RATES OF CHEMICAL CHANGE
How can we measure how fast a reaction goes?
What makes a reaction go faster or slower?
How do scientists explain changes in the speed of a
reaction?
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CORE BOOKLET 6
1
Rates of Chemical Change
You should:
Tick
Explore the effects of changing concentration,
temperature and surface area on the rates of chemical
reactions, using I.C.T where appropriate; this should
include:
 Planning and collection of reliable data, analysing
the data, drawing conclusions and evaluating the
procedures used
 Understanding the advantages of I.C.T tools in
terms of recording, continuous monitoring and
instantaneous display
 Explaining the outcomes in terms of the particle
theory
Understand the meaning of the term catalyst and know
that the development of better catalysts is extremely
important as it can lead to new ways of making
materials that may use less energy, use renewable raw
materials or use fewer steps.
SUMMARY –
all you need to know for this topic – pg 2-8 – read + highlight
Some reactions happen very quickly for example;
 potassium reacting with water
 the formation of a precipitate
 An explosion
On the other hand, some reactions are very slow for example;
 the rusting of iron
 The setting of concrete.
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Many chemists in industry spend their careers trying to speed up or slow
down the rate of chemical reactions.
How do scientists explain changes in the rate of a
reaction?
In any chemical reaction we begin with REACTANTS. The components of
the reactants collide then rearrange. We end up with PRODUCTS e.g.
REACTANT AB + REACTANT CD
PRODUCT AC + PRODUCT BD
The Collision Theory

For a reaction to occur the reactant particles must COLLIDE.

For a collision to result in a reaction it must have enough ENERGY.

The amount of energy required for a successful collision is specific
to each reaction and is called the ACTIVATION ENERGY. The
reason that activation energy is specific to each reaction is that
reactant particles may have bonds between them that need to be
broken. Every different type of bond requires a specific amount of
energy to break it.
E.g.
To break an H-H bond would take 436 kJ
To break a Cl-Cl bond 242kJ
What makes a reaction go faster or slower?
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Altering TEMPERATURE
SURFACE AREA of a SOLID
CONCENTRATION of a SOLUTION
By using CATALYSTS
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Temperature
Increasing the temperature of reactant particles increases the rate of a
chemical reaction, this is because:
a.
b.
The reactant particles gain energy and move faster. They
therefore collide more often PER UNIT TIME.
More particles are likely to reach the activation energy so that
more collisions would be successful.
Surface Area
This applies to reacting solids. If you have 1 lump of a solid only the
outer particles can collide and react with other substances. If you break
up the lump then outer and now inner particles will be able to collide and
react. The more collisions there are PER UNIT TIME the faster the
reaction. Increasing surface area does not affect the energy of the
reacting particles.
Concentration
This applies to reacting solutions. In a solution such as hydrochloric acid
solution there will be HCl particles but there will also be water particles.
A high concentration of HCl will contain lots of HCl particles and less
water.
A low concentration of HCl will contain fewer HCl particles and more
water.
The higher the concentration the faster the reaction! This is because
there are more relevant particles (HCl particles in our example) to collide
PER UNIT TIME. Increasing concentration does not affect the energy
of the particles.
Catalysts
Catalysts are substances that can be added to a chemical
reaction to make it go faster. The catalyst itself will not take
part in the reaction and can therefore be reused. Catalysts
actually work by lowering the activation energy for a reaction.
They provide a surface for the reactant molecules to get
together on therefore speeding up the number of collisions so
that there are more of them PER UNIT TIME.
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Advantages of catalysts include:


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Very small quantities of the most active catalysts can convert
thousands or millions of times their own weight of chemicals.
A catalyst may increase the rate of only one reaction out of many
competing reactions.
Catalysts including biological catalysts such as enzymes can often
work at low temperatures therefore reducing cost.
Catalysts including biological catalysts such as enzymes can often
work at low pressures therefore reducing cost and risk.
Because they provide a different pathway for a reaction to occur,
catalysts can reduce the number of steps required for a reaction
to take place.
Examples of where catalysts are used include:



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Catalytic converters made from platinum and rhodium break down
some of the more harmful byproducts of automobile exhaust.
Haber process (synthesis of ammonia from nitrogen and hydrogen,
where ordinary iron is used as a catalyst)
Catalytic cracking - breaking long-chain hydrocarbons into smaller
pieces using a zeolite catalyst
Sulphuric acid production using the catalyst vanadium(V) oxide
How can we measure how fast a reaction goes?
Generally, in order to measure the rate of a chemical
reaction we have to carry out experiments. During
these experiments we monitor either the rate of loss
of a reactant or the rate of evolution of a product for
example:
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The rate of loss of a reactant example 1
Or, the rate of evolution of a product example 2
In the above two examples the reactions have been monitored
over a period of time and a graph drawn giving a profile of
the reaction. The rate in each case can be determined at any
point on the graphs by calculating rise over run.
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Instruments used to measure rate include:
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a top pan balance as in example 1 if mass loss/gain is being
measured
A gas syringe as in example 2 if volume of a gas is being measured
The method below is called downward displacement, again used if
volume of gas is being measured
When some substances react for example, a chemical called sodium
thiosulphate reacts, a precipitate will form.
This reaction can be timed by watching to see how long it takes for
a cross to disappear under the reaction.
Watch stopped
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Data logging is another way in which the rate of a chemical reaction can
be determined. Here is a picture of a data logger;
Data logging is a much more accurate way of measuring the rate of a
chemical reaction. You can either write down the results directly from
the display or the logger can be connected to the computer which will
then display all of your results in a table, as a graph etc.
Advantages of data logging
1. If you ‘fetch’ results using computer software they are instant.
They can also be instantly turned into tables and graphs to show
trends.
2. They are able to continuously monitor your experiment. For
example they can read the drop in light intensity caused by a
precipitate every minute or every second depending upon what’s
needed.
Useful searches
Rates of Reaction and Collision Theory - GCSE Chemistry - Sciencepages.co.uk
S-Cool! - GCSE Chemistry Revision Guide
crocodile clips :: Absorb Chemistry for GCSE
And many more! Simply type ‘rates of reaction at g.c.s.e’ and lots of sites will appear. The diagrams
in this booklet have been taken from Doc Brown’s Chemistry Clinic.
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Experiment 1 – using a data logger
Increasing:
Temperature
Reaction:
Adding hydrochloric acid to sodium thiosulphate
Measuring:
Decreasing light intensity reaching a data logger
1.
2.
3.
4.
5.
Take 10cm3 hydrochloric acid temperature 40◦C.
Take 10cm3 sodium thiosulphate in a conical flask. Place the flask
over a light sensor attached to a data logger.
Add your hydrochloric acid to the thiosulphate solution
Download your data
Repeat using 80◦C hydrochloric acid
Log on>Prog>Science>LogIT Lab 2> Log It Lab.
Plug logger into computer.
Click onto the FETCH icon.
Click OK
Now complete tasks 1, 2 and 3 below.
1. Stick your graphs into your booklet. Work out the rate
of each reaction,
Key skill - numeracy
Rate = RISE
RUN
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Display your results on the grid below.
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2. Write about 2 advantages of using data loggers. Use the
information on page 8 to help you;
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3. In the space on the next page use the particle theory
and particle diagrams to explain which experiment had
the largest rate, the 40◦C or the 80◦C.
Use the
explanation on pages 3+4 to help you as well as the class
textbooks.
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Draw diagrams showing a low and higher temperature hydrochloric acid particles reacting with the sodium
thio sulphate particles in this space;
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Experiment 2 – looking at catalysts
Looking at the action of…
a catalyst
Reaction a
The decomposition of hydrogen peroxide
Observing:
The action of the inorganic catalyst manganese dioxide on hydrogen peroxide
This type of reaction is a decomposition reaction. A reaction where there
is only one reactant and two or more products is a decomposition reaction.
Add a quarter of a spatula full of manganese dioxide to a test tube
one third full of hydrogen peroxide in a test tube rack!
1. Write the equation to describe this reaction
2. What is a decomposition reaction?
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3. Write your observations
describe what you see, hear, smell, and feel.
4. What if anything happened to the catalyst by the end of the
experiment?
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Looking at the action of…
a catalyst
Reaction b
The decomposition of hydrogen peroxide
Observing:
The action of a biological peroxidise enzyme present in liver, on hydrogen peroxide
This type of reaction is a decomposition reaction. A reaction where there
is only one reactant and two or more products is a decomposition reaction.
Add a small piece of liver to a conical flask containing hydrogen
peroxide!
Info!
Enzymes are catalysts produced by living cells.
The liver contains enzymes that can speed up the decomposition of
hydrogen peroxide into water and oxygen.
Write down your observations -
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Using page 5 to help you, write down 5 advantages of a catalyst
in your own words.
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Give three examples of where catalysts are used in industry:
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