1.3 Factors affecting reaction rates

Chemical reactions don't happen at the same speed. Temperature, concentration, and catalysts play crucial roles in determining how fast reactions occur. Understanding these factors helps us control and optimize reactions in labs and industries.

Let's dive into how these factors work. Temperature speeds up reactions by increasing molecule energy. Higher concentrations mean more collisions between molecules. Catalysts provide an easier path for reactions to happen. These concepts are key to mastering reaction kinetics.

Factors Affecting Reaction Rates

Factors influencing reaction rates

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• Temperature
  • Increasing temperature raises average kinetic energy of molecules leads to more frequent and energetic collisions (boiling water vs room temperature water)
  • Higher fraction of molecules possess energy greater than or equal to activation energy barrier at elevated temperatures enables more successful collisions
• Concentration
  • Increasing reactant concentrations results in higher number of molecules per unit volume (concentrated vs dilute hydrochloric acid)
  • Greater concentration leads to more frequent collisions between reactant molecules per unit time yields faster reaction rates
• Catalysts
  • Provide alternative reaction pathway characterized by lower activation energy (enzymes, metal surfaces)
  • Reduced activation energy allows larger proportion of molecules to possess sufficient energy for reaction
  • Increase reaction rates without being consumed enables reuse and cost-effectiveness in industrial processes (Haber-Bosch process, catalytic converters)

Temperature effects on reactions

• Collision theory states reactions occur when reactant molecules collide with sufficient energy and proper orientation
• Increasing temperature raises average kinetic energy of molecules results in faster motion and more frequent collisions (cooking food at high heat vs low heat)
• Elevated temperatures increase fraction of molecules with energy equal to or greater than activation energy leads to more successful collisions
• Arrhenius equation quantifies the exponential relationship between temperature and reaction rate constant: $k = Ae^{-E_a/RT}$
  • $k$ = reaction rate constant
  • $A$ = pre-exponential factor related to collision frequency and orientation
  • $E_a$ = activation energy
  • $R$ = gas constant (8.314 J/mol·K)
  • $T$ = absolute temperature (K)

Concentration and reaction rates

• Reaction rates directly proportional to reactant concentrations according to rate law equation: Rate = $k[A]^m[B]^n$
  • $k$ = rate constant
  • $[A]$, $[B]$ = concentrations of reactants A and B
  • $m$, $n$ = reaction orders (determined experimentally)
• Doubling concentration of one reactant typically doubles reaction rate, assuming other factors constant (2 M HCl reacts twice as fast as 1 M HCl with Mg)
• Higher concentrations lead to greater number of reactant molecules per unit volume results in more frequent collisions
• Increased collision frequency per unit time directly translates to faster reaction rates (flammability of concentrated vs dilute gasoline vapor)

Catalysts in reaction processes

• Catalysts increase reaction rates without being consumed allows for regeneration and reuse (Ziegler-Natta catalysts for polymerization)
• Provide alternative reaction pathway with lower activation energy makes it easier for reactants to overcome energy barrier
• Reduced activation energy increases fraction of successful collisions by lowering energy requirement (catalytic hydrogenation of vegetable oils)
• Specific to certain reactions and can be homogeneous (same phase as reactants) or heterogeneous (different phase)
  • Homogeneous catalysts: enzymes in biological systems, acid catalysts in esterification reactions
  • Heterogeneous catalysts: metal surfaces in Haber-Bosch process for ammonia synthesis, platinum in catalytic converters
• Catalysts do not alter overall thermodynamics or equilibrium of reaction, only kinetics (catalyzed vs uncatalyzed decomposition of hydrogen peroxide)