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Introduction

During a neutralisation reaction heat energy is released and the temperature of the reaction mixture increases. It is possible to measure the temperature rise when a known quantity of an alkali is neutralised and hence calculate the energy released when 1 mole of alkali is neutralised.

Task 1: Measuring the temperature rise in a reaction


The temperature rise on neutralising an alkali

  1. Open Yenka file Model 1.
  2. Pour 50 cm3 of 1 M sodium hydroxide (NaOH) solution into the beaker. What is the initial temperature of the alkali?
    Answer
    25 °C
  3. What ions must be present in solution for it to be alkaline?
    Answer
    OH​​- ions.
  4. What ions are present in all acidic solutions?
    Answer
    H​​+ ions.
  5. Now pour in 50 cm3 of 1 M hydrochloric acid (HCl). The temperature of the mixture rises and this is shown on the graph. Write a balanced formula equation for the reaction occurring.
    Answer
    NaOH + HCl = NaCl + H2O
  6. Once the temperature starts to fall, use the graph to determine the maximum temperature reached and record it below.
    Answer
    31.5 °C
  7. Now reload the simulation and perform the reaction with sodium hydroxide and nitric acid. Record all your results in the table below.
    Initial temperature Highest temperature Temperature rise
    Answer
    25 °C
    Answer
    31.5 °C
    Answer
    6.5 °C
  8. Does the acid that neutralises the alkali affect the energy released in the reaction?
    Answer
    No.
  9. Write an ionic equation for the reaction between those ions responsible for acidity and those ions responsible for alkalinity. The product is water.
    Answer
    H​​+ + OH​​- = H2O

Summary

The ionic neutralisation equation is the same no matter which acid reacts or which alkali reacts. It involves the formation of a bond between a hydroxide ion and a hydrogen ion. Bond formation is exothermic. It is therefore apparent why the temperature change will be the same for both combinations of acid and alkali tested in the experiment.

Task 2: Calculating the energy released in a neutralisation reaction

  1. Open the Excel file Calculator.
  2. The spreadsheet will calculate the energy released in the experiment for you. Simply enter your data into the appropriate cells (shaded yellow). Write down the energy released in the experiment (shaded light blue). Include units.
    Answer
    2730 J (based on 25-31.5°C rise).
  3. The second part of the spreadsheet calculates the energy released when 1 mole of alkali is neutralised. Write down the answer (shaded light green), including units.
    Answer
    54,600 J/mol (based on 25-31.5°C rise).

Summary

Simple temperature measurements made when a known mass of substance reacts can be used to calculate the energy change for a reaction. The energy change for a neutralisation appears to be independent of the acids and alkalis used.

Teacher Summary

  • The first task simply measures the temperature change when a strong acid reacts with a strong alkali, and introduces the idea of the ionic equation for neutralisation.
  • The second task allows the student to calculate the actual energy change for the reaction automatically. It does not require any mathematical ability or understanding.