F.6/7 Chemistry Practical: Enthalpy of Formation of Calcium Carbonate
Objective: To determine enthalpy of formation of calcium carbonate
--------------------------------------------------------------------------------------------------------------------Introduction
Molar enthalpy of formation of calcium carbonate is the enthalpy change for the formation of 1
mole of calcium carbonate according to the following equation at s.t.p:
In this experiment, enthalpy of formation of calcium carbonate is determined indirectly by
measuring the following molar enthalpy change
(1) Ca(s) + 2HCl(aq) -------------> CaCl2(aq) + H2(g)
H 1
(2) CaCO3(s) + 2HCl(aq) --------> CaCl2(aq) + CO2(g) + H2O(l)
H 2
----------------------------------------------------------------------------------------------------------------------Theory (Write this section with the help of the following questions)
1. Work out an energy cycle to relate Hf, H 1 and H 2 . [Hints: molar enthalpy of formation of
water, -285 kJ mol-1 and molar enthalpy of formation of carbon dioxide, -393 kJ mol-1 are
necessary data in the energy cycle.]
2. In procedure A, temperature change, , for the reaction of calcium and hydrochloric acid is
determined. Give a formula for the calculation of 'heat given out, Q' by using the temperature
change. (Use the following symbols in your formula: mw for mass of hydrochloric acid, Cw for
specific heat capacity of the resulting solution,  for the temperature change)
3. Show that calcium metal is the limiting reagent in step 3 of procedure A.
4. If x g of calcium is used to react with 150 cm3 of hydrochloric acid, express H1 in terms of Q
and x.
-------------------------------------------------------------------------------------------------------------------Chemicals:
0.9 g calcium, 2.5 g anhydrous calcium carbonate (in powder form), 300 cm3 of 2 M hydrochloric
acid
Apparatus: -10 - 110 oC thermometer, readable 0.5 oC, polystyrene cup with cover, capacity of 200
cm3 and electronic balance, readable 2 digits.
Procedure
A. Reaction of calcium with dilute hydrochloric acid
1.
Weigh out approximate 0.9 g of calcium, recording the mass to two decimal places.
2.
Using a measuring cylinder, measure out 150 cm3 of 2 M hydrochloric acid and place it in a
polystyrene beaker. Measure the temperature of the acid.
3.
Add the weighed portion of calcium metal into the acid and stir thoroughly with the
thermometer until all the metal has reacted.
4.
Record the maximum temperature attained by the solution. (If there is time, repeat the
experiment and determine the average temperature rise per gram of calcium used.).
B. Reaction of calcium carbonate with dilute hydrochloric acid
1.
Weigh out 2.5 g of dry, powdered calcium carbonate on a piece of clean paper. Transfer the
En03_CaCO3 /p.1
carbonate into a clean polystyrene beaker.
2.
3.
4.
Record the mass to one decimal places.
Place 150 cm3 of approximate 2 M hydrochloric acid in a measuring cylinder. Record its
temperature
Pour the acid on the carbonate in the beaker. Stir briskly with the thermometer and record the
maximum temperature reached by the solution. (If there is time, repeat the temperature and
determine the average temperature rise per gram of magnesium carbonate used.)
Data and results [You should prepare the table of data before going to the laboratory for
practical] A suggested format of data table is given below.
Procedure A
Procedure B
Mass of Ca + container /g
Mass of CaCO3+container /g
Mass of container /g
Mass of container /g
Mass of Ca used /g
Mass of CaCO3 used /g
o
Final temperature / C
Final temperature /oC
Initial temperature /oC
Initial temperature /oC
Temperature rise /oC
Temperature rise /oC
Calculation
Calculate molar enthalpy of formation of calcium carbonate.
Discussion (Write this section with the help of the following questions)
1. Which reagent is in excess in step 1-2? Calcium or hydrochloric acid? Show that one of them is
in excess by calculation.
2. It is desirable that one of the reagents is in excess. Explain.
3. What assumptions have you made in the calculation?
4. Give (a) random error sources and systematic error sources in this practical.
5. Try to estimate percentage error of H 1 and H 2 .
6. On which thermodynamic principle does Hess's law of constant heat summation depend?
7. Can you measure molar enthalpy of formation directly from the reaction between carbon,
oxygen and calcium? Why?
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En03_CaCO3 /p.2