reactions

The overall order of a reaction is the sum of the exponents for the individual reactants.

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Rate Laws

Higher-Order Reactions

The overall order of a reaction is the sum of the exponents for the individual reactants.

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Reaction Mechanisms

Reaction Mechanisms

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One-Step and Multistep Reactions

An elementary reaction is a reaction in which reactants are converted to products in a single step.

•  This type of reaction has only one activation-energy peak and one activated complex.

Reaction Mechanisms

A balanced equation does not tell you how a reaction occurred.

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One-Step and Multistep Reactions Most chemical reactions consist of two or more elementary reactions.

Reaction Mechanisms

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One-Step and Multistep Reactions Most chemical reactions consist of two or more elementary reactions.

•  The series of elementary reactions or steps that take place during the course of a complex reaction is called a reaction mechanism.

Reaction Mechanisms

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One-Step and Multistep Reactions An intermediate is a product of one step in a reaction mechanism and a reactant in the next step. •  more stable structure and longer lifetime than

an activated complex. •  do not appear in the overall chemical

equation for a reaction.

Reaction Mechanisms

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Interpret Graphs

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Rate-Determining Steps In a multistep chemical reaction, the steps do not all progress at the same rate.

•  The slowest step will determine, or limit, the rate of the overall reaction.

Reaction Mechanisms