5.2 Factors Affecting Reaction Rates

Chemical reactions are influenced by temperature and concentration. The Arrhenius equation shows how temperature affects reaction rates, while collision theory explains concentration's impact. Higher temperatures and concentrations generally lead to faster reactions by increasing molecular energy and collision frequency.

Physical factors like surface area also play a role in reaction rates. Catalysts, including enzymes in biological systems, speed up reactions without being consumed. They provide alternative pathways with lower activation energies, enabling efficient and selective reactions in various contexts.

Temperature and Concentration Effects

Temperature and Arrhenius equation

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• Arrhenius equation $k = A e^{-E_a/RT}$ relates rate constant to temperature
• Higher temperature increases rate constant exponentially
• Activation energy $E_a$ represents energy barrier reactants must overcome
• Lower $E_a$ leads to faster reactions (enzyme-catalyzed reactions)
• Increased temperature boosts molecular kinetic energy resulting in more frequent and energetic collisions

Concentration and collision theory

• Collision theory states reactions occur when molecules collide with sufficient energy and proper orientation
• Higher concentration increases collision frequency accelerating reaction rate
• Rate laws express reaction rate as function of concentrations Rate = $k[A]^m[B]^n$
• Overall reaction order sum of individual reactant orders determines how rate changes with concentration
• First-order reactions rate directly proportional to concentration (radioactive decay)

Physical Factors and Catalysts

Surface area in reaction rates

• Increased surface area leads to faster reactions by exposing more reactive sites
• Smaller particles have higher surface area-to-volume ratio enhancing reaction rates (nanoparticles in catalytic converters)
• Gases generally react faster than liquids which react faster than solids due to molecular mobility
• Reactions often occur at interfaces between phases increasing interfacial area can enhance rates (oil-water emulsions)

Catalysts and biological systems

• Catalysts increase reaction rate without being consumed providing alternative pathway with lower activation energy
• Homogeneous catalysts same phase as reactants (acid catalysis)
• Heterogeneous catalysts different phase from reactants (platinum in catalytic converters)
• Enzymes biological catalysts highly specific and efficient
• Enzyme catalysis enables reactions at physiological temperatures and pH regulating metabolic pathways
• Catalyst selectivity favors specific products or reaction pathways controlling biological processes (DNA replication)