The Effect of Temperature on the Rate of a Chemical Reaction
Length: 1614 words (4.6 double-spaced pages)
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In this investigation we are trying to find out how the temperature
affects the rate of reaction.
I believe that the higher the temperature, the faster the reaction the
reaction between the sodium thiosulphate and hydrochloric acid. This
will happen because kinetic energy and the heat energy makes the
particles move faster, collide faster and react faster; therefore they
can break bonds easier.
Firstly, I will explain the equation needed to follow out the
[IMAGE]Na2 S2O 3(aq) + 2HCL(aq) 2NaCl(aq) +SO2 (g)+ H2O (l)+S(s)
This is the experiment that will carried out, I will now put this
equation into word format:
Sodium Hydrochloric Sodium Sulphur
[IMAGE]Thiosulphate+ Acid Chloride+ Dioxide + Water + Sulphur
The sodium thiosulphate consists of: two atoms of sodium, two atoms of
sulphur and two atoms of oxygen. This combined with the hydrochloric
acid creates a chemical reaction and the products are 2 molecules of
sodium chloride, a molecule of sulphur dioxide, water and sulphur.
Rate of reaction:
The rate of reaction is the speed or velocity at which a chemical
reaction happens, expressed in terms of the amount of time taken for a
reaction to occur (usually in seconds).
We can also time how quickly reactants are used up. The quicker these
things happen, the faster the rate of the reaction. If a reaction has
a low rate that means the molecules combine at a slower speed than a
reaction with a high rate. Some reactions take hundreds, maybe even
thousands of years while other can happen in less than one second. The
rate of reaction depends on the type of molecules which are combining.
Scientists like to know the rate of the reaction. They measure
different kinds of rates too. Each rate that can be measured tells
scientists something different about the reaction. Here are a few
(1) Forward Rate: The rate of the forward reaction when reactants
(2) Reverse Rate: The rate of the reverse reaction when products
recombine to become reactants.
(3) Net Rate: The forward rate minus the reverse rate.
(4) Average Rate: The speed of the entire reaction from start to
(5) Instantaneous Rate: The speed of the reaction at one moment in
Scientists measure all of these rates by finding out the
concentrations of the molecules in the mixture. If you find out the
concentration at two different times you can find out what direction
the reaction is moving and how fast it is going.
Since many reactions happen with several steps, the rate for each step
needs to be measured. There will always be one step which is the
slowest. That slowest step is called the RATE LIMITING STEP (the one
reaction out of all of them that really determines how fast the
overall reaction can happen).
The collision theory says that the more collisions in a system, the
more likely combinations of molecules will happen. So if there are a
higher number of collisions in a system, more combinations of
molecules will occur, the reaction will go faster, and the rate of
that reaction will be higher. Key things decide whether a particular
collision will result in a reaction - in particular, the energy of the
collision, and whether or not the molecules hit each other the right
way around (the orientation of the collision). To make naming ions,
particles, molecules and free radicals easier, I will use the word
species. Reactions where a single species falls apart in some way are
slightly simpler because you won't be involved in worrying about the
orientation of collisions. Reactions involving collisions between more
than two species are going to be extremely uncommon.
Collisions with two different species:
It is pretty obvious that if you have a situation involving two
species they can only react together if they come into contact with
each other. They first have to collide, and then they may react.
Why "may react"? It isn't enough for the two species to collide - they
have to collide the right way around, and they have to collide with
enough energy for bonds to break.
Orientation of reaction:
Consider a simple reaction involving a collision between two molecules
- ethene, CH2=CH2, and hydrogen chloride, HCL, for example. These
react to give chloroethane.
As a result of the collision between the two molecules, the double
bond between the two carbons is converted into a single bond. A
hydrogen atom gets attached to one of the carbons and a chlorine atom
to the other.
The reaction can only happen if the hydrogen end of the HCL bond
approaches the carbon-carbon double bond.
Any other collision between the two molecules does not work. The two
simply bounce off each other.
Of the collisions shown in the diagram, only collision 1 may possibly
lead on to a reaction.
Why does collision 2 not work as well? The double bond has a high
concentration of negative charge around it due to the electrons in the
bonds. The approaching chlorine atom is also slightly negative because
it is more electronegative than hydrogen. The repulsion simply causes
the molecules to bounce off each other.
Activation energy is the energy needed for a collision to happen.
Even if the species are orientated properly, you still won't get a
reaction unless the particles collide with a certain minimum energy
called the activation energy of the reaction.
Activation energy is the minimum energy required before a reaction can
occur. You can show this on an energy profile for the reaction.
For a simple over-all exothermic reaction, the energy profile looks
If the particles collide with less energy than the activation energy,
nothing important happens. They bounce apart. You can think of the
activation energy as a barrier to the reaction. Only those collisions
which have energies equal to or greater than the activation energy
result in a reaction.
Any chemical reaction results in the breaking of some bonds (needing
energy) and the making of new ones (releasing energy). Obviously some
bonds have to be broken before new ones can be made. Activation energy
is involved in breaking some of the original bonds.
Where collisions are relatively gentle, there isn't enough energy
available to start the bond-breaking process, and so the particles
What can affect the rate of reaction:
Surface area can affect the rate of reaction. The more finely divided
the solid is, the faster the reaction happens. A powdered solid will
normally produce a faster reaction than if the same mass is present as
a single lump. The powdered solid has a greater surface area than the
single lump. You are only going to get a reaction if the particles in
the gas or liquid collide with the particles in the solid. Increasing
the surface area of the solid increases the chances of collision
The concentration of the reaction also affects the rate. For many
reactions involving liquids or gases, increasing the concentration of
the reactants increases the rate of reaction. In a few cases,
increasing the concentration of one of the reactants may have little
noticeable effect of the rate. In order for any reaction to happen,
those particles must first collide. This is true whether both
particles are in solution, or whether one is in solution and the other
a solid. If the concentration is higher, the chances of collision are
Pressure may also be a factor in a reaction. Increasing the pressure
on a reaction involving reacting gases increases the rate of reaction.
Changing the pressure on a reaction which involves only solids or
liquids has no effect on the rate. Increasing the pressure of a gas is
exactly the same as increasing its concentration. If you have a given
mass of gas, the way you increase its pressure is to squeeze it into a
smaller volume. If you have the same mass in a smaller volume, then
its concentration is higher. In order for any reaction to happen,
those particles must first collide. This is true whether both
particles are in the gas state, or whether one is a gas and the other
a solid. If the pressure is higher, the chances of collision are
As you increase the temperature the rate of reaction increases. As a
rough approximation, for many reactions happening at around room
temperature, the rate of reaction doubles for every 10°C rise in
temperature. You have to be careful not to take this too literally. It
doesn't apply to all reactions. Even where it is approximately true,
it may be that the rate doubles every 9°C or 11°C or whatever. The
number of degrees needed to double the rate will also change gradually
as the temperature increases.
To make this experiment a fair test I will be controlling some aspects
of the experiment and varying other aspects of the test. The variable
will be to differ the temperature, to see if the rate of reaction
between the sodium thiosulphate and the hydrochloric acid is any
different. The aspect I will be controlling will be the concentration
of hydrochloric acid and sodium thiosulphate and the volume of the
reactants, I will be using the same apparatus to keep the experiment
To keep the test safe, I will be equipped with goggles to shield my
eyes from any vigorous reactions and gloves to protect my hands.
The materials we will be using are:
Paper with an X
1. Firstly get all your apparatus together and make sure you have all
safety items at your disposal.
2. Get your measuring cylinder and pour 20ML of sodium thiosulphate
into a test tube, make sure you do this for each test.
3. Use the second measuring cylinder to measure 20ml of hydrochloric
4. Use the beaker to set up 5 different temperatures for 5 different
tests by putting the test tubes with sodium thiosulphate inside the
beaker. Make it so it is a water bath.
5. Once the thiosulphate is at a suitable temperature, place the paper
with an X under the cylinder with HCL. Prepare the stopwatch. As soon
as you pour the thiosulphate into the HCL start the stopwatch. Stop
the stopwatch when you are no longer able to see the X under the
6. Record your results for the 5 times you do the experiment with the
situations stated above.
There should be five different situations to experiment, they are:
1. Beaker with ice only.
2. Beaker with boiling water.
3. ½ boiling water and ½ tap water.
4. Only tap water.
5. A small amount of ice in tap water.
Temperature of thiosulphate
Time taken for X to go (seconds)
½ boiling water and ½ tap water
Only tap water
in tap water
I have concluded that my hypothesis was correct and that the higher
the temperature, the less time taken to react. As the temperature
steadily increased, the time taken steadily decreased. The graph shows
a smooth curve of best fit and shows that the two things being
compared to relate to each other.
I would improve on my experiment by repeating the test two more times
and finding the average so we can find the overall results and the
results can be more accurate. Maybe if we could also test
concentration on the rate of reaction by increasing and decreasing the
amount of reactants. The experiment was an overall success and I am
pleased with my results.