3d. What is the effect of
temperature on the rate or speed of a chemical reaction?
Doc Brown's
Chemistry KS4 science GCSE/IGCSE/O level Revision Notes - Factors
affecting the Speed-Rates of
Chemical Reactions -
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3.
The Factors affecting the Rate of Chemical Reactions
REACTION RATE and TEMPERATURE
Varying the TEMPERATURE of the reactants - increase
or decrease
3d
The effect of Temperature

Experimental methods for
investigating the effect of temperature on the rate of a chemical
reaction
Parts of the sections of
1. Introduction and 2. collision theory are repeated here, but with
extra experimental methods and theoretical details applied to
experiments and theories linked to the effect of changing the
temperature on the rate of a chemical reaction
-
(ii) The same apparatus can be used to
investigate the how the speed of the decomposition of hydrogen peroxide
varies at different temperatures in the presence of a fixed amount of
catalyst (e.g. manganese(IV) oxide, manganese dioxide).
- hydrogen peroxide ===> water + oxygen
- 2H2O2(aq) ===> 2H2O(l)
+ O2(g)
- You must keep the following variables
constant - the volume of hydrogen peroxide solution, the concentration
of the hydrogen peroxide, the mass of catalyst AND its particle size, and TRY to keep a gentle constant stirring rate as
you are noting down the time and volume of carbon dioxide gas formed.
- Gentle stirring is important, if you
don't, the bottom layers of hydrogen peroxide become depleted in acid
giving a falsely slow rate of reaction.
- You follow the reaction by measuring the
volume of oxygen gas formed.
- You repeat the experiment at different
temperatures using the same volume and concentration of hydrogen
peroxide and mass of the same catalyst to see the effect of temperature
on the rate-speed of the catalysed decomposition of hydrogen peroxide.
- More details of laboratory investigations
('labs') involving 'rates of reaction' i.e. experimental methods for
observing the speed of a reaction and including the effect of temperature are given in
the
INTRODUCTION
-
In both these cases, measuring the initial rate of gas formation (see left
and below diagrams) gives a reasonably accurate measure of how fast the reaction is for
that concentration.
-

- The initial gradient, giving the initial rate of
reaction, is the best method i.e. the
best straight line covering several results at the start of the
reaction by drawing the gradient line using the slope of the
tangent from time = 0, where the graph is nearly linear.
- Examples of graph data for two
experiments where one of the reactants is completely used up - all
reacted.
- The two graph lines represent two
typical sets of results to explain how the rate of reaction data can be
processed.
- Graph A (for a faster reaction) could
represent a higher temperature than in Graph B (a slower reaction).
-

- The
set of graphs above shows you some typical results.
- The rate of reaction order is X > E > Y >
Z, and could represent four increasing temperatures for fixed amounts of
solid and concentration of reactants.
- The greater the temperature,
the steeper the initial gradient, the faster the reaction.
-
For the effect of temperature on the rate of
reaction, under some circumstances graph W could represent the result
of taking twice the mass of solid reactant (e.g. double amount of marble
chips) or twice the concentration (same volume) of a soluble reactant, BUT
it does depend on which reactant is in excess, so take care in this
particular graph interpretation.
- The graph on the right
shows how the initial rate varies with increase in temperature of the
reaction mixture. The reciprocal of the reaction time can be taken as a
measure of the speed of the reaction at that particular temperature.
-
(iii) Investigating how varying the
concentration of either sodium thiosulfate or hydrochloric acid affects the
rate they react together to give a precipitate of sulfur
-
mix =>
ongoing
=> watch stopped
=>
-
You must keep the volumes of reactants
constant, the concentrations of the hydrochloric acid and sodium thiosulfate
constant, and the same person making all
the observations with the same size cross on white paper.
-
You must pre-heat both
solutions to the desired temperature, separately, before mixing them
together.
-
It is important you take the same total
volumes of reactant solutions to give the same depth of liquid you are viewing the
cross through.
- Everything should be mixed quickly and the
clock started, but there is no need to stir the mixture once it is fully
mixed.
- You note the time when the cross first
disappears.
- To vary the temperature of the
reactant solution mixture, you will have to pre-heat the solutions,
mix them, start the clock AND take the temperature. The reaction
mixture may cool a little, so you can re-take the temperature at the
end and use the average, not perfect, but more accurate than either
temperature reading.
- You repeat the experiment at different
temperatures to see their effect on the rate-speed of the acid promoted
decomposition of sodium thiosulfate to form a sulfur precipitate. Typical
results are shown below.
-
temperature (oC) |
20 |
25 |
30 |
35 |
40 |
45 |
time for X to be
obscured (s) |
240 |
220 |
190 |
150 |
100 |
40 |
-
 The graphs on the left
shows how the reaction time and rate varies to obscure the X with
increase in temperature of the hydrochloric acid and sodium thiosulfate
mixture. The reciprocal of the reaction time can be taken as a measure
of the speed of the reaction at that particular temperature.
- More details of laboratory investigations
('labs') involving 'rates of reaction' i.e. experimental methods for
observing the speed of a reaction and including the effect of temperature are given in
the INTRODUCTION
-
See also graphs 4.6, 4.7 and 4.8 for a
numerical-quantitative data interpretation AND the
introduction page
Theoretical interpretation of results of
the effect of changing temperature on the rate of a chemical reaction
For each factor I've presented
several particle diagrams to help you follow the text explaining how the
particle collision theory accounts for your observations of reaction rate
varying with the temperature of the reaction system (some 'work' better than
others!)

A picture of a particles (ions or
molecules) undergoing changes in a chemical reaction
-
HOW DOES TEMPERATURE AFFECT THE SPEED OF A CHEMICAL
REACTION?
-
IF SO, HOW AND WHY?
-
Why does a reaction go faster at a higher
temperature?
- The greater the temperature
of the reactants, the greater the average kinetic energy of the particles.
- Therefore, the more chance
of a successful more energetic 'fruitful' collision between two particles with sufficient
combined kinetic energy to overcome the activation energy barrier, break
bonds and form the products.
- The frequency of collision increases too,
but this is the lesser of the two factors which both contribute to an
increased rate of reaction on raising the temperature.
-
We are talking about an increased
frequency of fruitful collisions, mainly because of the higher
average kinetic energy of the molecules - leading to product formation.
-
Increased frequency means an increase
in 'rate/speed of reaction'.
-
When gases or liquids are heated the particles gain kinetic energy and
on average move faster (see diagrams below).
-
The increased speed increases the chance
(frequency) of collision between reactant molecules and the rate of
reaction increases.
-
BUT this is NOT
the main reason for the increased reaction speed, so be careful in your
theory explanations if investigating the effect of temperature, so read on
after the pictures!
==
inc. T ==>
The product molecules are
not shown, but just imagine how more energetic collisions will occur in
the right-hand diagram!
-
Most molecular
collisions do not result in chemical
change.
-
To further explain the effect of
temperature on the speed of a reaction, we need to consider the reaction
profile and the importance of the activation energy.
-
-
Before any change takes place on collision,
the colliding molecules must have a minimum kinetic energy called the
Activation Energy
shown on the energy level diagrams below (sometimes called
reaction profile/progress diagrams - shown below).
-
Going up and to the top 'hump' represents
bond breaking on reacting particle collision.
-
Going down the other side represents the
new bonds formed in the reaction products. The
red
arrow down represents the
energy
released - exothermic
reaction.
-
It does not matter whether the reaction is an exothermic or an endothermic
in terms of energy change, its the activation energy which is the most important
factor in terms of temperature and its effect on reaction speed.
-
Now 'hotter' molecules have a greater
average kinetic energy, and so at higher temperatures, a greater proportion of them have the required activation energy to
react i.e. their combined kinetic energy on collision is sufficient to
break open bonds and allow the reaction to proceed to product formation.
-
This means that the increased chance
(frequency) of 'fruitful' higher energy
collisions in a given time greatly increases the speed of the reaction, depending on the
fraction of molecules with enough energy to react.
-
For this reason,
generally speaking, and in the absence of catalysts or extra energy input,
a low activation energy reaction is likely to be fast and a high
activation energy reaction much slower, reflecting the trend that the
lower the energy barrier to a reaction, the more molecules are likely to
have sufficient energy to react on collision.
-
See also graphs 4.3, 4.4
and 4.8 for a numerical-quantitative data interpretation for a
numerical-quantitative data interpretation AND the
introduction page
-
In industry you would
try to run the reaction at the highest economic temperature.
-
BUT, the energy bill
should not be economically demanding,
-
the speed of reaction at
higher temperature must be under control and not too fast to be
dangerous,
-
if the reaction is
exothermic and an equilibrium is formed, operating at too high a
temperature might significantly reduce the yield.
APPENDIX Trying to
resolve an apparent confusion for a GCSE
(or A Level)
student!
- With increase in temperature,
there is an increased frequency (or chance) of collision due to the more
'energetic' situation - but this is the minor factor when considering why
rate of a reaction increases with temperature.
- The minimum energy needed for
reaction, the activation energy (to break bonds on collision), stays the same
on increasing temperature.
- However, the average increase in particle kinetic energy
caused by the absorbed thermal energy means that a much greater proportion of the
reactant molecules now has the minimum or activation energy to react.
- So, at a higher temperature, there are
more particles with the higher kinetic energies.
- Therefore there will be more particles
colliding with enough energy to overcome the threshold activation
energy.
- It is
this increased chance of a 'successful' or 'fruitful' higher energy collision
leading to product formation, that is the major factor, and this effect increases
more than the increased frequency of particle collision,
for a similar rise in temperature.
- This is usually only fully
discussed at Advanced A level Chemistry, but it may impress the teacher for GCSE coursework
if you look up the
Maxwell-Boltzmann
distribution of kinetic energies, though its quite difficult to get
over some of these ideas without considering graphs of probability versus
particle KE, but that's up to you!
- There is also the
Arrhenius Equation relating rate
of reaction and temperature - but this involves advanced level
mathematics.
Rates of
reaction notes INDEX
GCSE/IGCSE MULTIPLE CHOICE
QUIZ
on RATES of reaction
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