Chemistry
Sensors:
Loggers:
Temperature
Any EASYSENSE
Logging time: 5 minutes
Teacher’s notes
11 Measurement of enthalpy changes:
Endothermic reaction
Read
Heat released or absorbed by a reaction can be measured by the increase or decrease in temperature.
The measuring of the temperature changes in an experiment is calorimetry. In it simplest form
calorimetry only needs a reaction vessel and a thermometer.
Accuracy in the measurement comes from using non heat conducting materials and more accurate
thermometers. In this investigation the main improvement in the measurements come from the ability
to capture the whole heat change in the reaction. When using a thermometer the observer can often
miss the peak change, the data logger does not wander in its concentration.
Results for the simple experiments indicated produce good values close to published results.
Apparatus
1.
An EASYSENSE logger.
2. A Smart Q Temperature sensor.
3. 1.5 mol dm-3 Citric acid.
4. 10 g Sodium hydrogen carbonate (bicarbonate).
5. Polystyrene cup.
6. 250 cm3 beaker.
7. Measuring cylinders.
8. Accurate balance.
Set up of the software and logger
Each data collection is timed at 5 minutes; this allows a good margin for capturing the whole
experiment.
Use the setup file 11 Calorimetry Endothermic
The settings for logging are summarised in the table below.
Recording method
Time
Graph
5 minutes
L3 Chemistry
T11 - 1(V2)
Notes
The polystyrene cups usually fit into the 250 cm3 beakers with and air gap all round the cup. The air
gap adds to the insulating properties of the cup. If the polystyrene cups cannot be found then a
standard vending style cup can be used with additional insulating material to fill the air gap around the
cup.
A lid to go over the apparatus can make a significant difference to the accuracy of the results and
should be used if possible. The lid also helps prevent the acid vapour produced by the heat of the
reaction from escaping into the classroom.
The experiment is a very simple measurement of the heat produced by an endothermic reaction. The
practical can be extended by the logging of the temperature information and the mathematics used to
calculate the enthalpy change. Enthalpy does not have to be calculated for there to be value in the
practical.
Sample result
Graph of the endothermic reaction of Citric acid and Sodium bicarbonate.
Calculations
Heat released by reaction = heat gained by the solution
1.
2.
3.
4.
Heat gained by the solution = M x C x ΔT
M = Mass of solution (mass of acid + mass sodium hydrogen carbonate)
C = specific heat capacity, assumed to be 4.18 J g-1 K-1
ΔT = temperature change
Use the values of Tstart and Tfinal substituted into the calculation to find the energy transferred. Make
sure the volume of the acid and sodium hydrogen carbonate actually used are entered and not the
suggested volumes and masses.
L3 Chemistry
T11 - 2(V2)
Worked example
In a similar experiment 100.5 cm3 of acid were used, 4.2 g of sodium hydrogen carbonate added and a
temperature decrease of - 5.2oC was measured from the graph:
Substituting the experimental data into the calculation
M x C x ΔT
104.7 x 4.18 x -5.2 = - 2276 J (rounded up)
1. The formula mass of sodium hydrogen carbonate = 84
2. Moles of sodium hydrogen carbonate in the reaction is 0.500/24.3 = 0.0059 mol
Therefore, the heat absorbed per mole of sodium hydrogen carbonate = 2195/0.0206 = 110 473 J mol-1
or 110.4 kJ mol -1
Errors include,
1.
Assuming specific heat capacity of the reaction is mass of acid + mass of sodium bicarbonate =
same mass of pure water.
2. Accurate mass measurement of sodium bicarbonate.
3. Accurate measurement of acid volume (and hence mass).
4. Heat loss in the system, absorbed by sensor housing, lost to air by evaporation, lost to stirrer
if used.
The data collected can be used to calculate the entropy change of the system
Extension to calculations
ΔS (entropy) can be calculated
•
ΔS = -ΔH/T
Taking value of ΔH from this expt.
•
T = Kelvin temperature of room (298)
•
ΔS = - + 219.50 / 298 = + 0.736
Or + 736 J K-1 mol
–1
(This reaction will take place spontaneously)
L3 Chemistry
T11 - 3(V2)