One of the most useful things for chemists to understand about reactions is how fast they occur.
The rate of reaction is defined as "the change in concentration of the reactants or products
per unit time". Controlling the rate of reaction and understanding the processes
by which reactants change into products are essential skills for production chemists.
They need to be confident that when production is scaled up the rate of reaction remains the same
otherwise, there could be cost or health and safety implications. There are two main practical
approaches to measuring the rate of reaction: the initial rate method and the continuous monitoring
method. The simplest initial rate experiments involve measuring the time taken for some easily
recognisable event in the reaction to occur. An advantage of this method is that the concentration
remains close to that of the initial mixture and can be easily calculated. In this video, I'm going
to demonstrate a clock reaction. We will then carry out a second experiment using the continuous
monitoring method, by measuring the volume of gas produced by a reaction every 10 seconds.
The continuous monitoring method provides us with plenty of data however, further calculations are
required to determine the exact concentration at different points during the reaction. There
are many other examples of using continuous monitoring to measure different properties,
like colour change by colorimetry, pH by a pH sensor or meter, and temperature by a
temperature sensor or probe. Another approach is to monitor the change in concentration
by periodically taking a sample and determining the concentration for example, by titration.
To explore the initial rate method we are going to look at the iodine clock reaction,
where acidified hydrogen peroxide reacts with a solution containing potassium iodide,
sodium thiosulfate and starch. When carrying out initial rate experiments, it's always good
to know what change you're expecting. Here we're expecting a change from a colourless solution to
one that is blue-black. I'm just going to pop my goggles on before I start the experiment.
Wait for it...
As you can see our solution has changed from colourless to a blue-black colour.
I am now going to perform an experiment where I change the concentration of hydrogen peroxide
and measure how long it takes for the reaction to occur at each concentration. We're going to
use five different concentrations of hydrogen peroxide as such, I will need five conical flasks
which I will label A, B, C, D and E. I'm now going to prepare our reaction. To start with I
will add a solution containing potassium iodide and sodium thiosulfate to each conical flask.
I'm going to use a 20 centimetre-cubed pipette for this but first I need to rinse it out.
On the bulb pipette there is a line just above the bulb
this is 20 centimetres-cubed exactly and this is the line we are aiming for.
As you can see, I'm slightly over so I'm going to release a little pressure now... and there we go.
When dispensing the liquid, touch the pipette to the conical flask
to make sure the final drops end up in your flask. I am now going
to add two centimetres-cubed of two grams per hundred centimetre-cubed solution of starch
to each of our conical flasks. To do this I'm going to use a pipette.
Finally, I'm going to measure out 10 centimetres-cubed of 1.0 mole per
decimetre-cubed sulfuric acid into beakers using a burette. It's important to make
sure we clean our burettes using the same solution as we will be using in the burette,
so I'm going to clean it with one mole per decimetre cube sulfuric
acid. Make sure the tap of your burette is closed. To clean I slowly rotate the burette,
angling it upwards each time in order to make sure that the liquid goes all the way down the burette.
I can then open the tap and empty this into the waste beaker.
You are now ready to fill your burette. Don't forget to make sure the tap is closed before
you fill your burette. Luckily, my wonderful technician has already pre-cleaned and filled
my burette with sulfuric acid for me. I'm going to measure 10 centimetres-cubed now into each beaker.
Again, I will be at eye level to make sure I put exactly 10 centimetres-cubed into each one.
As we get closer to our ten centimetre mark, I close the tap of the burette slightly.
This slows the rate of flow and therefore allows me to be incredibly accurate
with my measurements. Now to prepare the hydrogen peroxide. You may have noticed that bottles of
hydrogen peroxide are often labelled as 5 vol or 20 vol. This is because the concentration of
hydrogen peroxide is given in volumes rather than moles per decimetre-cubed. The volume
refers to the volume of oxygen gas produced when hydrogen peroxide decomposes to produce oxygen
and water. We can calculate the concentration of hydrogen peroxide using the balanced equation
and the fact that one mole of gas occupies a volume of 24 decimetres-cubed at standard
temperature and pressure. I will now measure out our hydrogen peroxide. I'm going to measure out 30
centimetres-cubed of 20 vol hydrogen peroxide. I have labelled my measuring cylinder to avoid
confusion. My technician has pre-prepared other concentrations of hydrogen peroxide
by adding 25, 20, 15 and 10 centimetres-cubed of hydrogen peroxide to a measuring cylinder
and then making each of them up to 30 centimetres-cubed using distilled water.
We are now ready to start our reactions however, before we begin let's think about the chemistry.
The dramatic colour change we see is due to iodine reacting with starch.
Hydrogen peroxide oxidises the iodide ions to form iodine. However, we don't see this straight away.
This is because a second reaction is also occurring. As soon as iodine is formed,
it reacts with thiosulfate ions to form tetrathionate ions in a second faster
reaction. Effectively, the iodide ions are recycled. Once all the thiosulfate is used up,
free iodine remains in solution and can react with the starch, thus forming our blue-black complex.
So now I need to borrow some hands to help me get this started.
Okay. Three, two, one. Pour.
Don't forget to start your stopwatch as soon as you add your solution to your conical flask.
Three, two, one. Pour.
Don't forget to record all of your results in a table.
I now need to repeat the whole experiment multiple times to get more data and ensure my
results are reliable. You can see the results of my additional experiments in the table.
As my reactions are now finished, I need to dispose of the reaction mixture.
The reaction mixture contains iodine which can be harmful to aquatic life. As such,
I cannot pour it directly down the sink. I need to dispose of it in the correct method, which
in this case is to pour into a beaker containing sodium carbonate solution. The sodium carbonate
solution reduces the iodine to iodide ions. This can now be safely disposed of by a technician.
In this next experiment, we are going to use the continuous monitoring method.
We are going to follow the reaction between magnesium and hydrochloric acid,
recording the change in concentration. We will then repeat this experiment
at different temperatures to explore the effect of temperature on rates of reaction.
To monitor the change in concentration we're going to measure the volume of hydrogen gas produced.
I'm about to start my experiment so don't forget to put your safety goggles on. First, I'm going to
set up by measuring out 20 centimetres-cubed of 1.0 mole per decimetre-cubed hydrochloric acid,
and dispensing it into a sidearm boiling tube. I can use a measuring cylinder or pipette for this
but today I'm going to use a pipette. Just like in our previous experiment, I'm making
sure that the meniscus of the hydrochloric acid is exactly on the line above the bulb.
I am now going to record the temperature of the hydrochloric acid. You can use a temperature
probe or a thermometer to do this. I am using a thermometer. Again, I am at eye level to make
sure my measurement is accurate. I now need to make sure that the plunger can move freely in
the syringe. As you can see, it does. This means the pressure won't build up inside the syringe
causing it to explode. I am now going to measure out 0.09 g of magnesium ribbon.
To ensure that the balance measures the mass accurately I need to tare it first.
I'm now going to fold my magnesium ribbon into a v-shape. Holding my sidearm boiling tube
at a shallow angle, I'm going to place it just inside the top. I am then going to add the bung.
As I add the bung, the plunger may move slightly out of the gas syringe. We need
to make sure we record this measurement as this is our starting value we will be using.
I am now going to tilt the tube and start the stopwatch immediately.
I will measure the volume every 10 seconds until I have three readings of the same value.
Starting... now!
I am now going to repeat the experiment at different temperatures.
The temperature can be controlled using a thermostatically controlled water bath.
If this is not available, you can use a bunsen burner, a beaker of water, a tripod and gauze and
heat the water to the desired temperature. This is 60 degrees C. Starting... now!
This is 80 degrees C. Starting in three, two, one...
Now we have some results, the next step is for you to process them. When processing the data
to determine the rate, you will need to be able to draw a tangent to the curve and take
the gradient. You can find plenty of help with these calculations in the additional resources.
You can also find suggestions for how to take these experiments further.
Thank you for watching and have fun with your experiments! Inspiring your teaching and learning.