Kinetics Flashcards
What do the values of the rate constant k tell us about the speed of the reaction?
Large k = fast reaction
Small k = slow reaction
What are the units of k when the rate of reaction is given by k[A]?
s-1
What are the units of k when the rate of reaction is given by k[A]^2?
mol-1 dm3 s-1
State the typical rate law and annotate.
Rate = k [A]n [B]m
n and m: orders with respect to species A and B
n + m: overall order of the reaction
k: rate constant
For the reaction R –> P, state the zero order rate law and explain.
rate = k[R]0
Reaction rate is independent of the concentration of R.
For the reaction R –> P, state the first order rate law and explain.
rate = k[R]
Rate of reaction is directly proportional to the concentration of R.
For the reaction R –> P, state the second order rate law and explain.
rate = k[R]2
Doubling the concentration of R causes the rate of reaction to quadruple.
How would you plot a second order rate?
If the initial rate is plotted against the initial concentration and a straight line is not obtained, square the initial concentration value and if a straight line is obtained the rate is second order.
Give a summary of complex reactions (4).
- The rate is found to depend on concentration
Rate = k[A]m [B]n [C]p - m n and p are determined experimentally NOT by simply looking at the chemical equation.
- m n and p are usually integers but can also be fractions.
- The orders determined give chemists an insight into the mechanisms by which the reactions occur.
State the integrated rate equation for a zero order reaction.
[A]t = -k’t + [A]o
Derive the integrated rate equation for a zero order reaction.
d[A]/dt = -k’
d[A] = -k’dt
{d[A] = -k’ {dt *dont integrate k because it is a constant
[A]t - [A]o = -k’t
[A]t = -k’t + [A]o
State the integrated rate equation for a first order reaction.
ln[A]t = -k’t + ln[A]o
Derive the integrated rate equation for a first order reaction.
d[A]/dt = -k’[A]
d[A]/[A] = -k’dt
{d[A]/[A] = -k’ {dt
ln[A]t - ln[A]o = -k’t
ln[A]t = -k’t + ln[A]o
State the integrated rate equation for a second order reaction.
1/[A]t = k’t + 1/[A]o
Derive the integrated rate equation for a second order reaction.
d[A]/dt = -k’[A]2
d[A]/[A]2 = -k’dt
{d[A]/[A]2 = -k’ {dt
1/[A]t - 1/[A]o = k’t
1/[A]t = k’t + 1/[A]o
Draw the graphs of rate vs. concentration and concentration vs. time for a zero order reaction.
See notes
Draw the graphs of rate vs. concentration and concentration vs. time for a first order reaction.
See notes
Draw the graphs of rate vs. concentration and concentration vs. time for a second order reaction.
See notes
Why do we linearise equations?
Makes them easier to interpret.
What is meant by the term “half-life”?
t1/2
The time it takes for the concentration of the reactant to decrease by half of its original value.
What is t1/2 for a first order reaction?
t1/2 = ln2/k
For first order reactions, it is independent of the initial concentration, it only depends on k i.e. t1/2 is constant through the course of the reaction.
What is t1/2 for a zero order reaction?
t1/2 = [A]o/2k
For zero order reactions, the half life depends on initial concentration and is inversely proportional to k i.e. t1/2 value gets smaller over time.
What is t1/2 for a second order reaction?
t1/2 = 1/k[A]o
For second order reactions, the half life depends on the initial concentration AND k i.e. t1/2 gets bigger over time as the reaction slows down.
What are the units of k for a zero order reaction?
mol dm-3 s-1
