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Q2. What are “standard conditions” for thermochemical calculations?
A2. It is defined as elements or compounds in their normal physical states, a pressure of 1 atm (101325 Nm-2), and a temperature of 25 oC (298 K).
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Q3. Write the equation for the formation of calcium carbonate under standard conditions. (Call this Equation 1)
A3. Ca(s) + C(s) + 3/2O2(g) → CaCO3(s)
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Q4. Write an ionic equation for the reaction taken place.
(Call this Equation 2)
A4. Ca(s) + 2H+(aq.) → Ca2+(aq.) + H2(g)
Q5. Calculate, in each experiment, the heat change in the reaction between the calcium and the acid per gram of calcium.
A5. Energy evolved by the reaction
= Energy absorbed by the acid
By E = mcT,
1st experiment: Heat change = (0.1)(4200)(26.5)/1.07
= 10402J = 10.40kJ g-1
2ndexperiment: Heat change = (0.1)(4200)(25.5)/1.04
= 10298J = 10.30kJ g-1
Q6. Calculate the average heat evolved by one mole of calcium.
A6. Number of mole of calcium = 1/40.1 = 0.0249 mol
Average heat energy evolved = (10402+10298)/2 = 10350J
Average heat evolved by one mole of calcium
=10350/0.0249 = 415.7 kJ mol-1
Q7. Why is the exact concentration of the hydrochloric acid unimportant?
A7. While the mass of calcium metal is the limiting factor, the concentration of the hydrochloric acid just affects the rate of the reaction and should be excess. So, it is not important to the exact concentration.
Q8. Should we measure the volume of acid with a measuring cylinder? Why?
A8. We should not use the measuring cylinder. We need to obtain the volume of acid used to apply into E=mcT. While the scale of the measuring cylinder is far from accurate, we should use the pipette.
B. Reaction of calcium carbonate with dilute hydrochloric acid
Results :
Questions:
Q1. Write an ionic equation for the reaction taken place.
(Call this Equation 3)
A1. CO32-(aq.) + 2 H+(aq.) → CO2(g) + H2O(l)
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Q2. Calculate the average heat evolved by one mole of calcium carbonate.
A2. Energy evolved by the reaction
= Energy absorbed by the acid
By E = mcT,
1st experiment: Heat change = (0.1)(4200)(1.5)/2.52
= 250J = 0.25kJ g-1
2ndexperiment: Heat change = (0.1)(4200)(1.5)/2.50
= 252J = 0.25kJ g-1
Number of mole of calcium carbonate = 1/(40.1+12+16*3) = 0.01 mol
Average heat energy evolved = (250+252)/2 = 251J
Average heat evolved by one mole of calcium carbonate
=251/0.01 = 25.1 kJ mol-1
Q3. Draw an energy-cycle linking Equation 1,2 and 3 together, you must include reactions that had not been performed in the experiment.
A3.
Ca(s) + C(s) + 3/2O2(g) + HCl(aq.) → CaCO3(s) + HCl(aq.)
H2(g) + CaCl2(aq.) + C(s) + 3/2 O2(g) → CaCl(aq.) + CO2(g) + H2O(l)
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Q4. 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.
A4. The enthalpy change of formation of water: -286 kJ mol-1
The enthalpy change of formation of carbon dioxide: -393 kJ mol-1
Q5. Calculate the heat of formation of calcium carbonate.
A5. Enthalpy change of formation of calcium carbonate
= {enthalpy change of formation of carbon dioxide + enthalpy change of equation 2}-{enthalpy change of equation 3 + enthalpy change of formation of water}
= (-415.7)+ (-393)+(-286)-(-25.1)
= -1069.6 kJ mol-1
Q6. As far as you can, list out the major sources of inaccuracy in the experiment and suggest ways to improve them whenever possible.
A6. Source of errors:
- Heat loss to surrounding by evaporation, conduction and radiation.
- The thermometer absorbed some energy.
- There are two assumptions which are the specific heat capacity and the density of the solution are the same as those of the water.
- Some of the samples stick in the weighing bottle.
- The reading of the thermometer is not accurate enough.
Improvement:
- Use the vacuum flask calorimeter instead of the polystyrene foam cup or use the cotton wool to cover the outer surface of the cup.
- Use a more accurate reading thermometer. (i.e. smaller scale)
- Find out the specific heat capacity of the solution and the thermometer.
- Use the method of “weighing by difference” to find out the amount of the samples used.
Q7. State the law which you have used in order to answer Question (5).
A7. Hess’s law.
Hess’s law of constant heat summation states that the total enthalpy change accompanying a chemical reaction is independent of the route by which the chemical reaction takes place.
Q8. State the law which you have used in order to answer Question (7) depend? Why is this principle useful?
A8. The law of conservation of energy states that energy can neither be created nor destroyed but can be changed from one form to another.
The importance of Hess’s law lies in the fact that enthalpy changes can be calculated for reactions that cannot be carried out in the laboratory, or reactions that are too slow or involve the formation of side products. So, it is useful.
Discussion:
- The amount of the acid should be sure that it is excess with a view to using up calcium and calcium carbonate.
- In the assumption, the density and specific heat capacity of acid are supposed to be same as the water. For the aim to be more accurate, we can find out the density and specific heat capacity of the acid.
- The enthalpy of formation can also be found by the summary of all the average bond enthalpy. However, it is very inaccurate because the energy required to break a particular bond varies from one compound to another, and even within the same compound.
Conclusion:
The enthalpy of formation of calcium carbonate is -1069.6 kJ mol-1
Reference : Miss Sin’s Note
Comprehensive Chemistry Physical Chemistry I(5th Edition)
Chem-is-try(Physical Chemistry Questions Review)
