AQA AS and A Level

Chemistry

11

Practical 2: Measurement of an enthalpy change

AT a,b, kCPAC links Evidence Done 1a Correctly follows written instructions to

carry out the experimental techniques or procedures.

Practical procedure

2a Correctly uses appropriate instrumentation, apparatus and materials (including ICT) to carry out investigative activities, experimental techniques and procedures with minimal assistance or prompting.

Practical procedure

2b Carries out techniques or procedures methodically, in sequence and in combination, identifying practical issues and making adjustments when necessary.

Practical procedure

2c Identifies and controls significant quantitative variables where applicable, and plans approaches to take account of variables that cannot readily be controlled.

Practical procedure

2d Selects appropriate equipment and measurement strategies in order to ensure suitably accurate results.

Practical procedure

3a Identifies hazards and assesses risks associated with these hazards, making safety adjustments as necessary, when carrying out experimental techniques and procedures in the lab or field.

Practical procedure

3b Uses appropriate safety equipment and approaches to minimise risks with minimal prompting.

Practical procedure

4a Makes accurate observations relevant to the experimental or investigative procedure.

Results table

4b Obtains accurate, precise and sufficient data for experimental and investigative procedures and records this methodically using appropriate units and conventions.

Results table

Procedure

1 Place approximately 3 g of solid potassium carbonate into a test tube. Accurately weigh the test tube and its contents.

2 Use a burette to dispense 30 cm3 of 2 mol dm–3 hydrochloric acid into a polystyrene cup, which is supported in a beaker.

3 Measure and record the temperature of the acid.

4 Continue measuring the temperature while adding potassium carbonate to the acid and stirring. Record the highest temperature reached.

5 Re-weigh the empty test tube.

6 Repeat steps 1–5 using approximately 3.5 g of potassium hydrogencarbonate instead of potassium carbonate. This time, record the lowest temperature reached.

● Wear goggles.

● 2 mol dm–3 hydrochloric acid is an irritant.

● Potassium carbonate is an irritant.

Safety !

Equipment

● two test tubes

● 2 mol dm–3 dilute hydrochloric acid

● solid potassium carbonate

● solid potassium hydrogencarbonate

● thermometer able to read up to 50 °C or more

● polystyrene cup

● 250 cm3 or 400 cm3 beaker

● burette, clamp and stand

● stirring rod

● mass balance (2 d.p.)

● spatula

● To calculate the molar enthalpy change for two reactions and use Hess’s Law to determine the enthalpy change for the reactions

Objective

DRAFT

12

AQA AS and A Level

ChemistryPractical 2: Measurement of an enthalpy change

Learning tips

● The heat capacity of the final solution can be assumed to be the same as the heat capacity of water. The volume of water made in the reaction is so small it can be ignored.

● For exothermic reactions, the enthalpy change, △H, is negative.

● Make sure you do not use the equals sign incorrectly. It is very easy to end up stating that a negative number equals a positive number.

Analysis of results

1 Record your results in the tables provided.

The equations for the reactions 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)

Results (Use this space to record your results.)

Mass of test tube with potassium carbonate/g

Mass of test tube after emptying out potassium carbonate/g

Mass of potassium carbonate used/g

Start temperature/°C

Highest temperature/°C

Temperature change/°C

Mass of test tube with potassium hydrogencarbonate/g

Mass of test tube after emptying out potassium hydrogencarbonate/g

Mass of potassium hydrogencarbonate used/g

Start temperature/°C

Lowest temperature/°C

Temperature change/°C

DRAFT

13

AQA AS and A Level

ChemistryPractical 2: Measurement of an enthalpy change

2 Calculate the energy change for each reaction in J. The specific heat capacity of water is 4.2 J g–1 °C–1.

3 Calculate the enthalpy change, △H, for each reaction in kJ mol–1. Assume that the hydrochloric acid is in excess.

4 Use your results to calculate the enthalpy change for the thermal decomposition of potassium hydrogencarbonate:

reaction 3: 2KHCO3(s) → K2CO3(s) + CO2(g) + H2O(l)DRAFT

14

AQA AS and A Level

ChemistryPractical 2: Measurement of an enthalpy change

Questions

1 Why is it not possible to measure the enthalpy change for the decomposition of potassium hydrogencarbonate directly?

2 Show that the hydrochloric acid is in excess in both reactions.

3 Draw energy level diagrams for:

(a) reaction 1

(b) reaction 2

DRAFT

15

AQA AS and A Level

ChemistryPractical 2: Measurement of an enthalpy change

(c) reaction 3.

4 Explain why the reactions are conducted in a polystyrene cup rather than a glass beaker.

5 How might you alter your method if the reaction was highly exothermic?

6 How could the results from question 5 be used? Explain why.DRAFT