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Determination of the Heat of Formation of Calcium and Calcium Carbonate

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Chemistry

Laboratory report 6 –

Determination of the Heat of Formation of Calcium and Calcium Carbonate

Name: Suen Wing Sze

Name of partner: Sher Wai Kei

Date: 5/11/2008

Title of the experiment: Determination of the heat of formation of calcium and calcium carbonate

Aims: To determinate the amount of heat energy liberated when calcium reacts with dilute hydrochloric acid and when calcium carbonate reacts with dilute hydrochloric acid.

Introduction: 1.  Heat of formation is the enthalpy change of the reaction when one mole of substance is formed from its constituent elements in the standard states under standard conditions.

2.  Ca(s) + C(graphite) + image00.pngO2(g)→ CaCO3(aq)

3.  Standard conditions of thermochemistry are defined as :

(a) elements or compounds in their normal physical states,

(b) a pressure of 1 atm (101325 Nm-2), and

    (c) a temperature of 25 (298 K).

Reaction of calcium with dilute hydrochloric acid

Procedure:

  1. Approximately excess 1g of calcium metal had been weighed out. The weighing was then being recorded (two decimal places is sufficient accuracy).
  2. Measuring cylinder had been used to measure out 100cm3 of approximately 1M hydrochloric acid and place it in a vacuum flask.
  3. The temperature of the acid had been determined and recorded.
  4. Then the weighted portion of calcium was added and the solution had been stirred thoroughly with a stirrer until all the metal had been reacted.
  5. The maximum temperature attained by the solution had been record.

Data:

Mass of Ca used /g

1.03g

Final temperature /

54.50

Initial temperature /

26.00

Temperature rise /

28.50

Discussion:

4.  Ca(s) + 2H+(aq)→ Ca2+(aq) + H2(g)

5.  Heat of formation of calcium chloride will be evolved.

6.  E = mcT

= (100)(4.2)(28.5)

= 11970 J  

 The heat evolved in the reaction between calcium and acid is 11.970 kJ.

Number of mole of calcium used = image01.png

                           = 0.02569 mol

Heat would have been evolved by one mole of calcium atoms

= image02.png

=  465.94 kJ mol-1

  1. In the calculation, only the mass of calcium is the limiting factors. The concentration and the volume of hydrochloric acid do not involve in the calculation. It just affects the rate of reaction and can be excess. Therefore, the exact concentration of the hydrochloric acid is unimportant.

Reaction of calcium carbonate with dilute hydrochloric acid

Procedure:

1.  Approximately excess 1g of calcium carbonate had been weighed out. The weighing was then being recorded (two decimal places is sufficient accuracy).

  1. Measuring cylinder had been used to measure out 100cm3 of approximately 1M hydrochloric acid and place it in a vacuum flask.
  2. The temperature of the acid had been determined and recorded.
  3. Then the acid was added on the carbonate in the vacuum flask. The solution had been stirred thoroughly with a stirrer until all the metal had been reacted.
  4. The maximum temperature attained by the solution had been record.

Data:

Mass of CaCO3 used /g

2.73g

Final temperature /

27.00

Initial temperature /

26.00

Temperature rise /

1.00

Discussion:

  1. CO32-(aq) + 2H+(aq)→ CO2(g) + H2O(l)

9.  Heat of formation of carbon dioxide, heat of formation of water and hear of formation of calcium chloride.

10. E = mcT

= (100)(4.2)(1.00)

= 420 J

 The heat evolved in the reaction between calcium and acid is 0.42 kJ.

Number of mole of calcium carbonate used = image03.png

                                      = 0.027273 mol

    Heat would have been evolved by one mole of calcium carbonate

    = image04.png

    = 15.4 kJ mol-1

For the two experiments

11.  The powered calcium carbonate may be lost during experiment.

    The reading of the thermometer is not accurate enough.

    The thermometer and the vacuum flask have absorbed some energy.

12.  For the calculation in question 6,
the theoretical value of standard enthalpy change
= ΔH
f [CaCl2(aq)] - 2ΔHf [HCl(aq)]
= -795 – 2(-92.3)
= - 610.4 kJ mol
-1
while the experimentally determined value was- 465.94 kJ mol
–1
 the experimentally determined value was less then the standard enthalpy

        change by 144.46 kJ mol–1

    For the calculation in question 10,

the theoretical value of standard enthalpy change

    = ΔHf [CO2(g)] + ΔHf [H2O(l)] + ΔHf [CaCl2(aq)] - ΔHf [CaCO3(aq)] - 2ΔHf [HCl(aq)]
= -393.5 + (-285.8) + (-795) – (-1207) – 2(-92.3)
= -82.7 kJ mol
–1
while the experimentally determined value is –15.4 kJ mol
-1
 the experimentally determined value is less than the theoretical value by 67.3                       kJ mol-1

13.

14.  We should also find out the heat of formation of carbon dioxide and heat of formation of water.

15.  The heat of formation of calcium carbonate

    = △H1 + △Hf [CO2] + △Hf[H2O] - △H2

    = -65.94 + (-393.5) + (-285.8) + (-15.4)

    = -1160.64 kJ mol-14

16.  Hess’s law. It states that the total enthalpy change accompanying a chemical reaction is independent of the route by which the chemical reaction takes place. In the other words, the standard enthalpy change of a reaction depends only on the difference in standard enthalpy between the reactants and the products.

17.  The law of conservation of energy states that energy can neither be created nor destroyed but can be changed from one form to another.

18.  It is impossible to measure the enthalpy change of formation of CaCO3(s) from the reaction between Ca(s), C(graphite) and image00.pngO2 in the laboratory because these constituent elements do not combine to give CaCO3(s) as the only product. Therefore, by Hess’s law, we can link the equation to CO2(g), H2O(l) and CaCl2(aq) as the enthalpy change of formation of carbon dioxide, the standard enthalpy change of formation of water and the formation of calcium chloride can be determined experimentally.

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