Potassium bromide, KBr, is an ionic compound. A student constructs a Born-Haber cycle to calculate the lattice enthalpy of potassium bromide. Explain why the first ionisation energy of potassium is an endothermic process and why the electron affinity of bromine is an exothermic process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Potassium bromide has a lattice enthalpy of −672 kJ mol⁻¹. The first ionisation energy of potassium is +419 kJ mol⁻¹ and the first electron affinity of bromine is −325 kJ mol⁻¹.
Model answer (5 marks)
1. The first ionisation energy of potassium is the energy required to remove an electron from a neutral K atom, forming K⁺.
2. Removing an electron must overcome the attractive force between the electron and the nucleus, so energy is absorbed – the process is endothermic.
3. The first electron affinity of bromine is the energy released when a neutral Br atom gains an electron, forming Br⁻.
4. The added electron is attracted to the Br nucleus, so energy is released – the process is exothermic.
5. Because the magnitude of the ionisation energy (+419 kJ mol⁻¹) is larger than that of the electron affinity (−325 kJ mol⁻¹), the net electron transfer from K to Br requires a net input of energy, making the overall process endothermic.
2. Removing an electron must overcome the attractive force between the electron and the nucleus, so energy is absorbed – the process is endothermic.
3. The first electron affinity of bromine is the energy released when a neutral Br atom gains an electron, forming Br⁻.
4. The added electron is attracted to the Br nucleus, so energy is released – the process is exothermic.
5. Because the magnitude of the ionisation energy (+419 kJ mol⁻¹) is larger than that of the electron affinity (−325 kJ mol⁻¹), the net electron transfer from K to Br requires a net input of energy, making the overall process endothermic.
Examiner tips
- Use the terms "endothermic" and "exothermic" explicitly. Explain the direction of energy flow for each step. Show the sign of the ionisation energy and electron affinity. Mention the relative magnitudes to justify the overall endothermicity.
Common mistakes
- Confusing the sign of the electron affinity (writing it as +325 instead of −325). Saying the ionisation energy is exothermic. Forgetting to state that energy must be supplied to remove the electron.
Mark scheme (5 marks)
- First ionisation energy involves removing an electron from a potassium atom
- Energy must be supplied / input to overcome the attraction between the electron and the nucleus, making it endothermic
- First electron affinity of bromine involves adding an electron to a bromine atom
- The added electron is attracted to the nucleus of bromine, so energy is released, making it exothermic
- The magnitude of ionisation energy of potassium is greater than the magnitude of electron affinity of bromine, so the overall process of electron transfer from K to Br requires a net energy input (endothermic overall)
Key terms in this question
first ionisation energy · lattice enthalpy · endothermic · exothermic · Born-Haber cycle
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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