Rate equations

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Created by
Imogen Nunoo-Mensah

Cards (8)

  • The rate constant is a constant for a particular reaction at a specific temperature.
    • The overall order of the reaction is the sum of all the individual orders for all reactants.
    • The order of reaction with respect to a particular reactant is the power to which its concentration is raised in the rate equation. 
  • A+B→Products
    So, R=R=k[A]m[B]nk\left[A\right]^m\left[B\right]^n
    R = (Initial) Rate
    k = Rate constant
    m = The order of reaction with respect to reactant A
    n = The order of reaction with respect to reactant B
    (The orders m and n are restricted to the values 0, 1, and 2.)
  • Example: 2H2+2H_2+2NO→2H2O+2NO\rightarrow2H_2O+N2N_2
    R=R=k[NO]2[H2]1k\left[NO\right]^2\left[H_2\right]^1
    Second order with respect to NO means that the concentration of NO must be raised to the power of 2 in the rate equation, first order for H2 means that it will be raised to the power of 1.
    Overall order of the reaction is 2+1=3, meaning it is a third order reaction.
  • Example: NO2+NO_2+CO→NO+CO\rightarrow NO+CO2CO_2
    R=R=k[CO][NO2]2 k\left[CO\right]\left[NO_2\right]^2\
    In exams, it is common to be asked to deduce the order of a substance increase of creating this equation.
    So, by comparing experiments 1 and 2= Initial rate has increased by 16= Showing a second order with respect to NO2NO_2.
    Comparing experiments 1 and 3= Initial rate stays the same= Showing a zero order with respect to COCO.
    Therefore, the overall order of the reaction is 2+0=2, meaning it is a second order reaction.
  • Units of rate:
    Rate=Rate=ConcentrationTime\frac{Concentration}{Time} or Rate=Rate=mol dm−3s\frac{mol\ dm^{-3}}{s} so the units for rate are mol dm−3s−1mol\ dm^{-3}s^{-1}
  • General units of rate constant:
    Zero order: Units of rate = mol dm−3s−1mol\ dm^{-3}s^{-1}
    First order: Rate = k[A] = units of rateunits of concentration\frac{units\ of\ rate}{units\ of\ concentration}=mol dm−3s−1mol dm−3\frac{mol\ dm^{-3}s^{-1}}{mol\ dm^{-3}} = Cancel out repeats = s−1s^{-1}
    Second order: Rate = k[A]2k\left[A\right]^2=units of rateunits of concentration2\frac{units\ of\ rate}{units\ of\ concentration^2} = mol dm−3s−1mol dm−3 mol dm−3\frac{mol\ dm^{-3}s^{-1}}{mol\ dm^{-3\ }mol\ dm^{-3}}= mol dm−3s−1mol\ dm^{-3}s^{-1}