PAUL ROBERTS: Hello, I'm Paul Roberts, and I'm a chemistry teacher here at Andover College.
Today, we're going to be looking at the core practical of combustion of alcohols and measuring
their temperature rise. The alcohols we'll be looking at are ethanol, propanol, butanol,
and pentanol. This experiment is fairly straightforward. However, I'm going to show you some tips along
the way to get some clean, accurate results.
Regarding safety in this practical, you're obviously going to be handling a wide range
of highly flammable alcohols. There are also specific hazards associated with these alcohols.
More information of these can be found on the CLEAPSS website.
This core practical requires you to measure the temperature change of the water when the
alcohols-- ethanol, propanol, butanol, and pentanol-- are combusted. For this demonstration,
I'm only going to carry out the combustion of ethanol, as the other three alcohols will
be combusted in the same manner.
To start off with I'm going to measure and record the mass of the ethanol spirit burner,
taking care to weigh the burner with the cap on. Next, I'm going to measure out 100 centimeters
cubed of cold water into a measuring cylinder and then add this to the 250 centimeter cubed
conical flask. The conical flask is then clamped around its neck, and I'm going to lower the
base of the flask to around 5 centimeters above the height of the burner, which is on
a heatproof mat. You may choose to use draft shields to ensure more efficient energy transfer.
However, for the clarity of this practical, I am going to omit any draft shielding.
We are now ready to start the experiment. Firstly, I will place thermometer in the water
bath. It is briefly stirred and then left for 30 seconds. After this time, the initial
temperature of the water can be recorded.
Next, we can start the combustion. Making sure the burner is directly underneath the
conical flask. I'm going to remove the cap of the burner. And without delay, I will light
the burner, taking care not to knock off any pieces of wick. It is important that this
stage is done promptly. Otherwise, the alcohol will evaporate from the burner, leading to
unreliable results.
Once combustion has taken place, it is important that the water is constantly stirred and the
temperature is monitored. Failure to stir will result in hot and cold spots in the water
and produce unreliable results. Once the temperature has risen by approximately 40 degrees C, cap
the burner. This means if my initial temperature was 20 degrees C, I would cap the burner when
the temperature of the water bath reached 60 degrees C.
Because the base of the conical flask has been exposed to the flame, it will be hotter
than the water. It is therefore important to keep stirring the water even after the
flame has been extinguished. Expect the temperature to rise slightly more and record the maximum
temperature of the water. When you measure the temperature, ensure the bulb of the thermometer
is below the waterline. Resist the temptation to remove the thermometer from the water to
enable easier reading. Once you've recorded the maximum temperature of the water, you
can now weigh and record the mass of the spirit burner, including the cap.
Now, at this point, we have a conical flask full of hot water, which is a hazard. As a
tip, you can add 100 centimeters cubed of cold water to the flask, which should cool
it enough for you to be able to handle safely. Pour this water down the sink and rinse the
inside and outside of the flask with cold water to enable prompt cooling.
You can now repeat the experiment using the other alcohols-- propanol, butanol, and pentanol.
Try to use an identical experimental setup up for each alcohol, ideally, with the same
gap between burner and conical flask, the same draft shielding, and if possible, a similar
initial temperature and temperature rise.
Now, at the end of the experiment, you should have a range of differences in masses and
a range of temperature changes for the four alcohols you've combusted. You should also
be able to see a trend in which alcohol requires the least mass to produce a particular temperature
change.
So to summarize, I've shown you how to set up your experiment carefully, which has enabled
you to get safe and accurate results. We've also shown how you can measure the combustion
temperature change of one alcohol and use this as a template to obtain other results.