The equations for the reactions occurring are: reaction 1: K2CO3(s) + 2HCl(aq) → 2KCl(aq) + CO2(g) + H2O(l) reaction 2: KHCO3(s) + HCl(aq) → KCl(aq) + CO2(g) + H2O(l) 1. Calculate the energy change (Q) for each reaction in J. The specific heat capacity of water is 4.2 J g–1 °C–1. 2. Calculate the enthalpy change, ∆H, for each reaction in kJ mol–1 . Hint: Use the first law of thermodynamics considering a system at constant pressure. Potassium carbonate Potassium hydrogencarbonate Mass of weighing boat with the chemical (g) 5.35 6.23 Mass of empty weighing boat (at the end of the trial) 2.25 3.01 Mass of chemical used (g) 3.1 3.22 Start temperature (oC) 23.2 23.1 Highest (or lowest) temperature (oC) 28.4 19.2 Temperature change 5.2 3.9 3. Use your results and Hess’s law to calculate the enthalpy change for the thermal decomposition of potassium hydrogen carbonate: reaction 3: 2KHCO3(s) → K2CO3(s) + CO2(g) + H2O(l)
The equations for the reactions occurring are:
reaction 1: K2CO3(s) + 2HCl(aq) → 2KCl(aq) + CO2(g) + H2O(l)
reaction 2: KHCO3(s) + HCl(aq) → KCl(aq) + CO2(g) + H2O(l)
1. Calculate the energy change (Q) for each reaction in J.
The specific heat capacity of water is 4.2 J g–1 °C–1.
2. Calculate the enthalpy change, ∆H, for each reaction in kJ mol–1 .
Hint: Use the first law of
|
|
Potassium carbonate |
Potassium hydrogencarbonate |
|
Mass of weighing boat with the chemical (g) |
5.35 |
6.23 |
|
Mass of empty weighing boat (at the end of the trial) |
2.25 |
3.01 |
|
Mass of chemical used (g) |
3.1 |
3.22 |
|
Start temperature (oC) |
23.2 |
23.1 |
|
Highest (or lowest) temperature (oC) |
28.4 |
19.2 |
|
Temperature change |
5.2 |
3.9 |
3. Use your results and Hess’s law to calculate the enthalpy change for the thermal decomposition of potassium hydrogen carbonate:
reaction 3: 2KHCO3(s) → K2CO3(s) + CO2(g) + H2O(l)
Important: Pay attention to the signs of Q and delta H for endothermic and exothermic reactions.
4.Comment on why it is difficult to measure the enthalpy change in the decomposition of potassium hydrogen carbonate directly.
5. Is the final reaction endothermic or exothermic?
6. Does your result match the expected result from available data? If not, provide reasons that may have led to the discrepancy.
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