The reaction cannot be determined in the laboratory experimentally but by demining the enthalpy change when calcium carbonate reacts with hydrochloric acid, and applying the principles of Hess’s law, this enthalpy change can then be measured.
Safety
Calcium is reactive with both water and acids. Care should be taken when water is added and eye protection worn at all times. Calcium is highly flammable.
Apparatus
- Eye protection
- Polystyrene cup
- 250 cm3 beaker
- 100 cm3 measuring cylinder
- -50 to 50 °C thermometer
- 3 g calcium metal
- 8 g of powdered calcium carbonate
- 1.0 M HCL (aq) approximately 500 cm3
- Access to balance weighing ± 0.1g
Method
Experiment 1: Calculating the enthalpy change for the reaction between calcium metal and hydrochloric acid.
- 1.03 g of calcium was measured, number of moles of calcium used was calculated:
- g ÷ 40 = 0.02575 mol.
- 100 cm3 of 1.0 M HCL (aq) was measured out into a polystyrene cup using a measuring cylinder.
- The temperature of the acid was measured at 22°C.
- After taking the temperature, calcium metal was added to the acid, and the highest temperature rise was recorded at 42.2°C.
- Thus indicating that the reaction was exothermic and the temperature increase was:
ΔT= 20.2 °C.
Experiment 2: Calculating the enthalpy change for the reaction between calcium carbonate and hydrochloric acid.
- 2.65 g of was measured and the number of moles of calcium carbonate powder used was: 2.65 ÷ 100 = 0.0265 mol.
- 100 cm3 of 1.0 M HCL (aq) was measured out into a polystyrene cup and placed into a 250 cm3 beaker.
- The temperature of the acid was initially recorded at 21.6°C, and the final rise was recorded at 22.4°C, thus indicating a 0.8°C rise in temperature.
Results
The enthalpy change for the reaction between calcium metal and hydrochloric acid was calculated using the equation:
Enthalpy change = m X c X ΔT.
100 cm3 X 4.2 kJ kg-1 K-1 X 20.2K = -8484 J.
Enthalpy change in kJ mol-1 based on the number of moles of calcium metal used is:
-8484 X 0.02575 = -218.463 kJ mol-1.
The enthalpy change for the reaction between calcium carbonate and hydrochloric acid was calculated using the equation:
Enthalpy change = m X c X ΔT.
100 cm3 X 4.2 kJ kg-1 K-1 X 0.8K = -336 J.
Enthalpy change in kJ mol-1 based on the number of moles of calcium carbonate used is:
-336 X 0.265 = -89.04 kJ mol-1.
Reaction for the reactions in the experiments are:
Ca(s) + 2HCl → CaCl2 + H2
CaCO3 +2HCl → CaCl2 + H2O + CO2
