Core Practical 3: Enthalpy Determination

Key Reaction

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

One mole of water is formed per mole of acid and alkali reacting.

Apparatus

• Polystyrene (expanded polystyrene) cup
• 250 cm³ beaker (to support the cup and reduce heat loss)
• Two 25 cm³ measuring cylinders
• Thermometer (0.1 °C precision preferred) or electronic temperature probe
• Stopwatch or timer
• Stirring rod or glass rod
• Pipette (optional, for more precise volume transfer)
• Burette and clamp (optional alternative to measuring cylinders for better volume control)
• Graph paper or data logger software (for plotting temperature vs time)

Chemicals

• Hydrochloric acid (HCl), 1.00 mol dm⁻³
• Sodium hydroxide (NaOH), 1.00 mol dm⁻³

Method

1. Measure 25.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid (HCl) using a 25 cm³ measuring cylinder. Pour it into a polystyrene cup.
2. Use a thermometer (or data logger) to measure and record the temperature of the acid every minute for 3 minutes.
3. At minute 4, quickly add 25.0 cm³ of 1.00 mol dm⁻³ sodium hydroxide (NaOH) (pre-measured using a measuring cylinder) into the cup and stir immediately. Do not record the temperature at this point.
4. Continue recording the temperature every minute until 10 minutes.
5. Plot a temperature vs time graph.

Draw a best-fit line through points before and after mixing. Extrapolate both lines to the 4th minute (the mixing point) and determine the temperature change (ΔT) at this point.

Example Calculations

Example Data:

• Initial temp (average before mixing): 21.5 °C
• Final temp (extrapolated): 27.0 °C
• ΔT = 27.0 – 21.5 = 5.5 °C

1. Calculate heat released (q)
   Use q = mcΔT
   Assume density of solution = 1.00 g cm⁻³
   Total volume = 25 + 25 = 50.0 cm³ → mass = 50.0 g
   Specific heat capacity (c) = 4.18 J g⁻¹ K⁻¹
   q = 50.0 × 4.18 × 5.5 = 1149.5 J = 1.15 kJ
2. Calculate moles of water formed
   Limiting reagent = either HCl or NaOH (both 25.0 cm³ of 1.00 mol dm⁻³)
   Moles = 0.0250 mol
3. Calculate ΔH_neut (per mol of water formed)
   ΔH = –q ÷ n = –1.15 kJ ÷ 0.0250 mol = –46.0 kJ mol⁻¹

Sources of Error

• Heat loss to surroundings (e.g., through the polystyrene cup or when mixing).
• Assumed specific heat capacity (4.18 J g⁻¹ K⁻¹ used for the whole solution, though this is actually the value for water).
• Inaccurate temperature measurement (can be reduced with electronic thermometers or data loggers).

Improvements

• Use a digital thermometer or data logger for higher accuracy.
• Use a lid on the polystyrene cup to minimise heat loss.
• Stir the mixture thoroughly to ensure even temperature distribution.

Measuring Enthalpy Change of Combustion (ΔH_c)

The enthalpy of combustion is the energy change when one mole of a substance burns completely in oxygen.

Method

1. Measure a known volume of water in a calorimeter (beaker or copper can).
2. Record the starting temperature of the water.
3. Weigh the spirit burner containing the fuel.
4. Light the burner and allow it to heat the water.
5. Stir and measure the final temperature of the water.
6. Reweigh the burner to determine mass of fuel burned.
7. Calculate q using q = mcΔT, then use ΔH = q ÷ n.

Sources of Error (Combustion)

• Heat loss to surroundings (e.g., air, beaker).
• Incomplete combustion (producing CO instead of CO₂).
• Evaporation of fuel from the wick.

Summary

• Enthalpy change of neutralisation is determined by measuring temperature change in a polystyrene cup calorimeter.
• Graphical extrapolation reduces error by accounting for heat loss during mixing.
• Key calculation: q = mcΔT, then ΔH = –q ÷ moles reacted.
• Enthalpy of combustion can be measured using a spirit burner and calorimeter, though results are often less accurate due to heat losses and incomplete combustion.