What is meant by the charge density of an ion?

It is the charge carried by the ion in relation to its size, so a small ion with a high charge has a high charge density.

How does increasing charge density affect the lattice enthalpy of an ionic compound?

It makes the lattice enthalpy more exothermic.

Why does greater charge density make the lattice enthalpy more exothermic?

The ions attract each other more strongly because the charges are larger and closer together, so more energy is released when the lattice forms.

How does increasing charge density affect the enthalpy change of hydration of an ion?

It makes the hydration enthalpy more exothermic.

Why does greater charge density make the hydration enthalpy more exothermic?

The ion attracts polar water molecules more strongly, so the ion–dipole bonds formed are stronger and release more energy.

Place Li⁺, Na⁺ and K⁺ in order of increasingly exothermic hydration enthalpy and explain the order.

K⁺, then Na⁺, then Li⁺; the ionic radius falls from K⁺ to Li⁺ while the charge stays at 1+, so the charge density and the strength of the ion–dipole attraction both rise.

OCR A-Level Chemistry B H433

Development of Practical Skills in Chemistry

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Elements of Life

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Elements From the Sea

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Polymers and Life

Oceans

Topic overview Solubility in aqueous and non-aqueous solvents Lattice, hydration and solution enthalpies and how they relate Measuring an enthalpy change of solution Charge density and the size of enthalpy terms Entropy: qualitative treatment Total entropy change and the condition for feasibility The limiting temperature, and why a feasible reaction may not happen Calculating the entropy change of the system The solubility product expression, its units and calculations Predicting precipitation and the common ion effect Bronsted-Lowry theory of acids and bases Strong acids and strong bases Weak acids, Ka and pKa pH of strong acids and strong bases pH of weak acids, dilution and the limits of the approximation What a buffer is and how it resists a change in pH Buffer calculations, capacity and the natural buffers How the greenhouse effect works and which gases cause it Why a molecule absorbs, concentrations in ppm, and ocean chemistry

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Colour by Design

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Chemical Literacy

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Mathematical Skills in Chemistry

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Oceans

Charge density and the size of enthalpy terms

2 things to know
Components 01, 02 and 03

Overview

Recall questions on why greater charge density makes both lattice and hydration enthalpies more exothermic, and comparing named compounds.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

Why does greater charge density make lattice and hydration enthalpies more exothermic, and why is the lattice enthalpy of MgO far more exothermic than that of NaCl?

Key term

Why is the lattice enthalpy of sodium fluoride more exothermic than that of sodium iodide?

Questions from this subtopic

Each one with the model answer underneath.

1
What is meant by the charge density of an ion?
Model answer
It is the charge carried by the ion in relation to its size, so a small ion with a high charge has a high charge density.
2
How does increasing charge density affect the lattice enthalpy of an ionic compound?
Model answer
It makes the lattice enthalpy more exothermic.
3
Why does greater charge density make the lattice enthalpy more exothermic?
Model answer
The ions attract each other more strongly because the charges are larger and closer together, so more energy is released when the lattice forms.
4
How does increasing charge density affect the enthalpy change of hydration of an ion?
Model answer
It makes the hydration enthalpy more exothermic.
5
Why does greater charge density make the hydration enthalpy more exothermic?
Model answer
The ion attracts polar water molecules more strongly, so the ion–dipole bonds formed are stronger and release more energy.
6
Place Li⁺, Na⁺ and K⁺ in order of increasingly exothermic hydration enthalpy and explain the order.
Model answer
K⁺, then Na⁺, then Li⁺; the ionic radius falls from K⁺ to Li⁺ while the charge stays at 1+, so the charge density and the strength of the ion–dipole attraction both rise.

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