Reaction Rates Flashcards

Reaction Rates

  • Reaction rates are discussed in the context of the reaction 2 NO2 \rightarrow O2 + 2 NO.

  • The rate of reaction can be expressed in terms of the change in concentration of reactants or products over time.

Convention for Expressing Rates

  • Rates are typically expressed in terms of reactants.

  • Rates are defined to be positive values.

  • Disappearing reactants have a negative sign associated with their rate expression.

  • Appearing products have a positive sign associated with their rate expression.

  • The rate changes with concentration over time.

  • It is important to use instantaneous rates rather than average rates.

Instantaneous Rate

  • Instantaneous rate

  • The instantaneous rate is the initial rate at t=0.

  • Graphically, the instantaneous rate is the slope of the line tangent to the curve at a specific point.

Rate Law

  • Rate Law is a mathematical expression that relates reaction rates to reactant concentrations.

  • Two types of rate laws are:

    • Differential Rate Law: Rate depends on concentration, where k is the rate constant and n is the reaction order.

    • Integral Rate Law: Obtained by integrating the differential rate law

  • All rate laws must be obtained from experimental rate data; they cannot be derived from the stoichiometry of the reaction.

  • The reaction mechanism is reflected in the rate law.

Reaction Orders

  • Consider a general reaction: aA + bB \rightarrow Products

  • The differential rate law is given by: Rate = -\frac{d[A]}{dt} = k[A]^m[B]^n

  • The exponents m and n give the order of the reaction with respect to reactants A and B, respectively.

  • These exponents can be whole or fractional numbers, or even zero.

  • The overall order of the reaction is the sum of the exponents (e.g., 2 + 1 = 3, so it's a 3rd order reaction).

  • The reaction is 2nd order with respect to A and 1st order with respect to B.

Differential Rate Law by Method of Initial Rates

  • Given reaction rate data for:

  • The method involves the following steps:

    • Write the general differential rate law:

    • Take ratios of sets of data to cancel out k and one reactant.

    • Repeat.

    • Solve for k (including units!). Units of 0th order rate law are M/s. Units for 1st order rate law are 1/s. Units for 2nd order rate law are 1/Ms