OCR A2 Chemistry

Unit 5, Module 2 - Energy

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  • Created by: Claire
  • Created on: 21-03-11 20:00


q = mc∆T 

∆H = -mc∆T

  q = enthalpy change of water               m = mass of water

c = specific heat capacity of water

∆T = change in temperature of water

∆H = enthalpy change of the reactants

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Definitions I need to learn

Enthalpy change of formation

Enthalpy change of atomisation of an element

Enthalpy change of atomisation of a compound

Enthalpy change of neutralisation

Ionisation energy                                 Electron affinity

Enthalpy change of hydration

Enthalpy change of solution

Lattice enthalpy

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Lattice Enthalpy

Standard Lattice Enthalpy: the enthalpy change when 1mole of a solid ionic compound is formed from its gaseous ions under standard conditions. It's a measure of ionic bond strength.

Higher ion charge = more energy released forming a lattice, so the enthalpy will be more negative.

Smaller ionic radii = more exothermic lattice enthalpy. Smaller ions pack closer together, meaning a stronger attraction because of the high charge density.

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Born-Haber Cycles

Born-Haber cycles are used to find out what the enthalpy change would be if you took a less direct route....

1) Start with the Enthalpy change of Formation at the bottom

2) Put the enthalpies of atomisation and ionisation above

3) The electron affinity goes on the top right

Hess's Law:

Sum of clockwise arrows = Sum of anti-clockwise arrows

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