Chemical Kinetics

• Chemistry is concerned with change, where substances with defined properties are converted into other substances with different properties through chemical reactions
• Chemists aim to determine:

  • The feasibility of a chemical reaction, predicted by thermodynamics (reaction with ΔG < 0 at constant temperature and pressure is feasible)
  • The extent to which a reaction will proceed, determined by chemical equilibrium
  • The speed of a reaction, i.e., the time taken to reach equilibrium
• Chemical kinetics is the branch of chemistry that deals with the study of reaction rates and their mechanisms
• Thermodynamics focuses on the feasibility of a reaction, while chemical kinetics provides information about the rate of a reaction
• Chemical kinetics helps in understanding how chemical reactions occur
• In chemical kinetics, you will learn to:

  • Define the average and instantaneous rate of a reaction
  • Express the rate of a reaction in terms of changes in concentration of reactants or products with time
  • Differentiate between elementary and complex reactions
  • Define rate constant
  • Discuss the dependence of reaction rates on concentration, temperature, and catalyst
  • Derive integrated rate equations for zero and first-order reactions
  • Determine rate constants for zeroth and first-order reactions
  • Describe collision theory
• The speed of a reaction or the rate of a reaction is defined as the change in concentration of a reactant or product in unit time
• The average rate of a reaction depends on the change in concentration of reactants or products and the time taken for that change to occur
• The units of rate of a reaction are concentration time^(-1)
• Instantaneous rate of a reaction is obtained when considering the average rate at the smallest time interval, approaching zero
• For a reaction where stoichiometric coefficients of reactants and products are the same, the rate of the reaction is given by the rate of disappearance of any reactant being the same as the rate of appearance of the products
• In reactions where stoichiometric coefficients are not equal to one, the rate of disappearance of any reactant or the rate of appearance of products is divided by their respective stoichiometric coefficients
• The rate of a gaseous reaction at constant temperature can be expressed as the rate of change in partial pressure of the reactant or the product
• The rate of a chemical reaction may depend on factors like the concentration of reactants, temperature, and catalyst
• The rate law or rate expression represents the rate of reaction in terms of the concentration of the reactants
• The rate of a reaction generally decreases as the concentration of reactants decreases, and increases when reactant concentrations increase
• The rate expression for a reaction with stoichiometric coefficients aA + bB → cC + dD is Rate ∝ [A]^x [B]^y, where x and y may or may not be equal to the stoichiometric coefficients of the reactants
• The rate law expression is the expression where the reaction rate is given in terms of the molar concentration of reactants with each term raised to some power, which may or may not be the same as the stoichiometric coefficient
• The order of a reaction is determined by the sum of powers of the concentration of the reactants in the rate law expression
• The order of a reaction can be 0, 1, 2, 3, or even a fraction
• For example, if Rate = k [A]^1/2 [B]^3/2, the order would be 1/2 + 3/2 = 2, making it a second-order reaction
• The sum of powers of the concentration of the reactants in the rate law expression is called the order of that chemical reaction
• A zero order reaction means that the rate of reaction is independent of the concentration of reactants
• To calculate the overall order of a reaction with a rate expression like Rate = k [A]^1/2 [B]^3/2, you add the exponents of the reactants to find the order
• Molecularity of a reaction refers to the number of reacting species that must collide simultaneously in an elementary reaction to bring about a chemical reaction
• Unimolecular reactions involve one reacting species, bimolecular reactions involve two species, and trimolecular reactions involve three species
• Reactions with a molecularity of three are very rare and slow to proceed
• Complex reactions involving more than three molecules in the stoichiometric equation must take place in more than one step
• The rate of a complex reaction is determined by the slowest step, known as the rate-determining step
• The order of a reaction is an experimental quantity and can be zero or even a fraction, while molecularity cannot be zero or a non-integer
• Order is applicable to both elementary and complex reactions, while molecularity is only applicable to elementary reactions
• For a reaction with the rate law r = k [A]^1/2 [B]^2, the order of the reaction would be 3/2
• In zero order reactions, the rate is proportional to zero power of the concentration of reactants
• The integrated rate equation for a zero order reaction is [R] = -kt + [R]0
• Zero order reactions are relatively uncommon but they occur under certain conditions
• Zero order reactions are relatively uncommon but occur under special conditions
• Examples of zero order reactions include some enzyme-catalyzed reactions and reactions on metal surfaces
• The decomposition of gaseous ammonia on a hot platinum surface is a zero order reaction at high pressure
• In zero order reactions, the rate of the reaction is independent of the concentration of the reactant
• First order reactions have a rate proportional to the first power of the concentration of the reactant