Chemical Kinetics flash cards

1. 1.What does chemical kinetics study?

   The rate (speed) of chemical reactions, the factors affecting them (concentration, temperature, catalyst, surface area, etc.), and the mechanism by which reactions occur.

   Hint: How fast + how (pathway).

2. 2.Define the rate of reaction.

   The change in concentration of a reactant or product per unit time. Units: $\text{mol L}^{-1}\text{s}^{-1}$.

   Hint: $\Delta[\,]/\Delta t$

3. 3.Distinguish average rate and instantaneous rate.

   Average rate $=-\dfrac{\Delta[R]}{\Delta t}$ over a finite interval. Instantaneous rate $=-\dfrac{d[R]}{dt}$ at a particular instant (limit as $\Delta t\to 0$).

   Hint: Finite interval vs. a single moment (derivative).

4. 4.For $aA+bB\to cC+dD$, write the rate of reaction in terms of each species.

   $\text{Rate}=-\dfrac{1}{a}\dfrac{d[A]}{dt}=-\dfrac{1}{b}\dfrac{d[B]}{dt}=+\dfrac{1}{c}\dfrac{d[C]}{dt}=+\dfrac{1}{d}\dfrac{d[D]}{dt}$

   Hint: Divide each by its stoichiometric coefficient; minus for reactants.

5. 5.For $N_2+3H_2\to 2NH_3$, relate the rates of consumption/formation.

   $-\dfrac{d[N_2]}{dt}=-\dfrac{1}{3}\dfrac{d[H_2]}{dt}=+\dfrac{1}{2}\dfrac{d[NH_3]}{dt}$

   Hint: Coefficients 1, 3, 2.

6. 6.QUESTION: In $N_2+3H_2\to2NH_3$, if $H_2$ is consumed at $0.06\ \text{mol L}^{-1}\text{s}^{-1}$, at what rate is $NH_3$ formed?

   $\dfrac{d[NH_3]}{dt}=\dfrac{2}{3}\times 0.06=0.04\ \text{mol L}^{-1}\text{s}^{-1}$.

   Hint: Rate of $NH_3=\tfrac{2}{3}\times$ rate of $H_2$.

7. 7.Why does reaction rate generally decrease with time?

   Reactant concentrations fall as the reaction proceeds, and for most reactions rate depends on reactant concentration, so rate drops with time.

   Hint: Fewer reactant molecules left.

8. 8.What is the rate law (rate equation)?

   An experimentally determined expression relating rate to reactant concentrations, e.g. $\text{Rate}=k[A]^x[B]^y$, where $x,y$ are orders.

   Hint: Rate $=k\times$ (concentrations raised to powers).

9. 9.Can the rate law be predicted from the balanced equation?

   No. Orders $x,y$ must be found experimentally; they generally differ from stoichiometric coefficients (except for a single elementary step).

   Hint: Experiment decides, not stoichiometry.

10. 10.Define the rate constant (specific reaction rate) $k$.

    The proportionality constant in the rate law; numerically equal to the rate when all reactant concentrations are unity ($1\ \text{mol L}^{-1}$).

    Hint: Rate at unit concentrations.

11. 11.List key properties of the rate constant $k$.

    (i) Independent of reactant concentration. (ii) Depends on temperature (and catalyst). (iii) Its units depend on the overall order. (iv) Increases with temperature.

    Hint: Depends on T and catalyst, not concentration.

12. 12.Define order of reaction.

    The sum of the powers of the concentration terms in the experimentally determined rate law. For $\text{Rate}=k[A]^x[B]^y$, order $=x+y$.

    Hint: Sum of exponents in rate law.

13. 13.Define molecularity.

    The number of reacting species (atoms, ions, molecules) that collide simultaneously in an elementary reaction step. Always a whole number $\geq 1$.

    Hint: Molecules colliding in one elementary step.

14. 14.Give four differences between order and molecularity.

    Order: experimental, for overall reaction, can be zero/fractional/integer. Molecularity: theoretical, only for elementary steps, always a positive integer, never zero or fractional.

    Hint: Order can be 0/fractional; molecularity cannot.

15. 15.Can order of reaction be zero or fractional? Molecularity?

    Order can be zero, fractional, or negative (experimental). Molecularity is always a positive whole number ($1,2,3$).

    Hint: Only order can be non-integer.

16. 16.Why is molecularity of an overall reaction meaningless?

    Overall reactions occur through several elementary steps; molecularity is defined only for a single elementary step. For the overall reaction, only order is meaningful.

    Hint: Multi-step reaction → no single molecularity.

17. 17.What is the rate-determining step (RDS)?

    The slowest elementary step in a reaction mechanism; it governs the overall rate, so the rate law is derived from it.

    Hint: Bottleneck; slowest step controls rate.

18. 18.Derive units of $k$ for a general order $n$ reaction.

    $k=\dfrac{\text{Rate}}{[A]^n}$, so units $=(\text{mol L}^{-1}\text{s}^{-1})(\text{mol L}^{-1})^{-n}=\text{mol}^{1-n}\,\text{L}^{n-1}\,\text{s}^{-1}$.

    Hint: $\text{conc}^{1-n}\,\text{time}^{-1}$

19. 19.State the units of $k$ for zero, first, and second order reactions.

    Zero: $\text{mol L}^{-1}\text{s}^{-1}$. First: $\text{s}^{-1}$. Second: $\text{L mol}^{-1}\text{s}^{-1}$.

    Hint: Use $\text{mol}^{1-n}\text{L}^{n-1}\text{s}^{-1}$.

20. 20.QUESTION: A rate constant has units $\text{L mol}^{-1}\text{s}^{-1}$. What is the order?

    Second order. Units $\text{mol}^{1-n}\text{L}^{n-1}\text{s}^{-1}$ match $n=2$: $\text{mol}^{-1}\text{L}^{1}\text{s}^{-1}$.

    Hint: Match to $\text{mol}^{1-n}\text{L}^{n-1}\text{s}^{-1}$.

21. 21.QUESTION: The rate constant of a reaction is $2\times10^{-2}\ \text{s}^{-1}$. What is its order?

    First order — the unit $\text{s}^{-1}$ (time$^{-1}$) corresponds to $n=1$.

    Hint: $\text{s}^{-1}$ ⇒ first order.

22. 22.Write the differential and integrated rate law for a zero-order reaction.

    Differential: $-\dfrac{d[A]}{dt}=k$. Integrated: $[A]=[A]_0-kt$.

    Hint: Rate independent of concentration; straight line $[A]$ vs $t$.

23. 23.For a zero-order reaction, what is plotted to get a straight line, and what is the slope?

    $[A]$ vs $t$ gives a straight line with slope $=-k$ and intercept $=[A]_0$.

    Hint: Concentration vs time; slope $=-k$.

24. 24.Give the half-life of a zero-order reaction.

    $t_{1/2}=\dfrac{[A]_0}{2k}$. It is directly proportional to initial concentration.

    Hint: $\propto [A]_0$.

25. 25.Name a real reaction that is (approximately) zero order.

    Decomposition of $NH_3$ on a hot platinum surface, or of $HI$ on gold; also photochemical reactions and enzyme reactions at high substrate concentration.

    Hint: Catalyst surface saturated → rate independent of concentration.

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