Enthalpy of solution and hydrationEdexcel A-Level Chemistry: Flashcards
What these 13 flashcards ask
- Define enthalpy change of solution.
- Define enthalpy change of hydration.
- What does 'infinite dilution' mean?
- Why is the enthalpy change of hydration always exothermic?
- State the equation linking ΔH(sol), lattice energy and hydration enthalpies.
- Why does lattice energy appear with a negative sign in the equation for ΔH(sol)?
- How do you treat MgCl₂ in a ΔH(sol) cycle?
- Calculate ΔH(sol) for NaCl given lattice energy −787, Na⁺ −406 and Cl⁻ −364 kJ mol⁻¹.
- How does ionic charge affect lattice energy and hydration enthalpy?
- How does ionic radius affect lattice energy and hydration enthalpy?
- Which has the more exothermic hydration enthalpy, Mg²⁺ or Na⁺, and why?
- What are the two ways ΔH(sol) can be described?
- What does an energy level diagram for dissolving show?
Exam questions on Enthalpy of solution and hydration
- A student investigates an instant cold pack in which potassium chloride dissolves in water and the temperature of the water falls. In energy cycles, lattice energy is taken as the enthalpy change when one mole of the solid is formed from its gaseous ions. For potassium chloride the lattice energy is −717 kJ mol⁻¹, and the enthalpy changes of hydration are −322 kJ mol⁻¹ for K⁺ and −364 kJ mol⁻¹ for Cl⁻.Calculate the enthalpy change of solution of potassium chloride and use your answer to explain why the temperature of the water falls.2 marks
- A chemist compares how strongly different ions are attracted to water molecules when they dissolve. She is looking at the enthalpy change of hydration of a series of gaseous ions, and wants to explain the differences in terms of ionic charge and radius.Explain why the enthalpy change of hydration of Mg²⁺ is much more exothermic than that of Na⁺.2 marks
- Energy data (in kJ mol⁻¹) are given for sodium chloride and magnesium chloride. Lattice energy (formation of the solid from gaseous ions): NaCl −787; MgCl₂ −2526. Enthalpy change of hydration: Na⁺ −406; Mg²⁺ −1920; Cl⁻ −364. The enthalpy change of solution of NaCl is +17 kJ mol⁻¹.Calculate the enthalpy change of solution of magnesium chloride.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).