NVCC Chem 112 - Chemical Kinetics

Fast reactions

Occur in 10⁻¹² to 10⁻⁶ seconds.

Slow reactions

Cement curing takes days.

Extremely slow reactions

Graphite turning to diamond takes Millions of years

Reaction Rate

Change in concentration of product or reactant over time (M/s).

Rate Law(General)

General Form: Rate = k[A]^x[B]^y.

What is the reaction rate equation for A -> B?

Rate = - ∆[A]/∆t or ∆[B]/∆t

What are some different ways to measure reaction rate?

Spectroscopic (Change in color), Change in electrical conductance (Ion formation), Pressure measurements (Gas production)

How is Reaction Order determined?

Experimentally, not by coefficients.

Thermodynamics

Determines if a reaction occurs.

Kinetics

Determines how fast a reaction occurs.

First-order reaction

Graph: ln[A]/time → straight line.

Second-order reaction

Graph: 1/[A] vs. time → straight line.

Zero-order reaction

Graph: [A] vs. time → straight line.

Half-Life (First-order)

Constant, independent of initial concentration.

Half-Life (Second-order)

Depends on initial concentration.

Collision Theory

Reactions need sufficient energy and orientation.

Activation Energy (E_a)

Minimum energy to initiate a reaction.

Arrhenius Equation

k = A e^(-E_a / RT).

Temperature Effect on Rate

Higher temperature increases reaction rate.

Elementary steps

Individual steps leading to product formation.

Intermediates

Species formed and consumed in reactions.

Rate-determining step

Slowest step controls overall reaction rate.

Catalysts

Lower activation energy without being consumed.

Heterogeneous Catalysis

Catalyst in a different phase than reactants.

Homogeneous Catalysis

Catalyst in the same phase as reactants.

Enzyme Catalysis

Biological catalysts that speed up reactions.

Lock-and-key model

Substrate fits enzyme exactly.

Induced fit model

Enzyme changes shape to fit substrate.