KINETICS NOTES
(Topics 6 & 16)
6.1 Rates of reaction
6.1.1 Define the term rate of reaction
Rate of reaction is concerned with how quickly a reaction reaches a certain point, and can be
defined as:
The decrease in concentration of the reactants per unit of time, or
The increase in concentration of the reactants per unit of time
6.1.2 Describe suitable experimental procedures for measuring rates of reaction.
Change in concentration can be calculated through a variety of methods, such as:
Change in mass
Change in volume
Change in pH for reactions involving acids and bases
Using a data logger to collect data and produce graphs
Measuring the volume produced and dividing by the amount of time taken to obtain rate of
reaction
6.1.3 Analyse data from rate experiments
*Students should be familiar with graphs of changes in concentration, volume and mass against time
The graph below shows the rate of reaction of a substance over time.
To find the rate of reaction at a certain time, you simply have to find the gradient, which is the change in
volume of gas produced divided by the time.
Example:
Which has a faster rate of reaction?
Lets just say the blue line produces 200 g of gas and the red line 150 g. They both do so in 50
seconds.
If we take the gradient, the blue lines rate of reaction is 4 g /sec and the red line 3 g /sec, so the
average rate of reaction will be faster in the red line.
6.2 Collision theory
6.2.1 Describe the kinetic theory in terms of the movement of particles whose average energy is
proportional to temperature in kelvins.
Same temperature = same energy (e.g. at 400K, which atom will have the greatest speed?
Ans. At the same temperature, they all have the same energy)
At same energy, the lighter atom will have the greater speed
6.2.2 Define the term activation energy Ea
Activation energy the minimum energy required by colliding particles in order to produce
successful reactions. It is given the symbol Ea.
The energy of particles is expressed by their speed.
6.2.3 Describe the collision theory
Collision theory: reactions take place as a result of particles (atoms or molecules) colliding and then
undergoing a reaction. However, not all collisions cause reaction, even in a system where the
reaction is spontaneous.
Rate of reaction depends on:
Collision frequency
Number of particles with E Ea
Appropriate collision geometry/orientation
6.2.4 Predict and explain, using the collision theory, the qualitative effects of particle size,
temperature, concentration and pressure on the rate of a reaction.
Condition Effect on rate Explanation
Temperature Increases the rate of a
reaction
Two reasons:
1. There are more particles with sufficient
energy to react (most important) - more
successful collisions
2. There are more collisions
Concentration Increases the rate of
the reaction (usually)
There are more collisions as there are more
particles in closer proximity
Particle size The smaller the
particles the faster the
reaction. (note: the
solute particles in
solutions have the
smallest particle size
possible. and so
solutions react fastest)
Collisions occur at the surface of particles. The
larger the particle size the smaller the surface
area and the fewer collisions can occur.
Catalysts The presence of a
catalyst increases the
rate of a reaction
Catalysts provide an alternative mechanism
with a lower activation energy
Pressure Increasing pressure
increases number of
collisions per unit of
time
Reactant molecules are forced into tighter
space, meaning they are packed more closely
together, and hence increases chances of
collisions occuring
Surface area Increases the rate of
reaction
More surface area provides more possibility of
collisions occurring between reactants due to
exposed space
6.2.5 Sketch and explain qualitatively the Maxwell- Boltzmann energy distribution curve for a
fixed amount of gas at different temperatures and its consequences for changes in reaction rate.
Maxwell-Boltzmann curve shows the no. of reactant particles that have passed the Ea
threshold and the distribution of energy across particles
Increasing temperature of a substance increases average speed (energy) of the particles
Consequently number of particles colliding with sufficient energy to react increases
At higher temperatures there are more successful collisions hence faster reaction rate
Area under curve = total number of molecules, which remains consistent for every curve
and does not change at different temperatures
6.2.6 Describe the effect of a catalyst on a chemical reaction.
Adding a catalyst increases the rate of reaction because catalysts lower the activation
energy or provide an alternate pathway for the reacting particles.
6.2.7 Sketch and explain Maxwell-Boltzmann curves for reactions with and without catalysts
At the original
activation energy, a small area of the curve exceeds Ea threshold hence a small number of
particles have sufficient energy to collide and react
However, with the addition of a catalyst, which lowers activation energy, there is now a
greater area under the curve of particles with sufficient energy to react, provided they
collide in the correct orientation.
Hence, catalyst increases no. of particles with enough energy to collide and react
HIGHER LEVEL
16.1 Rate expression
16.1.1 Distinguish between the terms rate constant, overall order of reaction and order of
reaction with respect to a particular reaction.
This equation is experimentally determined in that values for n, m and k can only be found
through experimentation and not theoretical means
The rate expression shows the relationship between the speed of a reaction and the
concentration of the individual reactants.
Once orders are found they provide information regarding the specific reaction mechanism.
Rate constant (k) is a fixed value in a reaction, which quantifies the speed of a chemical reaction. It
can be affected by external factors such as temperature, pressure, particle size and catalysts.
This value remains constant and can be calculated by rearranging Rate = k[A]x[B]y to :
Units for rate constant:
Order Units
1 s-1
2 dm3mol-1s-1
3 dm6mol-2s-1
Rate = k[A]
m
[B]
n
Order of reaction with respect to a particular reactant is often represented by m, n as powers
over [A] and [B] (the reactants).
EXAMPLE CALCULATIONS:
1) If [A] is doubled, [B] kept constant, and rate stays the same, what is the order of reaction?
[2]x = [A] = amount of times number has changed, in this case, it has doubled so A=2)
[1] = [B]
[2]x[1]0 = 1
[2]x =1
x= 0
Hence, the order of reaction here is zero.
Rate Expression is therefore: k[A]0[B]
= k[B]
2) If [A] is kept constant and [B] is doubled, then the initial rate also doubles
[1]0[2]y = 2
Y = 1
Hence, the order of reaction here is one.
Rate Expression= k[B]1
3) If [A] is doubled, [B] is kept constant, the rate increases 4 times.
[2]x[1]0= 4
X=2
Hence, the order of reaction here is two.
Rate Expression = k[A]2
4) Lastly, if [A] is doubled, [B] is kept constant and the rate increases 8 times.
[2]y[B]2 = 8
y= 3
Hence, the order of reaction here is three.
Rate Expression = k[A]
3
[B]
2
Overall order of reaction is the sum of the individual order components of the reaction expression.
q = m + n
E.g. If rate expression = k[A]3[B]2
m = 3, n = 2
Therefore overall order = 3+2 = 5
16.1.2 Deduce the rate expression for a reaction from experimental data.
Lets try to answer these questions from the experimental data below:
1) What is the rate expression of the reaction?
2) Overall Rate of Reaction
3) Rate constant and its units at 298K.
Note: The data below is contrived and is used solely for exam purposes.
Experiment number Initial concentration of
[A] (g) / moldm-3
Initial Concentration of
[B] (g)/ moldm-3
Initial rate of
formation of
[C]/moldm-3s-1
1 3.010-3 5.010-3 4.010-3
2 3.0103 1.010-2 8.010-3
3 2.010-3 3.010-3 1.010-3
4 8.010-3 3.010-3 1.6 x 10-2
1) Lets closely examine the experimental data. If we look at experiment 1 and 2, if we keep *A+
constant at 3.010-3, and we double the concentration for [B], the rate of formation [C] also doubles.
Lets now map this out mathematically.
[A] = 1 (as the concentration remains constant)
[B] = 2 (as the concentration doubles)
[C] = 2 (as the rate doubles)
[1][2]x = 2
Using some simple maths, we can deduce x= 1, so the reaction for [B] is first order.
If we look at experiment 3 and 4, [B] is kept constant whilst [A] is quadrupled (x4). As a result, we can the
rate of reaction is (1.610-2/1.010-3), which equals 16.
Hence, from experiment 3 and 4
[A] = 4 (as the concentration increases by 4 times)
[B]= 1 (as it is kept constant)
[C] = 16 (rate increases 16 times)
[4]y[1] = 16
Y =2, hence [A] is a 2nd order reaction,
Hence, the rate expression is:
Rate = k [A]
2
[B]
1
, where k is the rate constant.
16.1.3 Solve problems involving the rate expression
Find order of *A+, *B+
Rearrange the equation
k = rate/[A]
m
[B]
n
[C]
p
Example:
3) Rate= k [A]
2
[B]
1
K = Rate/ [A]
2
[B]
1
Now, simply take in any of the experimental values in the table and plug in the values for rate, [A]
2
,
and [B] to arrive at the rate constant.
Ill get the results from Experiment 1.
Since the overall reaction is 3
rd
order, the units we will use are: dm
6
mol
-2
s
-1
K= (4.010
-3
) / (3.010
-3
)
2
x (5.010
-3
)
K= 8.888 x 10
4
K= 8.888 x 10
4
dm
6
mol
-2
s
-1
16.1.4 Sketch, identify and analyse graphical representations for zero-, first- and
second-order reaction
1. Zero order rate stays constant,
regardless of concentration
2. First order rate is proportional to
the concentration
3. Second order the graph is a curve,
as there is a quadratic relationship between
rate and concentration
Half-life the half-life of a reaction is the time it takes for the concentration of a substance to fall to
half of its original value.
16.2 Reaction mechanism
16.2.1 Explain that reactions can occur by more than one step, and that the slowest step
determines the rate of reaction (rate-determining step)
Very few reactions occur in one step = most are multi=step processes in which each step
rarely involves more than two molecules
The reaction mechanism is the actual step by step process by which a reaction occurs
Each step is called an elementary step or elementary process
The molecularity of an elementary step describes how many molecules participate in that
step; 1 = unimolecular, 2 = bimolecular and 3 = termolecular
NB molecularity of 3 is highest known if an intermolecular step is included, mechanism is
unlikely anything higher is impossible
Often, intermediate species are formed, which are a fundamental part of the process, but
do not appear in the final reaction equation; they are formed in one step and used in the
next step
If theoretical and experimental rate expressions do not match, it indicates that the reaction
occurs in more than one step
E.g. 2NO
2(g)
+ F
2(g)
2NO
2
F
(g)
Rate expression should be Rate = k[NO
2
]
2
[F
2
]
However, experimentally determined expression is Rate = k[NO
2
][F
2
]
Rate expressions dont match. Therefore, the reaction must occur in two steps.
16.2.2 Describe the relationship between reaction mechanism, order of reaction and rate-
determining steps
The reaction mechanism is a series of reactions between the particles of a reaction that eventually
lead to the final products. The order of reaction gives information about the particles involved in the
slow step (RDS), which in term determines the rate of the overall reaction because it is the step
requiring the most E
a.
Reaction mechanism
The actual step by step process by which a reaction occurs
Rate determining step
The slowest step in a reaction because it has the highest
activation energy. It determines the rate of the overall reaction.
Molecularity
The number of particles reacting in the rate determining step of a
reaction.
Activated complex
As two particles collide (with sufficient energy to react and in the
correct orientation) they form an intermediate called the
activated complex...not literally a chemical substance, but an
intermediate in which the bonds are in the process of being
broken and formed.
The order of the
reaction
This gives information about the particles involved in the rate
determining step (which is one step in the mechanism). For
example, if two of one type of particle is colliding, the order with
respect to that particle will be 2 (and zero to any others).
16.3 Activation energy
16.3.1 Describe qualitatively the relationship between the rate constant (k) and the temperature
(T)
Arrhenius equation is:
where:
A is a constant related to the number, orientation and frequency of collisions occurring
between the particles in the reaction
k is the rate constant
R is the universal gas constant
T is the absolute temperature
As T increases, k also increases and as T decreases, k also decreases
16.3.2 Determine activation energy (E
a
) values from the Arrhenius equation by a graphical
method
Now, Linear Form is y= mx+c. Arrhenius Equation can also be expressed in linear form, Ill show you
how below:
Here, the 1/T is the x value, -E
a
/R is the gradient, an Ln A is the y- intercept.
If we are to plot this graphically, in y=mx+c form, it would look something like this:
ln A is the y-intercept.
Calculating E
a
is not much harder.
Since the Gradient = Rise/ Run, calculate the gradient of any two points, well call this point (x), and
then:
Since R is already a known value, the universal gas constant.
