Exothermic and Endothermic Reactions
Exothermic and endothermic reactions describe the direction of energy transfer between a chemical reaction and its surroundings: an exothermic reaction transfers energy to the surroundings, raising their temperature, while an endothermic reaction takes in energy from the surroundings, lowering their temperature. Each type can be identified from temperature data or a reaction profile diagram, which also shows activation energy and the effect of a catalyst.
Before you start
Make sure you're comfortable with these topics first:
Method
- Identify a reaction as exothermic if the temperature of the surroundings rises, and endothermic if it falls; learn a named example of each type of reaction beyond just combustion and thermal decomposition, so you can justify your answer for a reaction you have not seen before.
- On a reaction profile, compare the energy level of the products with the energy level of the reactants: if the products are drawn lower than the reactants, the reaction is exothermic (energy is released overall); if the products are drawn higher, it is endothermic (energy is absorbed overall).
- Read the activation energy from a reaction profile as the energy difference between the reactants and the peak of the curve; this is the minimum energy that particles need in order to react.
- Remember that a catalyst lowers the activation energy by providing a different reaction pathway, which is shown on a reaction profile as a lower peak; the catalyst does not change the energy level of the reactants or the products, so it does not change the overall energy change of the reaction.
- For the required practical on temperature changes in reacting solutions, name the independent variable being tested (for example the volume or concentration of one reactant), the dependent variable (the maximum or minimum temperature reached), and the variables that must be controlled to keep the test fair.
- Know the apparatus used: the reaction is usually carried out in a polystyrene cup, sometimes placed inside a beaker for support, with a lid with a small hole for the thermometer, to reduce heat loss or gain between the reacting solution and the surroundings.
- Identify the main source of error in this practical as heat transfer between the reacting solution and the surroundings through the cup, which makes the measured temperature change smaller than the true value; state the improvement as using a better-insulated container, adding a lid, or using a data logger to record temperature continuously and extrapolate back to the moment of mixing.
Worked example
A student adds 4 g of solid sodium hydroxide to 50 cm3 of water in a polystyrene cup and stirs until it dissolves. The starting temperature of the water is 19.5 degrees C. The maximum temperature reached is 32.0 degrees C. Calculate the temperature change, and state whether dissolving sodium hydroxide is exothermic or endothermic.
- Identify the starting temperature and the maximum temperature reached: 19.5 degrees C and 32.0 degrees C.
- Calculate the temperature change: 32.0 - 19.5.
- Temperature change = 12.5 degrees C (an increase).
- Since the temperature of the surroundings (the water) increased, energy has been transferred to the surroundings by the dissolving process.
- Final answer: the temperature increased by 12.5 degrees C, so dissolving sodium hydroxide is exothermic.
Practice questions
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Q1State the energy transfer that occurs during an exothermic reaction.Show answer
Answer: Energy is transferred from the reacting chemicals to the surroundings, so the temperature of the surroundings increases.
Q2State the energy transfer that occurs during an endothermic reaction.Show answer
Answer: Energy is taken in from the surroundings by the reacting chemicals, so the temperature of the surroundings decreases.
Q3A student heats solid copper carbonate strongly in a test tube, and it breaks down into copper oxide and carbon dioxide. Heating must be continued throughout the reaction for it to keep going. State whether this thermal decomposition reaction is exothermic or endothermic, and explain your reasoning.Show answer
Answer: Endothermic. Energy must be continuously supplied, from the Bunsen burner, to keep the reaction going, showing that the reaction takes in energy from its surroundings rather than releasing it.
Q4A student reacts excess zinc powder with 25 cm3 of copper sulfate solution in a polystyrene cup. The starting temperature is 21.0 degrees C and the maximum temperature reached is 58.5 degrees C. Calculate the temperature change.Show answer
Answer: 37.5 degrees C increase (58.5 - 21.0 = 37.5).
Q5State what is meant by the activation energy of a reaction.Show answer
Answer: The minimum amount of energy that colliding particles must have for a reaction to occur.
Q6On a reaction profile diagram, the reactants are drawn at a higher energy level than the products. State whether the reaction is exothermic or endothermic.Show answer
Answer: Exothermic (the products have less energy than the reactants, so energy has been released overall).
Q7Explain, in terms of the reaction pathway, why adding a catalyst increases the rate of a reaction without changing the overall energy change of the reaction.Show answer
Answer: A catalyst provides an alternative reaction pathway with a lower activation energy, so more collisions have enough energy to react, increasing the rate; the energy levels of the reactants and products are unchanged, so the overall energy change of the reaction stays the same.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A student mixes citric acid solution with solid sodium hydrogencarbonate in a test tube. The mixture fizzes and the temperature of the mixture falls from 20.5 degrees C to 16.0 degrees C. State whether this reaction is exothermic or endothermic, and explain your answer in terms of energy transfer.
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A reaction has two possible pathways: an uncatalysed pathway, where the reactants are at an energy of 40 kJ and the peak of the reaction profile is at 180 kJ, and a catalysed pathway for the same reaction, where the peak is at 120 kJ and the reactants and products are at the same energy as before. (a) Calculate the activation energy of the uncatalysed reaction. (b) Calculate the activation energy of the catalysed reaction. (c) State the effect the catalyst has on the rate of the reaction, and explain why.
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A student investigates how the volume of dilute hydrochloric acid affects the maximum temperature rise when it neutralises a fixed volume of sodium hydroxide solution. Describe how the student could carry out this investigation, including the apparatus used, the variables that should be controlled, and one way to make the results as accurate as possible.
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Free printable worksheet
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This topic is chapter 22 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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