HessLawmagnesiumsulphatePage 1 of 3
IB CHEMISTRY INTERNAL ASSESSMENT
Experiment: Using Hess Law (SL/HL): hydration of copper sulphate
Aim
To determine the enthalpy change for the reaction
CuSO4 (s) + 5H2O (l)  CuSO4.5H2O (s)
Introduction
It is impossible to measure the enthalpy change for this reaction directly because if we add 5
moles of water to 1 mole of copper (II) sulphate, we do not produce hydrated copper (II) sulphate
crystals. These can only be made by crystalisation.
However, you can measure the enthalpy change of solution for the following two solids shown in
the equation below:
CuSO4 (s) + 100H2O (l)
 CuSO4 (aq, 100H2O)
CuSO4.5H2O (s) + 95H2O (l)  CuSO4 (aq, 100H2O)
Assessment opportunities: practice data collection and processing
Aspect 1
Aspect 2
Defining the problem and
selecting variables
Aspect 3
Controlling variables
Levels/marks
Developing a method for
collection of data
Formulates a focused
Designs a method for the
Develops a method that allows
problem/research question and effective control of the variables. for the collection of sufficient
identifies the relevant variables.
relevant data.
Formulates a problem/research
question that is incomplete or
identifies only some relevant
variables.
Designs a method that makes
some attempt to control the
variables.
Does not identify a
problem/research question and
does not identify any relevant
variables.
Designs a method that does not Develops a method that does
control the variables.
not allow for any relevant data to
be collected.
0
1
2
0
1
Develops a method that allows
for the collection of insufficient
relevant data.
2
0
1
2
Requirements and materials

safety goggles

distilled water

spatula

teat pipette

CuSO4 (s)

thermometer, 0 – 50 C

balance

CuSO4.5H2O (s)

2 polystyrene cups with lid
Procedure A: Heat of solution of CuSO4 (s) (=dried/anhydrous)
Complete/2
Partial/1
Not at all/0
total:
/6
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Use 0.10 mol of anhydrous copper (II) sulphate and 100 cm3 of water.
Procedure B: Heat of solution of CuSO4.5H2O (s)
Use 0.10 mol of hydrated copper (II) sulphate but using only 91 cm3 of water as the hydrated salt
already contains 9 cm3 (0.5 mol) of water.
Data collection and processing
Under the heading “Measurements and Observations”, design an appropriate results table which
contains all relevant raw data which will allow you to calculate the enthalpy change using Hess’s
Law.
Under the heading “Calculation” , use the raw data recorded in the above results table to
calculate the enthalpy change for the reaction given. Draw an enthalpy cycle diagram. Show
clearly how all raw data has been processed and carry out a propagation of uncertainty.
Assessment opportunities: conclusion and evaluation
Aspect 1
Aspect 2
Concluding
Aspect 3
Evaluating procedure(s)
Levels/marks
Improving the investigation
States a conclusion, with
Evaluates weaknesses and
justification, based on a
limitations.
reasonable interpretation of the
data.
Suggests realistic
improvements in respect of
identified weaknesses and
limitations.
States a conclusion based on a Identifies some weaknesses
reasonable interpretation of the and limitations, but the
data.
evaluation is weak or missing.
Suggests only superficial
improvements.
States no conclusion or the
conclusion is based on an
unreasonable interpretation of
the data.
0
1
2
1
2
0
1
Partial/1
Not at all/0
Identifies irrelevant weaknesses Suggests unrealistic
and limitations.
improvements.
0
Complete/2
2
total:
/6
The accepted value = - 62 kJ mol-1.
Conclusion and Evaluation (CE)
!st aspect:




Write a conclusion (= reply to your aim) based on the experimental results.
Indicate how you have used/interpreted the experimental results to arrive at your conclusion.
Give an explanation for your conclusion using your Chemistry knowledge.
Compare your experimental results with literature values or expected values (if available) and
state if possible a % error and if it is positive (more) or negative (less)
2nd aspect:
HessLawmagnesiumsulphatePage 3 of 3
Evaluation of result
Describe any limitations in the way you have interpreted your results e.g. have you ignored any
variables which you have not measured or could not have measured?
Evaluation of procedure (=materials/equipment + design/method) consists of:
Identification of systematic errors which are errors due to the quality of the equipment and
materials, poor experimental design and ‘incorrect’ use of the equipment. These errors cannot be
calculated and are also difficult to evaluate. However, these systematic errors can be reduced by
using better equipment/materials or improved experimental technique.
a. Evaluation of materials/equipment: Measuring tools improperly calibrated? Accurate
enough? Incorrect concentration of reagents? Impure reagents? Amounts of reagents
used large enough?
b. Evaluation of method: Are there any weaknesses in the method which could have caused
an error greater than the % uncertainty? Did you make any errors when carrying out the
experiment e.g. did you not do some thing which you should have done or did you do it
incorrectly? Were some variables not controlled? Were readings duplicated?
c. Evaluation of result: describe any limitations to the way you have interpreted your results
e.g. have you ignored any variables which you could have measured or have not
measured? Have you used all the raw data?
For each identified limitation in (a) or (b), weakness or error indicate the direction of its effect on
the experimental result i.e. would it have caused your experimental result be more or less.
Limitation/weakness
How much did it affect my result
Materials/equipment


Design/method


Evaluation of quality of the result:
3rd aspect
For each suggested weakness, limitation or error suggest improvements
Both the 2nd and 3rd aspect can be done using a table as shown below. There is no need to
always fill something in each row.
Limitation weakness
1.
2.
improvement