Summary Chemical kinetics is the of reaction rates, the changes in the concentrations of reactants and products with time. 14.1 Factors That Affect Reaction Rates Learning Objective To understand the factors that affect reaction rates. Although a balanced chemical equation for a reaction describes the quantitative relationships between the amounts of reactants present and the amounts of products that can be formed, it gives us no information about whether or how fast a given reaction will occur. This information is obtained by studying the chemical kinetics of a reaction, which depend on various factors: reactant concentrations, temperature, physical states and surface areas of reactants, and solvent and catalyst properties if either are present. By studying the kinetics of a reaction, chemists gain insights into how to control reaction conditions to achieve a desired outcome. Concentration Effects Two substances cannot possibly react with each other unless their constituent particles (molecules, atoms, or ions) come into contact. If there is no contact, the reaction rate will be zero. Conversely, the more reactant particles that collide per unit time, the more often a reaction between them can occur. Consequently, the reaction rate usually increases as the concentration of the reactants increases. One example of this effect is the reaction of sucrose (table sugar) with sulfuric acid. Temperature Effects You learned that increasing the temperature of a system increases the average kinetic energy of its constituent particles. As the average kinetic energy increases, the particles move faster, so they collide more frequently per unit time and possess greater energy when they collide. Both of these
carbon-14 5.70 × 103 yr dating of artifacts sodium-24 14.951 h cardiovascular system tracer 14.26 days biochemical tracer 1.248 × 109 yr dating of rocks iron-59 44.495 days red blood cell lifetime tracer cobalt-60 5.2712 yr radiation therapy for cancer 6.006 h biomedical imaging iodine-131 8.0207 days thyroid studies tracer radium-226 1.600 × 103 yr radiation therapy for cancer 4.468 × 109 yr dating of rocks and Earth’s crust 432.2 yr smoke detectors *The m denotes metastable, where an excited state nucleus decays to the ground state of the same isotope.
Haber process Fe, K2O, Al2O3 N2 + 3H2 → 2NH3 NH3 Ostwald process Pt and Rh 4NH3 + 5O2 → 4NO + 6H2O HNO3 water–gas shift reaction Fe, Cr2O3, or Cu CO + H2O → CO2 + H2 H2 for NH3, CH3OH, and other fuels steam reforming Ni CH4 + H2O → CO + 3H2 H2 methanol synthesis ZnO and Cr2O3 CO + 2H2 → CH3OH CH3OH Sohio process bismuth CH 2 =CHCH 3 +NH 3 +3 2 O 2 → CH 2 2 O CH 2 =CHCN catalytic Ni, Pd, or Pt RCH=CHR′ + H2 → RCH2—CH2R′ partially hydrogenated oils for margarine, and so Table: Some Commercially Important Reactions that Employ Heterogeneous Catalysts Homogeneous Catalysis In homogeneous catalysisA catalytic reaction in the catalyst is uniformly dispersed throughout the reactant mixture to form a solution., the catalyst is in the same phase as the reactant(s). The number of collisions between reactants and catalyst is at a maximum because the catalyst is uniformly dispersed throughout the reaction mixture. Many homogeneous catalysts in industry are transition metal compounds (Table below), but recovering these expensive catalysts from solution has been a major challenge. As an added barrier to their widespread commercial use, many