Specification:
Techniques and procedures for measuring the energy transferred when reactions occur in solutions (or solids reacting with solutions) or when flammable liquid burn; the calculation of enthalpy changes from experimental results.
Experiment and apparatuses:
A calorimeter is used to work out the enthalpy change of combustion.
If you don’t know the enthalpy of combustion please check the notes for this.
You would have your apparatuses like on the right and this is how the experience is carried out.
You would measure the temperature of the water at the start with a thermometer and its initial mass.
Measure the mass of the fuel you would use with a balance.
You would then heat the water till the temperature doesn’t rise anymore.
Record the reading of the temperature of water at the end.
Record the mass of the fuel left at the end.
(Real image of a calorimeter)
You would then measure the energy released by this equation:
Q=McΔT
Q is for the energy transferred to the water,
M is for the mass of water,
C is for the specific capacity (4.18Jg-1K-1)
ΔT is for the change in temperature
To find the Enthalpy change from the energy you would use this equation:
Enthalpy=Energy/moles
ΔH=Q/n
Example:
100g of water was heated from 23 C to 57 C by 1.8g of ethanol. Calculate the energy transferred and hence the enthalpy change of the fuel.
57-23=34 This is the temperature change.
use the equation Q=McΔT and put the value you know.
100✕4.18✕34
This gives you the energy of 14212J or 14.212KJ
You would find the moles of ethanol. Moles=mass/Mr
The Mr of ethanol (C2H5OH) is 46. ((12 ✕2) +(1✕6) +16)
Work out the Moles 1.8/46 and this gives 0.039 moles.
Now we need to find the enthalpy change.
14212/0.039 and this gives us 364.4 KJMol-1.
One last thing is you need to put -363.4 KJMol-1 because this reaction has to be exothermic as it’s a combustion experiment.
How to improve the experiment?
You could improve the experiment by:
-Add drought shield. This would reduce the movement of the air or the movement of the flame which means the energy is released but not for the water.
-Use metal can or calorimeter. This would conduct heat to the water quickly.
-Add a lid. This would prevent convection of hot air, so all the energy stays as much as possible in the can/ cold air doesn’t interact.
-Holes in the drought shield. This is to allow oxygen to enter to make a complete combustion.
Where could the student have gone wrong?
-Incomplete combustion
-Evaporation of the fuel.
-Didn’t stir in the experiment.
Exam question you could get asked (1 or 2 mark):
Why is the student value different from the data book values?
For this question you only need to say that the experiment wasn’t done under standard conditions and need to use a Bomb calorimeter.
The Energy from the calorimetry experiment of solutions.
This experiment is often for the enthalpy change of neutralisation.
If you don’t know the enthalpy of neutralisation please go check the notes for this.
You would have your apparatuses like on the left and this is how the experience is carried out.
Add the acid in the polystyrene cup.
Measure the temperature with a thermometer. This can take a bit of time.
Then you would add the alkali or the solid and stir the mix.
Measure the temperature change when the temperature stops rising.
You would as well need to know the volume of your solution you have added as both of them added together will be your mass.
As you can see on the diagram we need a lid and the polystyrene to prevent heat loss as well.
You would calculate the energy and the enthalpy change with the same equation as above:
Q=McΔT and ΔH=Q/n
You will probably need to use the concentration and the volume to find the moles of your solution. C=n/V
Extrapolating the data to find the change in temperature due to rapid increase in temperature.
This would be the graph you could get if you have to plot the temperature change over the time of the reaction. As you can see the temperature will rise very quickly from the start of the reaction to the end. This could be the temperature change. Therefore you would get an uncertain temperature rise like is shown with the uncorrected temperature rise.
Example:
25.0 cm3 of 1 moldm-3 HCl had a temperature of 20°C. 25 cm3 of 1 moldm-3 of NaOH was added and this raised the temperature to a maximum of 26°C. Calculate the enthalpy of neutralisation for hydrochloric acid.
Find the mass of the whole solution: 25.0 +25.0=50 cm3
Use the equation Q=McΔT and put the value you know.
50✕4.18✕6= 1254J or 1.254 KJ.
Find the moles of HCl using the concentration formula, C=n/V
1✕25✕10-3 =0.0250 moles Make sure your unit is in dm-3 so divide by 1000.
Find the enthalpy using the equation, Q/n
1.254/0.0250=50.16 KJ mol-1
You would put a negative sign in front -50.16 KJ mol-1 as this reaction is exothermic.
If you don’t know when to put a negative sign for an exothermic reaction or endothermic look at the temperature change. If the temperature increases its exothermic but if the temperature decreases this is endothermic.
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