Temperature changes required practical

Aim: To investigate how a variable, such as the volume of sodium hydroxide added to hydrochloric acid, affects the temperature change of a reaction in solution.

Variables

• Independent: Volume of sodium hydroxide solution added (cm³)
• Dependent: Highest temperature reached (or temperature change) in °C
• Control: Volume and concentration of hydrochloric acid; Concentration of sodium hydroxide solution; Starting temperature of the solutions; Type of cup, lid and insulation

Equipment

• Polystyrene cup with a lid, standing in a 250 cm³ beaker for support
• Thermometer (or temperature probe)
• Measuring cylinders (10 cm³ and 50 cm³)
• Dilute hydrochloric acid (HCl)
• Dilute sodium hydroxide solution (NaOH)
• Glass stirring rod

Method

1. Stand a polystyrene cup inside a beaker so that it does not tip over.
2. Measure 30 cm³ of dilute hydrochloric acid into the cup using a measuring cylinder.
3. Place the thermometer in the acid through a hole in the lid and record the starting temperature.
4. Add 5 cm³ of sodium hydroxide solution, put the lid on and stir.
5. Record the highest temperature reached.
6. Add another 5 cm³ of sodium hydroxide, stir and record the highest temperature again.
7. Continue adding 5 cm³ portions until a total of 40 cm³ has been added.
8. Repeat the whole experiment at least twice more and calculate a mean temperature for each volume.

Safety

• Wear eye protection: sodium hydroxide is an irritant or corrosive depending on concentration, and hydrochloric acid is an irritant.
• Stand the cup in a beaker so it cannot tip over and spill.
• Take care not to break the thermometer; stir with a glass rod, not the thermometer.
• Wipe up spills straight away.

Results and calculations

Record the volume of alkali added and the highest temperature for each repeat, then calculate the mean. Plot highest temperature (y-axis) against volume of sodium hydroxide added (x-axis); draw two straight lines of best fit (rising and falling) and read off where they cross to find the volume needed for neutralisation and the maximum temperature. Temperature change = highest temperature − starting temperature.

Common mistakes and improvements

• Energy is lost to the surroundings, so the temperature rise is lower than it should be; use an insulated polystyrene cup with a lid.
• Reading the thermometer before the highest temperature is reached; stir and watch until the reading stops rising.
• Measuring volumes inaccurately; use a measuring cylinder of a suitable size, read at eye level, or use a burette for the alkali.
• Starting temperatures of the two solutions being different; let both stand in the room until they reach the same temperature.

Exam tips

• Explain the polystyrene cup and lid: polystyrene is a good thermal insulator and the lid reduces energy loss by evaporation and convection.
• State whether a reaction is exothermic (temperature of surroundings rises) or endothermic (temperature falls), and give examples such as neutralisation (exothermic) and citric acid with sodium hydrogencarbonate (endothermic).
• Explain the shape of the graph: the temperature rises while acid and alkali react, then falls after neutralisation as extra cold alkali is added and no more reaction takes place.
• Identify anomalous results and describe how repeating and calculating a mean improves repeatability.
• Suggest improvements such as using a temperature probe and data logger for more precise readings.