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Endothermic & Exothermic Reactions (CIE IGCSE Chemistry)
Revision Note
Exothermic & endothermic reactions
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
- This process involves the transfer of thermal energy into and out of reaction mixtures
- The terms used to describe this are:
- System: the reacting chemicals
- Surroundings: anything other than the chemicals reacting
- The energy within the system comes from the chemical bonds themselves which could be considered as tiny stores of chemical energy
Exothermic reactions
- In exothermic reactions, thermal energy is transferred from the chemical energy store of the chemical system to the surroundings
- The energy of the system decreases, which means that the energy change is negative
- The temperature of the surroundings increases because thermal energy is given out / released
- The overall transfer is from the system to the surroundings
Diagram showing the transfer of heat energy outwards from an exothermic reaction
- Typical examples of exothermic reactions include:
- Combustion
- Oxidation
- Neutralisation
- Hand warmers used in the wintertime 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
Endothermic reactions
- In endothermic reactions, thermal energy is transferred from the surroundings system to the system surroundings
- The energy of the system increases, which means that the energy change is positive
- The temperature of the surroundings decreases because thermal energy is taken in / absorbed
- The overall transfer is from the surroundings to the system
Diagram showing the transfer of heat energy from the surroundings into an endothermic reaction
- Endothermic reactions are less common than exothermic reactions
- Typical examples of endothermic reactions include:
- Electrolysis
- Thermal decomposition
- The first stages of photosynthesis
- Cold packs for sports injuries are based on endothermic reactions, designed to take heat away from a recently injured area to prevent swelling
Worked example
A student was investigating the temperature change for four different chemical reactions. The table shows the chemicals that the student combined for each reaction along with the initial and final temperatures of the reaction.
Experiment |
Chemicals |
Initial temperature |
Final temperature |
|
1 |
10 cm3 NaOH |
10 cm3 HCl |
19 |
21 |
2 |
10 cm3 NaHCO3 |
2 g citric acid |
20 |
16 |
3 |
10 cm3 CuSO4 |
0.5 g Mg powder |
20 |
26 |
4 |
10 cm3 H2SO4 |
3 cm Mg ribbon |
19 |
31 |
Explain whether each reaction is endothermic or exothermic.
Answers:
- Reactions 1, 3 and 4 are exothermic reactions because they show a temperature increase
- Reaction 2 is an endothermic reactions because it shows a temperature decrease
Reaction pathway diagrams
- Reaction pathway diagrams are graphical representations of the relative energies of the reactants and products in chemical reactions
- On a reaction pathway diagram:
- Progress of the reaction is shown on the x-axis
- Energy is shown on the y-axis
- The difference in height between the energy of reactants and products is the overall energy change of a reaction
Reaction pathway diagram of an exothermic reaction and an endothermic reaction
- In exothermic reactions:
- Energy is given out to the surroundings
- The energy of the products will therefore be lower than the energy of the reactants
- The overall energy change is negative
- This is represented on the reaction profile with a downwards-arrow as the energy of the products is lower than the reactants
- In endothermic reactions:
- Energy is taken in from the surroundings
- The energy of the products will be higher than the energy of the reactants
- The overall energy change is positive
- This is represented on the reaction profile with an upwards-arrow as the energy of the products is higher than the reactants
Exam Tip
- To help you remember whether a chemical system is exothermic or endothermic:
- In EXothermic reactions heat Exits the system and in ENdothermic reactions heat ENters the system.
- Exothermic reactions always give off heat and they feel hot
- Endothermic reactions always take heat in and they feel cold.
- Core candidates will be expected to interpret reaction pathway diagrams
- Extended candidates will be expected to draw and interpret reaction pathway diagrams
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