Chapter 13 - Chemical Kinetics

13.1 - The Rate of a Reaction

• Because the term "kinetic" implies movement or change, we define kinetic energy as the energy available as a result of an object's motion.
  • Chemical kinetics is the branch of chemistry that studies the rates at which chemical reactions take place.
  • The rate of a reaction, also known as the reaction rate, is defined as the change in the concentration of a reactant or a product over time (M/s).
• where k is the rate constant, which is a proportionality constant between the reaction rate and the reactant concentration.
  • rate = −Δ[A] Δt or rate = Δ[B] Δt
• In aqueous solutions, molecular bromine reacts with formic acid (HCOOH) as follows: Br2(aq) + HCOOH(aq) ⟶ 2Br−(aq) + 2H+ (aq) + CO2(g)

13.2 - The Rate Law

• The rate of a reaction is proportional to the concentration of reactants, and the rate constant is the proportionality constant k.

  • The rate law expresses the link between a reaction's rate and the rate constant, as well as the concentrations of the reactants raised to certain powers.
  • aA + bB ⟶ cC + dD
  • rate = k[A]x [B]y

• The relationships between the concentrations of reactants A and B and the reaction rate are specified by the exponents x and y.

  • When they're added together, we get the overall reaction order
  • which is defined as the sum of the powers to which all of the reactant concentrations in the rate law are elevated.

  

13.3 - The Relation Between Reactant Concentration and Time

• A first-order reaction is one in which the rate of the reaction is proportional to the concentration of the reactant raised to the first power.

• The concentration of the reactant(s) lowers as the reaction progresses.

  • The half-life, t1, is another way to assess the rate of a reaction by linking concentration to time.
  • The half-life, t1 2, is the time it takes for a reactant's concentration to drop to half of its initial concentration.

• A second-order reaction is one in which the rate of the reaction is determined by the concentration of one reactant raised to the second power or the concentrations of two separate reactants raised to the first power.

  

13.4 - Activation Energy and Temperature Dependence of Rate Constants

• We believe that for colliding molecules to respond, their total kinetic energy must be equal to or greater than the activation energy (Ea),
  • The activation energy (Ea) is the smallest amount of energy needed to start a chemical reaction.
  • When molecules collide, an activated complex is generated, which is a transitory species formed by the reactant molecules as a result of the collision before the product is formed.

13.5 - Reaction Mechanisms

• An overall balanced chemical equation does not provide much information about how a reaction occurs.
• In many circumstances, it is simply the sum of multiple primary stages, or elementary reactions, a succession of small reactions that describe the molecular progress of the entire reaction.
  • The word reaction mechanism refers to the series of basic stages that lead to the production of a product.
• Intermediate species, such as N2O2, are so-called because they occur in the reaction mechanism but not in the overall balanced equation.
  • The number of molecules reacting in an elementary step determines the reaction's molecularity.
  • A bimolecular reaction is a basic step in which two molecules interact.
  • A unimolecular reaction is a simple reaction in which only one reacting molecule is involved.
  • Because the simultaneous collision of three molecules is a significantly less common event than a bimolecular collision, very few termolecular events, or processes involving the participation of three molecules in one elementary step, are known.
• The rate-determining step, which is the slowest in the sequence of events leading to product production, determines the overall rate law.

13.6 - Catalysis

• A catalyst is a chemical that lowers the activation energy of a process, increasing the pace of the reaction.
• Enzyme catalysis is the most spectacular and important of all the complicated mechanisms that have evolved in living systems.
  • Enzymes are enzymes that act as biological catalysts.