Determination of the enthalpy change for the thermal
decomposition of potassium hydrogencarbonate
Principle
When potassium hydrogencarbonate, KHCO3, is heated, it decomposes to form potassium
carbonate, K2CO3. The object of this experiment is to determine the enthalpy change during
this reaction. This enthalpy change is difficult to measure directly, so an indirect method is
used.
1. Explain the following terms:
a) enthalpy
b) enthalpy change
2. Write an equation including state symbols for the thermal decomposition of the potassium
hydrogencarbonate to potassium carbonate, showing the products in their usual states
under standard conditions.
3. What are the standard conditions for thermochemistry?
4. Why is it difficult to determine this enthalpy change directly?
You are provided with 2 mol/dm3 hydrochloric acid, solid potassium carbonate and solid
potassium hydrogencarbonate. By determining the enthalpy change of reaction between
potassium carbonate and hydrochloric acid and that between potassium hydrogencarbonate
and hydrochloric acid, it is possible to obtain indirectly the enthalpy change for the
decomposition of potassium hydrogencarbonate.
Procedure
CARE Eye protection must be worn for both experiment 1 and experiment 2.
Experiment 1: The reaction of potassium carbonate with hydrochloric acid
The teacher will provide you with 20 cm3 of approximately 2 mol/dm3 hydrochloric acid into
a beaker. Take the temperature of the acid and record this in a table similar to table 1 (see next
page).
Accurately weigh between 2,5 g and 3,0 g of anhydrous potassium carbonate (K2CO3) in a
plastic beaker. Record the mass in a table similar to table 1.
Now add the acid to the weighed portion of K2CO3 and stir the mixture carefully with the
thermometer until all the solid has reacted. Rapid effervescence will occur. Be careful not to
lose any of the reaction mixture by spilling. Record the maximum temperature of the solution
after mixing.
5. Write an equation, including state symbols, for the reaction between potassium carbonate
and hydrochloric acid.
6. From your results in table 1, calculate the energy released or absorbed during the reaction
between the potassium carbonate and the acid.
(Assume that the specific heating capacities of all the solutions are the same as that of water
(i.e. 4,2 J/(gK)), and that the solutions have a density of 1,0 g/cm3. Assume that the specific
heating capacity of the plastic beaker is negligible.)
Table 1
Mass of potassium carbonate used
Temperature of the acid initially
Temperature of solution after mixing
Temperature change during reaction
g
o
C
C
o
C
o
7. Calculate the enthalpy change for one mole of potassium carbonate.
8. Why is the concentration of the acid unimportant?
Experiment 2: The reaction of potassium hydrogencarbonate with hydrochloric acid.
Repeat experiment 1 using an accurately weighed sample of potassium hydrogencarbonate
between 3,25 g and 3,75 g in place of potassium carbonate. Record all masses and
temperatures in a table similar to table 2.
Table 2
Mass of potassium hydrogencarbonate used
Temperature of the acid initially
Temperature of solution after mixing
Temperature change during reaction
g
o
C
C
o
C
o
9. Write an equation, including state symbols, for the reaction between potassium
hydrogencarbonate and hydrochloric acid.
10. From the results in table 2, calculate the energy released or absorbed during the reaction
between the potassium hydrogencarbonate and the acid. (You should make the same
assumptions as in question 6.)
11. Calculate the enthalpy change for one mole of potassium hydrogencarbonate.
12. Mention three major sources of error in your experiments.
13. Draw an enthalpy diagram linking the reaction of K2CO3 (s) with HCl(aq), the reaction of
KHCO3 (s) with HCl(aq) and the decomposition of KHCO3 (s).
14. What is the enthalpy change for the decomposition of potassium hydrogencarbonate?
15. What law have you used in answering question 14 and on what thermodynamic principle
does this law depend?