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REACTION RATE LAWS

REACTION RATE LAWS. Goal of kinetics experiment is to measure concentration of a species at particular time during a rxn so a rate law can be determined. stated: rates are obtained from concen. vs fct of time.

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REACTION RATE LAWS

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  1. REACTION RATE LAWS Goal of kinetics experiment is to measure concentration of a species at particular time during a rxn so a rate law can be determined stated: rates are obtained from concen. vs fct of time …. is an eqn which links the rxn rate w/ concentrations or P of react & pdts Order of rxn cannot be deduced from chemical eqn. of rxn Rate law expressions - calculate rate of rxn from rate constant & reactant concen - convert into eqn to determine concen of reactants @ any time

  2. i.e. rxn: A + 2B = X rate of rxn: r = k[A]m[B]n reaction rate rate constant m & n: - independent of concen & T - not stoich. coeff - obtained from rate exp. exponent m: - referred as the order of rxn w/ respect to reactant A - not necessarily an integer overall rate: - sum of orders for reactants - in our i.e.-----> m + n

  3. ZERO ORDER Has a rate which is independent of concentration of reactant(s), therefore, increasing concen. of rxning species not speed up rate Zero-order rxns found when material that is required for rxn to proceed, is saturated by reactants Rate is: Rate is a CONSTANT If zero-order occurs, 1) closed system 2) no net build of intermediates 3) no competing rxns shown by solving Mass balance for the system

  4. integration gives eqn called integrated zero-order rate law [A] = -kt + [A]o concentr of chemical @ particular time Initial concentr calculate k from plot of graph; straight line plot of [A] vs time,t; slope = -k Determine units: half-life describes time needed for half of reactant to be depleted

  5. FIRST ORDER Depends on concentration of only 1 reactant, if other reactants present but each will be zero-order 1st order rate constant, units of 1/time eqn for first-order reaction A -----> pdts rate is: know: integration gives eqn called integrated first-order rate law ln[A] = -kt + ln[A]o eqn line: y = m x + b

  6. [A] ln[A] time, t time, t calculate k from plot of graph; plot of ln[A] vs time,t; gives straight line slope = -k Determine units: half-life describes time needed for half of reactant to be depleted

  7. SECOND ORDER second-order rate law A. A ------ > pdts depends on concentration of 2nd-order reactant rate is: integrated in the form: [A]O @t = 0 & [A] @ t: B. or, A + B = pdts two 1st-order reactants: Another way to represents rate laws, take ln of both sides: ln r = ln k + 2 ln[A]

  8. Plot 1/[A] vs time,t; slope = 2nd-order rate constant Determine units: half-life for 2nd order dependent on one 2nd order reactant:

  9. Summary for reaction orders 0, 1, 2, & n Zero-Order First-Order Second-Order nth-Order Rate Law Integrated Rate Law Units of Rate Constant (k) Linear Plot to determine k Half-life [A] = [A]Oe-kt ln[A] = ln[A]O - kt [A] = [A]O - kt ln[A] vs t [A] vs t

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