Enthalpy of formation of calcium carbonate

       ∴By E = mc△T,

      △H per gram of calcium
= [(100/1000)(4200)(28)]/ 1.0909
= -10780J g-1
= -10.780kJ g-1

For the second experiment:

∵Energy evolved by the reaction= Energy absorbed by the acid

∴By E = mc△T,

     △H per gram of calcium = [(100/1000)(4200)(26)]/ 1.0538
= -10362J g-1
= -10.362kJ g-1

3. Calculate the average heat evolved by one mole of calcium.
   For the first experiment:
   △H per one mole of calcium
   =[(100/1000)(4200)(28)]/ [1.0909/40.08]
   =-432066 J mol-1
   =-432.066 kJ mol-1

For the second experiment:
△H per one mole of calcium
=[(100/1000)(4200)(26)]/ [1.0538/40.08]
=-415329 J mol-1
=-415.329 kJ mol-1

Average △H per one mole of calcium  (ΔHx)
 =(432.066 +415.329)/2
=-423.698 kJ mol-1

7. Why is the exact concentration of the hydrochloric acid unimportant?

    Approximate concentration of hydrochloric acid is not considered as one of the errors. In the reactions, limiting agents, i.e. calcium and calcium carbonate, must be reacted completely. Only these matter in the reaction but not the amount of H+(aq) provided in hydrochloric acid. Indeed, the concentration of the hydrochloric acid just affects the rate of the reaction. Therefore the exact concentration of hydrochloric acid is unimportant provided that there is enough H+(aq) to react with limiting agents completely.

8. Should we measure the volume of acid with a measuring cylinder? Why?
   
   The measuring cylinder should not be used to measure the volume of acid. This is because the heat absorbed by the acid must be counted when calculating the heat change of the reaction. As the acid has very high specific heat capacity, the errors in calculating the heat change of the reaction will be very significant if the heat absorbed by acid is not taken into account. Thus, the volume of acid should be obtained accurately to apply into E=mc△T in order to calculate the heat absorbed by the acid accurately. As the scale of the measuring cylinder is far from accurate, the pipette should be used instead.
   

2. Reaction of calcium carbonate with dilute hydrochloric acid

1.        3.0940g of dry powdered calcium carbonate was weighed out accurately and placed directly into a clean dry plastics cup.

2.        100 cm3 of approximately 1 M hydrochloric acid was pipetted into another beaker.

3. The acid was poured on the carbonate in the plastic beaker.
4. The solution was stirred briskly with the thermometer and the maximum temperature reached by the solution. was recorded.
5. The experiment was repeated with 2.7400g of dry powdered calcium carbonate.

Results:

Calculations and Discussion:

1. Write an ionic equation for the reaction taken place. (Call this Equation 3)
   
   CO32-(aq.) + 2 H+(aq.)  CO2(g) + H2O(l)

2. Calculate the average heat evolved by one mole of calcium carbonate.

(Making the same assumptions as in Part A)
For the first experiment:
∵Energy evolved by the reaction= Energy absorbed by the acid

∴By E = mc△T,

  △H per mole of calcium carbonate
= [(100/1000)(4200)(2)]/[(3.0940/(40.08+12.01+16x3)]
=840  /[(3.0940)/(100.09)]
=-27174 J mol-1
=-27.174 kJmol-1

                                        For the second experiment:

∵Energy evolved by the reaction= Energy absorbed by the acid

∴By E = mc△T,

  △H per mole of calcium carbonate
= [(100/1000)(4200)(2)]/[(2.7400/(40.08+12.01+16x3)]
=840  /[(2.7400)/(100.09)]
=-30685 J mol-1
=-30.685 kJmol-1

Average △H per one mole of calcium   (ΔHy)
=(27.174 +30.685)/2
=-29.080kJ mol-1

                                                                 

1. Draw an energy-cycle linking Equations 1, 2 and 3 together, you must include reactions that had not been performed in the experiment.

           2H+(aq.) +Ca(s) + C(s) + 3/2 O2(g)                  CaCO3(s)  + 2H+(aq.)

            ΔHx                                                  ΔHy

                             

         Ca2+(aq.) + H2(g) + C(s) + 3/2 O2(g)                            Ca2+(aq.) +   CO2(g) + H2O(l)

1. Besides your experimental results, what other information do you need to enable you to calculate the heat of formation of calcium carbonate? Look up these necessary data from any suitable source.
   
   The enthalpy change of formation of water: -286 kJ mol-1
   The enthalpy change of formation of carbon dioxide: -393 kJ mol-1
   
   
   

5.   Calculate the heat of formation of calcium carbonate.

ΔHθf [CaCO3(s) ]

=ΔHx  +ΔHθf [CO2(g)]  +ΔHθf [H2O(l) ]  -ΔHy

=-423.698-393-286+29.080
=-1073.6 kJ mol-1

6.  As far as you can, list out the major sources of inaccuracy in the experiment and suggest ways to improve them whenever possible.
   
Sources of error:
- Heat loss to surrounding by evaporation, conduction and radiation.       
-The thermometer absorbed some energy.
- The specific heat capacity and the density of the solution are not actually the same as those of the water.
-Some of the samples failed to dissolve completely       
- The reading of the thermometer is not accurate enough.
-The experiment was not exactly carried out under standard conditions.
-Some heat was gained by the gases, a considerable amount of heat is loss to surroundings when the gases are releasing.
-The lid covering the plastic cups cannot prevent heat loss efficiently.
-Heat capacities of plastic cups and thermometer were ignored.
-The samples were not pure calcium and pure calcium carbonate due to air oxidation and reaction with moisture in air.

Improvements
-Use the vacuum flask calorimeter with a cork stopper instead of the polystyrene foam cup
-Use a more accurate reading thermometer (e.g. Beckmann thermometer)
- Find out the specific heat capacity of other materials other than water.
-Use sand paper to remove the oxide layer on calcium metal.
-Use powdered calcium instead of the granules in order to increase the rate of reaction ,so that the heat loss to the surroundings can be reduced

7. State the law which you have used in order to answer Question (5).
   
   The heat of formation of calcium carbonate cannot be determined directly by calorimetric experiments as there are several experimental difficulties    :
   -The extent of the reaction cannot be controlled
   -Heat evolved cannot be separated into appropriate terms
   -Direct combustion of calcium can be violent
   -Side reactions may arise, for example,
   2Ca(s) + O2(g)  2CaO(s)

C(s) + O2(g)  CO2(g)
Then ,Hess’s Law is applied to calculate the heat of formation of calcium carbonate. Hess’s law states that the energy change for any chemical or physical process is independent of the pathway or number of steps required to complete the process provided that the final and initial reaction conditions are the same. In other words, an energy change is path independent, only the initial and final states being of importance. This path independence is true for all state functions

3. State the law which you have used in order to answer Question (7) depends? Why is this principle useful?
   
   The law of conservation of energy states that energy can neither be created nor destroyed but can be changed from one form to another.In order to achieve the answer, Hess’s law is used. Hess’s law states that the total enthalpy change of a reaction is independent of the route by which the reaction takes place. In other words, the standard enthalpy change of a reaction depends on the differences in standard enthalpy between the reactants and the products. It means that the enthalpy of the reaction system is conserved. As the absolute enthalpy of a substance is not possible to be determined and only the difference between reactants and products can be measured experimentally. This law helps us to define the standard enthalpy change of a reaction.
   

Conclusion
The enthalpy of formation of calcium carbonate is -1073.6kJ mol-1

Reference

http://hk.knowledge.yahoo.com/question/question?qid=7007111800043
http://www.answers.com/8.%09Should+we+measure+the+volume+of+acid+with+a+measuring+cylinder%3F+Why%3F
http://hk.search.yahoo.com/search/kp?ei=UTF-8&p=word%E6%89%93%E5%88%86%E7%B7%9A&rd=r1&fr2=tab-web&fr=FP-tab-web-t
F.6 Chemistry Notes Section III by Ms Sin W L

THE END