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11.2 Reaction Rate and Concentration

11.2 Reaction Rate and Concentration. YOU ARE EXPECTED TO BE ABLE TO: Define the initial rate of a reaction as the instantaneous rate at the start of the reaction.

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11.2 Reaction Rate and Concentration

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  1. 11.2 Reaction Rate and Concentration YOU ARE EXPECTED TO BE ABLE TO: • Define the initial rate of a reaction as the instantaneous rate at the start of the reaction. • Write a rate law expression for a given reaction given experimental results that show the variation of initial rate with concentration. • Define the concept of order of reaction and determine the order of a given reaction from initial rate vs concentration data Chemistry 1011 Slot 5

  2. Initial Rate of Reaction • The initial rate of reaction is the instantaneous rate measured at the start of the reaction • The initial rate can be used as a benchmark • Measurements of the initial rate for a reaction at different starting concentrations can be used to determine the effect of concentration on rate Chemistry 1011 Slot 5

  3. The Rate Expression or Rate Law • In general: • Reactions occur as a result of collisions • The higher the concentration of molecules, the greater the rate • In some reactions, the rate is directly proportional to the concentration • In other cases, the rate may increase much more quickly than the concentration • The relationship between rate and concentration is known at the rate law or rate expression Chemistry 1011 Slot 5

  4. Determining the Rate Law Expression • The rate law MUST be determined from experiment – it is not related to the stoichiometry of the equation for the reaction • Examine the concentration vs time graph for N2O5 • Determine instantaneous rates at different times and concentrations of N2O5 • Plot instantaneous rate vs [N2O5 ] • This is a straight line, (equation y = mx + b) • Rate = k[N2O5] ; this is the rate law expression; k is the rate constant Chemistry 1011 Slot 5

  5. Using Initial Rates to determine the rate Law • Measurements of the initial rate of reaction at different initial concentrations can be used to determine the rate law expression • If a graph of initial concentration vs initial rate is a straight line, then rate is directly proportional to concentration: rate = k [reactant] • If a graph is a curve, then rate = k[reactant]m Chemistry 1011 Slot 5

  6. Order of Reaction • The rate law expression is of the form: Rate = k[A]m • The power to which the concentration is raised is called the order of reaction • If m = 0, then “zero order” • If m = 1, then “first order” • If m = 2, then “second order” Chemistry 1011 Slot 5

  7. Calculating the Order of Reaction when there is One Reactant • If m is not equal to 1, the order of the reaction can be determined by comparing two rates at two different concentrations: • Rate1 = k[A]1m • Rate2 = k[A]2m Rate1 = k[A]1m = [A]1 m Rate2 k[A]2m [A]2 Chemistry 1011 Slot 5

  8. Example 11.1 • CH3CHO(g) CH4(g) + CO(g) [ ] 0.10M 0.20M 0.30M 0.40M Rate (mol/L.s) 0.085 0.34 0.76 1.4 • Choose the first two concentrations Rate2 = 0.34= 4.0 Rate1 0.085 [CH3CHO]2 = 0.20 = 2.0 [CH3CHO]1 0.10 4.0 = (2.0)m ; m = 2 Rate = k[CH3CHO]2The reaction is second order Chemistry 1011 Slot 5

  9. Calculate the Rate Constant • Rate = k[CH3CHO]2 • Choose one set of data 0.085mol/L.s = k x (0.10 mol/L)2 k = 0.085mol/L.s = 8.5 L/mol.s (0.10 mol/L)2 Chemistry 1011 Slot 5

  10. Order of Reaction with More Than One Reactant • Most reactions involve >1 reactant aA + bB  products • General rate law expression is: rate = k[A]m x [B]n • m is “order with respect to A • n is order with respect to B • Overall order is m + n Chemistry 1011 Slot 5

  11. Determining the Order • Hold concentration of one reactant constant and vary other - measure initial rate • Repeat, holding concentration of other reactant constant • See Example 11.2, page 308 Chemistry 1011 Slot 5

  12. Determining the Order of Reaction Graphically • As an alternative to calculating the order of reaction by comparing two pieces of data, a graphical process can be used • Plot rate vs concentration • If the result is a straight line, there is a first order reaction, slope = k • If the result is a curve, it is necessary to construct a graph of (natural) log rate vs (natural) log concentration to determine m Chemistry 1011 Slot 5

  13. Why Use a Logarithm? • If rate = k[A]m • Then ln rate = ln k + m ln [a] • This will be a straight line with slope m Chemistry 1011 Slot 5

  14. Zero Order Reactions • In rare cases, the rate of reaction is independent of the concentration of reactant(s) • In these cases • Rate = k[A]0 • A plot of rate vs [A] would be a horizontal straight line Chemistry 1011 Slot 5

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