INTRODUCTION TO REACTION RATES
1. What do we mean by
the speed or rate of a chemical reaction? how do we measure it?
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GCSE/IGCSE Multiple choice
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Advanced A Level KINETICS
affecting the Speed-Rates of
Why is it important to
know how fast reactions occur?
How can we measure the
speed or rate of a chemical reaction?
INTRODUCTION to methods of measuring how fast a
reaction is going!
1a. Introduction to the INTRODUCTION !
Why do we want to know about the speeds (rate) of Chemical Reactions?
Chemical reactions occur at vastly
different rates over periods of a fraction of a second to days or years
Although the reactivity of chemical
reactants is a significant factor in how fast chemical reactions proceed,
there are many other variables that can be controlled in order to speed them
up or slow them down.
Chemical reactions may also be
reversible and therefore the effect of different variables needs to be
understood and controlled in order to maximise the yield of desired product
e.g. very important for the efficient economics of a chemical process in
Understanding energy changes that
accompany chemical reactions is another important factor of this process.
In industry, chemists and chemical
engineers determine the effect of different variables on reaction rate to
maximise the yield of product.
Sometimes compromises have to be made in
order to optimise the chemical processes in the chemical industry to ensure
that enough product is produced within a sufficient time, and in an energy
efficient way too.
What do we mean by 'Rate'?
WHAT DO WE MEAN BY SPEED OR RATE IN THE CONTEXT
OF A CHEMICAL REACTION?
IS IT TO FAST OR TO SLOW TO MEASURE THE SPEED?
WHAT SORT OF WAYS CAN WE MEASURE THE SPEED OF A
The phrase ‘rate of reaction’ means ‘how fast
or how slow is the reaction’
or 'the speed of the reaction'. It can be measured as the 'rate of formation of product' (e.g. collecting
a gaseous product in a syringe) or the 'rate
of removal of reactant'. The speeds of reactions vary greatly, with some
pretty extreme situations of both slow and fast!
chemical weathering of rocks is
an extremely very slow reaction and like fossil formation by mineralisation
processes, it might take hundreds of thousands or millions of years!
Rusting is a
you hardly see any change looking at it, but after a few weeks or months
you notice the difference in a piece of exposed iron or steel! However,
its a lot slower with aluminium, so your greenhouse should last a few
Corrosion of stonework,
especially limestone buildings, by acid rain is slow, but over the last
few hundred years of the industrial revolution many a medieval building
has suffered from its effects!
The fermentation of sugar to
alcohol is quite slow but you can see the carbon dioxide bubbles forming in
the 'froth' in a laboratory experiment or beer making in industry!
A faster reaction
example is magnesium
reacting with hydrochloric
acid to form magnesium chloride and hydrogen or the even faster
reaction between sodium and water to form sodium hydroxide and
These reactions take a few minutes.
A 'use of words'
revision note: Reacting and/or dissolving? Chemical or physical
If you take the solids
magnesium chloride or sodium hydroxide and mix them with water they dissolve to form a solution, but no chemical reaction to form new
substances takes place i.e. dissolving on its own is basically a physical change.
However, the two
substances mentioned above are formed in a chemical reaction change,
where the word 'dissolving' on its own is inadequate. The phrases reaction with ... or
reaction between ... are much more
appropriate, but there is no denying that the sodium dissolve
in water or magnesium dissolves in acid, BUT only because they have formed a water soluble compound
e.g. sodium hydroxide or magnesium chloride.
much faster would be magnesium burning in air, over in seconds!
e.g. when a fuel burns in air or oxygen, are very fast reactions.
Explosive reactions would be described as ‘very fast’,
in fact they are some of the fastest reactions known
e.g. the pop of a hydrogen-air mixture on applying a lit splint or the
production of a gas to inflate the air bags safety feature of many cars.
are the fastest chemical reactions you will encounter and are over in a
fraction of second, but a bunsen burner flames is a sort of 'controlled
explosion' and the chemical reactions of the fuel combustion are very rapid
indeed! but the rapidly burned fuel is continuously replaced by the gas
Explosive materials like
dynamite or TNT produce powerful explosions in a fraction of a second.
1c Why is the study of 'Rates of
Reaction', and the factors involved, so important?
MEASURING THE RATE OF A REACTION
- To measure the ‘speed of a chemical reaction’
or the ‘rate’ of a reaction depends on what the reaction
is, and what is formed that can be measured as the reaction proceeds.
- How do we define the rate or speed of a
- Mean rate of reaction = amount of product
formed / time interval involved
- Mean rate of reaction = amount of reactant
used up / time interval involved
- The use of the word mean is important.
- The rate of a reaction is constantly changing, in fact
it is always slowing down as the reactants are being used up.
- Therefore, the best you can do is get an average
value at a particular stage in the reaction and preferably as near as
possible to the initial rate of the reaction - the speed at the
start, after time = zero!
- This sort of definition will become pretty
obvious when analysing graphs of date e.g. volume of gas formed at various
time intervals, so the rate might be defined as ..
- Mean rate of reaction = cm3 gas
formed / time taken for gas to form
- so the rate units might be cm3/s or
- If the method of following the rate of reaction
involves weigh the reactants or products e.g. ..
- Mean rate of reaction = mass loss / time taken
- so the rate units might be g/s or g/min
- The measured gas volumes (e.g. cm3)
or mass losses (e.g. g) can be converted into moles so the rate can
be expressed as mol/s or mol/min.
EXAMPLES of experimental set-ups to follow the rate of a chemical reaction
are described below.
Examples of results data, graphs and their interpretation are also given.
Measuring the volume of gas evolved to measure the rate of a reaction
MEASURING GAS VOLUMES -
following the formation of a gaseous product of a reaction by collecting the gas
a gas is formed
from a solid reacting with a solution, it can be collected in a
gas syringe (see
the apparatus diagram below
and the graph of results).
- Using e.g. a gas syringe system (illustrated
below), you measure the volume of gas given off at regular time intervals.
- From the neat table of results, you then
plot the results on graph paper (or with a software package like excel)
making time the horizontal x-axis and the volume gas of gas the
- The initial gradient of the graph e.g. in
cm3/min (speed or rate) gives an accurate measure of how fast
a gaseous product is being formed e.g. in a metal carbonate - acid
reaction giving carbon dioxide, or metal - acid reaction giving hydrogen
- It is best to measure the initial rate
because the reaction mixture is becoming depleted in reactants so changing
the initial concentrations or surface area if there is a solid reactant
- You can measure the gas
formed every e.g. 30 or 60 seconds (0.5 or 1.0 minutes is typical) and plot the graph and measure the initial
gradient in e.g. cm3/min or cm3/sec.
- The most accurate measurements are
made early on in the reaction when the gas volume versus time is almost
- You can take a series of measurements and draw the graph (origin
0,0) to get the rate from the gradient (e.g. cm3/min),
measure the time to make a fixed volume of gas (* see below).
- If the reaction is allowed to go on, you can measure the final maximum volume of gas and the time at which the reaction
stops, though this a very poor measure of rate, because the reaction just
gets slower and slower as the reactant amounts/concentrations are
decreasing - so don't use this as a method of measuring reaction
- The steeper the initial slope/gradient,
the faster the initial reaction - its that simple!
- The gradient of the graph become less steep
because the reaction will naturally slow down due to the cumulative loss of
- For a series of experiments the initial gradient is
proportional to the relative rate and you measure it by marking and
measuring the gradient of the line from 0,0 tangentially over the first few
minutes of the reaction when the graph line is usually reasonably linear.
diagrams showing how to measure the initial rate of reaction e.g. gas
formation or mass loss.
- using graph paper
graph of volume of gas formed versus time represents a typical set of
results, this particular graph is discussed in more details in
More examples of graphs and their interpretation.
- When the gradient first becomes zero, i.e.
the graph line becomes horizontal, one or more of the reactants has been
used up and the reaction has then stopped.
- e.g. in the case of experiment run E, at 4
mins, at a maximum volume of 20 cm3 of gas formed.
- In terms of initial rate, the 'speed order'
is W > X > E > Y > Z, with decreasing initial graph gradient.
- Graphs X, E, Y and Z can be the result of
decreasing concentration from X to Z.
- The same pattern might be seen by decreasing
the temperature from X to Z.
- Graph W might be the result of doubling
the amount or concentration of one of the reactants compared to E,
steeper initial gradient - twice as fast at the start - doubling
initial rate and twice the amount of gaseous product formed.
- Graphs X could represent the effect of
adding a catalyst to experiment E.
- Graphs Z to X could be produced by
progressively crushing the same amount of solid reactant e.g. lumps <
granules < powder etc.
- Note: Complying with 'a fair test'
experiment design is discussed in detail for the experiments in each of the
sections for the effect of 3a.
3c surface area, 3d
temperature and 3e.
- Apart from the initial gradient, you can use
as a measure of the rate of reaction, how much gas is formed for a constant
time e.g. product gas volume after 2 minutes.
- (*) The reciprocal of the reaction time,
1/time, can also be used
as a measure of the speed of a reaction.
- The time can represent how long
it takes to form a fixed amount of gas after the first few minutes of a metal/carbonate
- acid reaction,
- or the time it
takes for so much sulphur to form to obscure the X in the sodium thiosulphate - hydrochloric
- The reaction time can be in minutes or seconds, as long as you stick
to the same unit for a set of results e.g. a set of experiments varying the
concentration of one of the reactants.
- however, I think the initial gradient method
to calculate the initial rate is more accurate because after a 'fixed time'
the reaction is slowing down with the natural depletion of reactants in
forming the products.
- For more details for A Level students see
Examples of reactions where you can measure the volume of gas evolved
and the sorts of investigations
you can do - the rate factors - typical results
- Examples of reactions involving gas
formation - DIAGRAM of APPARATUS and graph shown below
- You can follow the speed/rate of a
reaction that produces a gaseous product by collecting the gas and measuring
the total volume at suitable time intervals.
- (i) metals dissolving in acid ===> hydrogen gas, (test is lit splint => pop!),
- e.g. magnesium + sulphuric acid ==>
magnesium sulphate + hydrogen
- Mg(s) + H2SO4(aq)
===> MgSO4(aq) + H2(g)
- You can measure the volume of
hydrogen gas formed with a gas syringe.
- You can do a similar experiment with
- Mg(s) + 2HCl(aq) ===>
MgCl2(aq) + H2(g)
- (ii) carbonates dissolving in acids ===> carbon dioxide gas,
(test is limewater => cloudy),
- calcium carbonate (marble
chips) + hydrochloric acid
==> calcium chloride + water + carbon dioxide
- CaCO3(s) + 2HCl(aq)
===> CaCl2(aq) + H2O(l) + CO2(g)
- You can measure the volume of carbon
dioxide gas formed with a gas syringe.
- (iii) the manganese(IV) oxide catalysed decomposition of hydrogen peroxide (oxygen
gas, test is glowing splint => relights)
- hydrogen peroxide ===> water + oxygen
- 2H2O2(aq) ===> 2H2O(l) + O2(g)
- You can measure the volume of oxygen
gas formed with a gas syringe.
- All of these gas forming reactions can all be followed with the gas syringe method
suitable for use in schools and colleges.
- You can do all sorts of investigations
to look at the effects of varying ...
- (a) the reactant solution concentration,
and in industry pressure is varied, which is effectively varying the
concentration of gaseous reactants.
- (b) the temperature of the reactants
(solids plus solutions etc.),
- (c) the size of the solid reactant particles (surface area effect),
- You could also vary the particle
size of a solid catalyst to vary its surface area.
- (d) the effectiveness of
a catalyst on hydrogen peroxide decomposition.
- all of which are controlling factors
on the speed or rate of a chemical reaction.
- BUT don't forget the forgotten
factor if there is a solid reactant like limestone or a solid
catalyst - stirring.
- The mixture should be gently and
steadily stirred, preferably with a magnetic stirrer system.
- If you don't, the bottom layers of
the solution become depleted more than the upper layers as the solid
reactant will sediment out.
- This leads to erratic and inaccurate
- The shape of the graph is quite
of the progress of a chemical reaction.
the diagram above and notes below).
- The reaction is fastest at the start when the reactants are at a maximum (steepest gradient in cm3/min).
- The gradient becomes progressively less as reactants are used up and the reaction slows down.
- Finally the
graph levels out when one of the reactants is used up and the reaction stops.
- The amount of product depends on the
amount of reactants used.
- The initial rate of reaction is obtained
by measuring the gradient at the start of the reaction. A tangent line
is drawn through the first part of the graph, which is usually
reasonably linear from the x,y origin 0,0.
- This gives you an initial rate of
reaction in cm3 gas/minute,
- Typical results from a gas
producing reaction are shown below, for different amounts or
concentrations of reactants. How to calculate the reaction rate is
- e.g. for run q
[ ], after 2 mins, 20
cm3 of gas formed, so the rate of reaction is 20/2 = 10
- From the graph of results
you can measure the relative rate of reaction from ...
- (i) the initial
gradient in cm3/min (see on diagram above)
- (ii) you can
estimate from the graph the volume of gas formed after a particular
time e.g. 3 minutes
- (iii) you can estimate the time it takes to
form a particular volume of gas.
- (i) 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.
Results using Excel
- You can use application software
like Excel to process and portray your data in a graphical format,
or you can just plot your data, but accurately, on ordinary graph
- Keeping the temperature
constant is really important for a 'fair test' if you are
investigating speed of reaction/rate of reaction factors such as
concentration of a soluble reactant or the particle size/surface
area of a solid reactant.
- On the advanced gas calculations page,
temperature sources of error and their correction are discussed in
calculation example Q4b.3,
although the calculation is above GCSE level, the ideas on sources
of errors are legitimate for GCSE level.
- Note that if the temperature
of a rates experiment was too low compared to all the other
experiments, the 'double error' would occur again, but this time the
measured gas volume and the calculated speed/rate of reaction would
be lower than expected.
- Extra note on graph interpretation
to derive other rate data from the same graphs as above.
graph W: From the two blue crosses marked at 1 and 3 minutes
respectively you can calculate the average speed/rate of the reaction W
between the 1st and 3rd minutes of the reaction.
- Average rate = (volume at 3 mins - volume at 1 min) /
- Average rate = (37 - 22) / 2 = 9/2 = 4.5 cm3
graph E: From the purple tangent drawn at 2 minutes you can get the rate of
reaction E at 2 minutes. This mathematically is the more correct method.
- Without thinking, its pure coincidence I've used the
1st and 3rd minute again!
- Make sure you adjust the ruler until the space
between the ruler and the curve is equal on both sides of the point selected
for tangent measurement. You then extended the line to get two reasonably
accurate times and volumes on which to base your rate calculation.
- From the tangent line: volume at 1 min = 12 cm3,
volume at 3 min = 20 cm3
- Rate at 2 minutes = (volume at 3 mins - volume at 1
min) / time elapsed
- Rate at 2 minutes = (20 - 12) / 2 = 8/2 = 4.0 cm3
Measuring the rate of reaction my measuring the loss in mass of a reactant
- following the mass loss due to the escape of a gaseous product
- The rate of a reaction that produces a gas can also be measured by following the mass loss as the gas is formed and escapes from the reaction
- The method is ok for reactions producing carbon dioxide
(limestone + acid),
but I don't think it is as good as the gas syringe method for several
- I don't consider this as accurate as the
gas syringe method because the mass of gas is small, particularly in the
case of hydrogen (molecular mass 2), it might be just ok for carbon
dioxide (molecular mass 44) with a very accurate balance.
- Another problem is stirring, when
dealing with a solid reactant and solution, the mixture, ideally, should
be gently stirred to avoid local depletion of reactants, so how can you
possible do it on a sensitive balance!
- The reaction rate is expressed
as the rate of loss in mass from the flask in e.g. g/min based on the
initial gradient (see graph below).
The total mass of the flask
and contents are recorded at selected suitable time intervals and
subtracted from the initial mass, therefore you have then recorded the
actual mass loss which equals the mass of product formed.
The initial gradient of mass
loss gives the rate of the reaction (you can ignore the negative sign,
since technically its a negative gradient).
The speed of the reaction
would be measured as g/min or g/s, mass loss of reactant per unit of
time, or mass of product formed per unit of time.
This graphs represents the sort of results you might get.
The faster the reaction, the
greater the mass loss in a given time and the negative gradient of the
graph is steeper.
You can investigate
different concentrations of soluble reactants or different sized pieces of
a solid reactant or catalysts.
the initial gradient, the faster the reaction.
In terms of rate of
reaction, X > E > Y > Z, which you see for a series of increasing
temperatures or a series of increasing reactant concentrations for a
Graph W, the steepest, might
represent using double amount of a solid reactant or doubling the
concentration of a reactant solution.
Following the rate of a reaction from a visual precipitation effect
PRECIPITATION - following
the amount of a precipitate formed
- When sodium thiosulphate reacts with an acid, a yellow precipitate of sulphur is
formed and forms the basis of a good project for assessment.
- To follow this reaction in your
investigation you can measure how long it takes for a certain amount of sulphur
- You do this by observing the reaction down through a conical flask, viewing a black cross on white paper (see diagram below).
- The reaction can be followed in this way
because both solutions, and after mixing, are colourless and more
importantly, they are clear and transparent.
- Its the cloudiness that causes the cross
- The X is eventually obscured by the sulphur
precipitate and the time noted.
- sodium thiosulfate + hydrochloric acid
==> sodium chloride + sulfur dioxide + water + sulfur
+ 2HCl(aq) ===> 2NaCl(aq) + SO2(aq) +
H2O(l) + S(s)
Note: You do not see gas bubbles
because the very nasty sulphur dioxide gas is very soluble in water but
still take care you do not inhale any of the air
near the flask when you are doing the experiment or washing out the
=> watch stopped
- By using the same flask and paper X you can
obtain a relative measure of the speed of the reaction in forming the same amount of
- The speed or rate of reaction can expressed
as 'x amount of sulphur'/time, so the rate is proportional to 1/time for a
particular run of the experiment. In other words since you don't know
the absolute mass of sulphur formed, the reciprocal of the time is taken
as a measure of the relative rate of reaction.
- You can investigate the effects of
- (a) the hydrochloric acid or sodium
- (b) the temperature of the reactant solution
- to show the effects of changing one of the variables
you can plot graphs of e.g.
- reaction time versus temperature or
- or rate of reaction (1/reaction time)
versus temperature or concentration.
- You can also measure the speed of this
reaction by using a light gate to detect the precipitate formation. The
system consists of a light beam emitter and sensor connected to computer and
the reaction vessel is placed between the emitter and sensor. The light
reading falls as the sulphur precipitate forms.
- This would be a superior method than the naked
eye because direct observation is subjective, different students might have
different perceptions as to when the cross disappears!
- It is important you take the same total
volume of solution to give the same depth of liquid you are viewing the
- Note that unlike the other methods described
on this page, you do not get lots of data to plot to get the rate for a
particular experiment, its a sort of 'one of' measurement for a particular
concentration or temperature.
- So you get one graph when investigating one
factor! e.g. for the effect of temperature the results might look like this
|time for X to be
- ... from which you get one graph like
- Further examples
of graphs that may be obtained from the different methods.
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