Heat Exchange in Reactions

  • Chemical reactions occur so that elements can achieve a more stable energy state by gaining a full outer shell of electrons.
  • This is done by chemical bonding (we have already seen ionic and covalent bonding) where old bonds are broken and new bonds are formed.
  • This process involves the transfer of energy into and out of reaction mixtures.
  • The energy comes from the chemical bonds themselves which could be considered as tiny stores of chemical energy.
  • In the majority of reactions the energy is in the form of heat energy, although sometimes other types of energy are produced such as light or sound.
  • The changes in heat can be observed and measured with a thermometer.
  • Note that the overall amount of energy doesn’t change as energy, like mass, is conserved in reactions.
  • This means that it cannot be created or destroyed but it can be transferred.
  • So if energy is transferred to the surroundings during a chemical reaction, then the product formed must have less energy than the reactants by the same amount as that transferred.
  • The following are some examples of heat changes in reactions.
  • Salts dissolving in water:
    • These can either take energy in or give it out.
  • Neutralisation reactions:
    • These always give energy out.
  • Displacement reactions:
    • These can either take energy in or give it out.
  • Precipitation reactions:
    • These always give energy out.

Exothermic & Endothermic Reactions

Exothermic Reactions

  • In exothermic reactions energy is given out to the surroundings so the temperature of the surroundings increases.
  • Combustion, oxidation and neutralisation reactions are typical exothermic reactions.
  • Hand warmers used in the winter time are based on the release of heat from an exothermic reaction.
  • Self-heating cans of food and drinks such as coffee and hot chocolate also use exothermic reactions in the bases of the containers.

Exothermic- reaction, IGCSE & GCSE Chemistry revision notes

Diagram showing the transfer of heat energy outwards from an exothermic reaction

Endothermic Reactions

  • In endothermic reactions energy is taken in from the surroundings so the temperature of the surroundings decreases.
  • These types of reactions are much less common than the exothermic reactions.
  • Electrolysis, thermal decomposition reactions and the first stages of photosynthesis are typical endothermic reactions.
  • Sports injury treatment often use cold packs based on endothermic reactions to take heat away from a recently injured area to prevent swelling.

Endothermic- reaction, IGCSE & GCSE Chemistry revision notes

Diagram showing the transfer of heat energy from the surroundings into an endothermic reaction

Required Practical 4: Investigating Temperature Changes

Objective:
To perform a calorimetric study of the reaction between HCl and NaOH.

Hypothesis:
The temperature change will vary depending on the amounts of solutions involved.

Materials:

  • dilute hydrochloric acid
  • dilute sodium hydroxide solution
  • styrofoam calorimeter & lid
  • 25cm3 measuring cylinder
  • thermometer & stirrer

Calorimeter - Styrofoam Cup, IGCSE & GCSE Chemistry revision notes

Diagram showing the apparatus for the calorimetry investigation for displacement, dissolving and neutralisation

Practical Tip:
The lid should be placed on the calorimeter as soon as the acid is added to minimise heat loss to the surroundings.

Method:

  1. Using a measuring cylinder, place 25 cm3 of the NaOH into the calorimeter.
  2. Measure and record the temperature of the solution.
  3. Add 5cm3 of the dilute HCl and stir.
  4. Measure and record the highest temperature reached by the mixture.
  5. Repeat steps 1 – 4 increasing the amount of acid added by 5cm3 each time.

Results:
Record your results in a suitable table, eg:

Investigating Temperature Changes Table, IGCSE & GCSE Chemistry revision notes
Evaluation:
Plot a graph of the results and draw a line of best fit, using the graph to determine what volume of acid causes the biggest change in temperature.

Conclusion:
The larger the difference in the temperature the more energy is absorbed or released.

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.