Energetics

Goal of the practical

• Use a polystyrene-cup calorimeter to record how the temperature of a neutralisation reaction varies with the volume of one reactant added, and use the data to identify the volume that gives the largest temperature change

Apparatus

• 100 cm³ polystyrene cup with a fitted lid
• Thermometer (0–110 °C, 0.5 °C divisions) and stirring rod
• 25 cm³ measuring cylinder (×2: one for each reactant)
• 25 cm³ dilute sodium hydroxide solution per run
• Dilute hydrochloric acid (same concentration as the alkali)

Method

• Measure 25 cm³ of NaOH solution into the polystyrene cup and place on the bench in the cup-and-lid assembly
• Read the temperature once it has settled — this is T_start
• Measure 5 cm³ of HCl with the second measuring cylinder, add it to the cup, fit the lid, and stir gently with the thermometer
• Read the highest temperature reached — this is T_max
• Empty and dry the cup, then repeat the run with 10 cm³ of HCl, then 15, 20, 25, 30 and 35 cm³ — stepping past the expected equivalence point so the peak of the curve can be seen clearly

Sample results table

• Plot ΔT (y-axis) against volume of HCl added (x-axis) and draw a smooth curve
• The peak of the graph indicates the volume of HCl at which the largest temperature rise occurs — for equal concentrations of acid and alkali this should be at 1 : 1 stoichiometry (25 cm³ in this example)
• Past the peak, additional acid cools the mixture relative to the maximum because the extra acid is not warming up further but is mixing in at room temperature