Energy Changes
Energy changes is the GCSE chemistry topic covering exothermic and endothermic reactions, reaction profiles and activation energy, how bond breaking and bond making transfer energy, and how chemical cells, batteries and hydrogen fuel cells generate electricity from chemical reactions. It includes the required practical on temperature changes in reacting solutions.
Before you start
Make sure you're comfortable with these topics first:
Method
- Learn the difference between exothermic reactions, which transfer energy to the surroundings so temperature increases (for example combustion, neutralisation), and endothermic reactions, which take in energy from the surroundings so temperature decreases (for example thermal decomposition, some dissolving processes).
- On a reaction profile, identify the reactants and products by their energy level: if products have less energy than reactants, the reaction is exothermic; if products have more energy, it is endothermic. Activation energy is the energy difference between the reactants and the peak of the curve.
- Calculate the overall energy change as (energy of products) - (energy of reactants), and remember a catalyst lowers the activation energy but does not change the overall energy change.
- Explain energy changes in terms of bonds: breaking bonds requires energy, so it is endothermic, and forming new bonds releases energy, so it is exothermic; whether the overall reaction is exothermic or endothermic depends on the balance between these two energy transfers.
- For the required practical on temperature changes, calculate the mean temperature change from repeat readings, excluding any anomalous results, and know that a lid or insulation reduces heat loss to improve accuracy.
- For cells and fuel cells, learn that a simple cell produces a voltage from two different metals in an electrolyte, and that a hydrogen fuel cell generates electricity from the reaction between hydrogen and oxygen, producing only water.
Worked example
The reaction profile for a reaction shows the reactants at 180 kJ, the products at 100 kJ, and the peak of the curve (activation energy point) at 260 kJ. (a) State whether the reaction is exothermic or endothermic, giving a reason. (b) Calculate the overall energy change for the reaction. (c) State the activation energy.
- Compare the energy of the products (100 kJ) with the reactants (180 kJ): the products have less energy, so the reaction is exothermic.
- Calculate the overall energy change: products - reactants = 100 - 180.
- Overall energy change = -80 kJ (80 kJ released).
- Find the activation energy: peak - reactants = 260 - 180.
- Activation energy = 80 kJ.
- Final answer: the reaction is exothermic, with an overall energy change of -80 kJ and an activation energy of 80 kJ.
Practice questions
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Q1State whether the combustion of methane is exothermic or endothermic.Show answer
Answer: Exothermic.
Q2Give one everyday example of an endothermic process.Show answer
Answer: A sports cold pack activating (also accept: thermal decomposition of a metal carbonate).
Q3A reaction profile shows the reactants at 220 kJ and the products at 340 kJ. State whether the reaction is exothermic or endothermic.Show answer
Answer: Endothermic (the products have more energy than the reactants).
Q4Using the values in the previous question, calculate the overall energy change for the reaction.Show answer
Answer: +120 kJ (340 - 220)
Q5A reaction profile shows the reactants at 150 kJ and the peak (activation energy point) at 195 kJ. Calculate the activation energy.Show answer
Answer: 45 kJ (195 - 150)
Q6Explain, in terms of bonds, why breaking bonds is an endothermic process.Show answer
Answer: Breaking bonds requires energy to be supplied/taken in from the surroundings to separate the atoms, so energy is absorbed rather than released.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A student adds solid ammonium nitrate to water in a test tube and the temperature of the water decreases. State whether this process is exothermic or endothermic, and explain your answer in terms of energy transfer.
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A reaction profile shows the reactants at 210 kJ, the products at 130 kJ, and the peak of the curve at 250 kJ. (a) Calculate the overall energy change for the reaction. (b) Calculate the activation energy for the reaction.
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In an investigation into temperature changes in reacting solutions, a student adds excess magnesium powder to dilute hydrochloric acid and records the following temperature changes across three repeats: Repeat 1 = 8.1 degrees C, Repeat 2 = 15.6 degrees C, Repeat 3 = 8.3 degrees C. (a) Identify the anomalous result. (b) Suggest one possible reason for this anomalous result. (c) Calculate the mean temperature change, excluding the anomalous result. (d) Suggest one improvement to the method that would reduce heat loss to the surroundings.
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Free printable worksheet
Want more practice on paper? Download the energy changes worksheet pack - 18 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 4 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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