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5.2Kinetics

Introduction to Rate Law — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 9 parts Lesson 5 of 14 1 quiz

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5.2 Introduction to Rate Law — AP CHEMISTRY notes: Write rate laws, state reaction orders and the rate constant, and determine orders from initial-rate data.Rate law — an equation relating the rate to reactant concentrations: rate = k[A]m[B]nWhat the symbols mean: rate (mol dm−3 …

  • Rate constant k
  • Order with respect to a reactant
  • Overall order
  • Units of k
  • First-order case
  • Concentration doubled and rate doubled

Section 1 of 9The rate law

Rate law
  • Rate law an equation relating the rate to reactant concentrations: rate = k[A]m[B]n
    What the symbols mean: rate (mol dm−3 s−1) · k = rate constant · [ ] = concentration (mol dm−3) · each power is the order with respect to that reactant
    • (units depend on the overall order)
  • k is the rate constant and m and n are the orders with respect to A and B.
  • Rate laws are found only by experiment, because they depend on the mechanism.
  • Intermediates do not appear in the experimental rate law.
Rate constant k
  • Rate constant k the proportionality constant linking rate to concentration
  • Its value depends on temperature and on the reaction, not on concentration.

Section 2 of 9Reaction order

OrderEffect of doubling [A]Appears in rate law?Units of k (per second)
ZeroNo change in rateNoM s-1
FirstRate doublesYes, [A]s-1
SecondRate increases fourfoldYes, [A]2M-1 s-1

Section 3 of 9Overall order and units of k

Overall order and units of k
Order with respect to a reactant
  • Order with respect to a reactant the power to which its concentration is raised in the rate law
Overall order
  • Overall order the sum of the orders of all reactants in the rate law
Units of k
  • Units of k found by rearranging rate = k[A]m
  • Units of k = units of rate ÷ (units of concentration)overall order.
  • The units of k therefore reveal the overall order.

Section 4 of 9Rate against concentration data

  • First-order case
    • a plot of rate against [D] is a straight line through the origin
    • Rate = k[D], so doubling [D] doubles the rate.
    • The gradient of the line equals k.

Section 5 of 9Determining orders from initial-rate data

1
Choose two experiments in which only one reactant concentration changes.
2
State how that concentration changed.
3
State how the initial rate changed.
4
Use (change in concentration)order = change in rate to find the order.
5
Repeat for each reactant, then write the rate law and calculate k from any experiment.

Section 6 of 9Reading the order from the exponent

Reading the order from the exponent
Concentration doubled and rate doubled
  • Concentration doubled and rate doubled first order, since 21 = 2
Concentration doubled and rate quadrupled
  • Concentration doubled and rate quadrupled second order, since 22 = 4
Concentration doubled and no rate change
  • Concentration doubled and no rate change zero order, so the reactant is left out of the rate law

Section 7 of 9More from the specification

1Empirical rate law and reaction order
  • Empirical rate law and reaction order measuring initial reaction rates across varied concentrations provides an experimental means to formulate rate laws.
2Hydrogen
  • A gas that gives a squeaky pop with a lighted splint.
3Activation energy
  • The minimum energy colliding particles need to react.
4Successful collision
  • A collision with the correct orientation and enough energy to react.
5Catalyst
  • A substance that speeds up a reaction and is not used up.
6Half-life
  • The time for the concentration of a reactant to halve.
7Product
  • A substance formed in a reaction.
8Decomposition
  • A reaction where one compound breaks down into simpler substances.

Section 8 of 9Key terms

TermMeaning
Rate lawEquation relating reaction rate to reactant concentrations.
Rate constant (k)Proportionality constant in the rate law, constant at a given temperature.
Reaction orderExponent of a reactant concentration in the rate law.
Overall orderSum of all individual orders in the rate law.
Initial rateRate measured at the start of a reaction before concentrations change appreciably.

Section 9 of 9Exam technique

1
Writing the rate law from the balanced overall equation — orders must be found from experiment, not from coefficients.
2
Including zero-order reactants in the rate law — a zero-order reactant has no effect on rate, so it is omitted.
3
Thinking k changes with concentration — k depends on temperature (and catalyst) only.
4
Changing two concentrations at once when finding an order — only one concentration may change between the two experiments compared.

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