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
What are the units of k for a first order reaction?
s-1
What are the units of k for second order reaction?
mol-1 dm3 s-1
State the 3 methods for determining the rate law.
- Isolation method.
- Tangents to the curve.
- The log-log plot.
Outline the isolation method (4).
- All the reactants except one present in such a large excess they can be assumed constant.
- Determine the order of the remaining reactant not in excess.
- Switch so the other reactant is excess and repeat for all reactants.
- Determine the order of the other reactants.
Outline the tangents to a curve method (4).
- Plot the rate vs [A].
- If the plot is straight then the reaction is first order.
- If the plot is NOT straight then plot rate vs [A]2.
- If this plot is straight then the reaction is second order.
Outline the log-log plot method (4).
- log (rate) = log(k’) + log[A]m
- log (rate) = m log[A] + log(k’)
- Compare to the equation of a straight line y = mx + c.
- The gradient of the line = m, the intercept = k’
What does a reaction mechanism describe and what must it include? (2).
- The sequence of elementary steps is the reaction mechanism which describes the changes we believe to take place as reactants are transformed into products.
- It must account for the overall stoichiometry, rate law and other known facts about the reaction.
What is meant by the term “molecularity”?
The molecularity of an elementary reaction is the number of reactants molecules or atoms taking part.
What is a unimolecular reaction?
Only ONE reactant molecule is taking part.
What is a bimolecular reaction?
TWO reactant molecules are taking part.
What is a termolecular reaction and how common are they?
An elementary step that involves the simultaneous collision of THREE reactant molecules.
Very low probability, only tends to occur at very high pressure.
What is a consecutive reaction?
One in which the product from the first step becomes the reactant for the second step.
What is the steady-state approximation?
The concentrations of all intermediates remain constant and small throughout the reaction.
What is an intermediate?
Any species that does not appear in the overall reaction but it does appear in the mechanism.
What is the rate-determining step?
The slowest elementary step in a multi-step reaction that governs the rate of the overall reaction.
State the term used to describe temperature dependence and annotate.
k = A e^-Ea/RT
A = pre-exponential factor, measure of the rate at which collisions occur, independent of T Ea = activation energy kJ mol-1
State the integrated temperature dependence equation.
ln k = ln A - Ea/RT
State the temperature dependence equation we would apply to two temperatures.
ln k2/k1 = Ea/R(1/T1 - 1/T2)
Define the term catalyst.
A substance that increases the rate of the reaction without being consumed in the reaction.
How do catalysts work?
- Catalysts cause the reactants to make products via a different mechanism in which the slowest step is faster than the original, speeding up the overall reaction rate.
- The new step has a lower activation energy therefore a greater proportion of reactant molecules have energy equal to or greater than the new lower energy barrier.
What would you plot to get a straight line temperature dependence graph?
ln k vs. 1/T
What is an elementary reaction?
In which one or more of the reactants react directly to form products in a single step and with a single transition state.
What is a complex reaction?
In which the mechanism comprises more than one elementary step.
