Chemistry 116 - Chapter 4

What is chemical kinetics?

The study of reaction rates and the factors that influence how fast reactions occur.

What is reaction rate?

The change in concentration of reactants or products per unit time.

Why is there a negative sign for reactants in rate expressions?

Because reactant concentrations decrease as the reaction proceeds

What is instantaneous rate?

The reaction rate at a specific moment, given by the slope of a tangent line on a concentration vs. time graph.

Why do we divide by stoichiometric coefficients in rate expressions?

To ensure all rate expressions equal the same overall reaction rate.

What is a rate law?

An equation relating reaction rate to reactant concentrations and the rate constant.

What is reaction order?

The exponent of a reactant concentration in the rate law.

What is overall reaction order?

The sum of all exponents in the rate law.

Why must rate laws be determined experimentally?

Reaction orders do not generally match stoichiometric coefficients.

How are rate laws determined experimentally?

By comparing initial rates when reactant concentrations are varied

What is the initial rate method?

Measuring the instantaneous rate at the start of the reaction to determine reaction order.

What is an integrated rate law?

An equation showing how reactant concentration changes with time.

Plot that gives a straight line for first‑order reactions

ln[A] vs. time (slope = –k)

Definition of half‑life

Time required for a reactant concentration to decrease to half its initial value.

Half‑life behavior of first‑order reactions

Constant; independent of initial concentration.

Plot that gives a straight line for zero‑order reactions

[A] vs. time (slope = –k)

Plot that gives a straight line for second‑order reactions

1/[A] vs. time (slope = k)

How does temperature affect reaction rate?

Increasing temperature increases reaction rate.

What is activation energy (Ea)?

The minimum energy required for a reaction to occur.

What does the Arrhenius equation describe?

The relationship between rate constant, activation energy, and temperature.

What is the transition state?

A high‑energy, unstable arrangement of atoms formed during a reaction.

What is a reaction mechanism?

A sequence of elementary steps describing how a reaction occurs.

What is an elementary step?

A single molecular event in a reaction mechanism.

What is molecularity?

The number of molecules involved in an elementary step.

What is the rate‑determining step?

The slowest step in a mechanism that controls the overall reaction rate.

What is a catalyst?

A substance that increases reaction rate without being consumed.

How do catalysts speed up reactions?

They provide a pathway with lower activation energy.

Do catalysts affect equilibrium?

No; they speed up forward and reverse reactions equally.

Difference between homogeneous and heterogeneous catalysis

Homogeneous: same phase as reactants; Heterogeneous: different phase.

What is enzymatic catalysis?

Catalysis carried out by biological molecules called enzymes.