Chapter 14: Kinetics – The Speed of Chemistry

Introduction to Chemical Kinetics

Chemical kinetics is the study of the speed (rate) at which chemical reactions occur and the factors that affect these rates. Understanding kinetics allows chemists to control reaction conditions, optimize yields, and predict how fast products will form.

• Key factors affecting rates: concentration, temperature, catalysts, and reaction mechanism.

• Key terms: rate law, reaction order, intermediate, catalyst.

What Happens During a Reaction?

During a chemical reaction, reactants are converted into products. This involves breaking and forming chemical bonds. The concentrations of reactants decrease while those of products increase over time.

• Generic reaction: A → B

• Concentration changes: [A] decreases, [B] increases

Reaction Rate

The reaction rate is the speed at which reactants are consumed or products are formed. It is typically measured as the change in concentration over time.

• Average rate: Calculated over a time interval.

• Instantaneous rate: The rate at a specific moment (the slope of the tangent to the concentration vs. time curve).

Formulas:

• For a general reaction (A → B):

Note: The negative sign for reactants indicates a decrease in concentration.

Calculating Average Rates

To find the average rate, use concentration data at two time points:

Example: Calculating the average rate for the disappearance of N2O5 between 400 and 500 seconds.

• Rate = M/s

Relative Rates and Reaction Stoichiometry

Relative rates use stoichiometry to relate the rates of disappearance of reactants and appearance of products.

• For the reaction:

Example: For , the rate of appearance of NO2 is double the rate of disappearance of N2O5.

Rate Law and Reaction Orders

The rate law expresses the relationship between the rate of a reaction and the concentrations of reactants, each raised to a power (the reaction order).

• General form:

• k is the rate constant; x and y are the orders for A and B, determined experimentally.

Overall reaction order: Sum of all exponents in the rate law.

Understanding Reaction Orders

0th Order

• Rate is independent of [A]:

• Graph: [A] vs. time is a straight line (constant slope).

1st Order

• Rate is proportional to [A]:

• If [A] doubles, rate doubles.

• Graph: [A] vs. time is a curve; rate decreases as [A] decreases.

2nd Order

• Rate is proportional to [A]2:

• If [A] doubles, rate quadruples.

• Graph: [A] vs. time is even more curved.

Experimental Determination of Reaction Orders

Reaction orders are found by comparing how changes in concentration affect the rate, using experimental data.

• Compare experiments where only one reactant's concentration changes.

• Alternatively, use logarithms:

Rate Constant (k)

The rate constant, k, is a proportionality factor in the rate law. It is independent of reactant concentration but depends on temperature.

• General form:

• Units of k depend on overall reaction order.

• "Fast" reactions: ; "Slow" reactions:

Example: For , given rate and concentrations, solve for k:

Predicting Rates

Given reaction orders, predict how changes in concentration affect the rate.

• If rate is 1st order in NO and 0th order in O2, doubling [NO] doubles the rate; changing [O2] has no effect.

Summary of Key Concepts

• Identify factors affecting reaction rates.

• Write and use rate laws.

• Determine reaction orders from data.

• Calculate average and instantaneous rates.

• Relate rates using stoichiometry.

• Calculate and interpret rate constants.

Equations and Relationships

• Concentration vs. time for 0th, 1st, and 2nd order reactions.

• Rate law:

• Relative rates:

• Order determination: