Thermochemistry

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Thermochemistry

• AH Chemistry, Unit 2(c)

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Introduction

• Thermochemistry concerns the study of energy
  changes in chemical reactions.
• First Law of Thermodynamics Energy is
  conserved.
• Hesss law states that the overall reaction
  enthalpy is the sum of the reaction enthalpies of
  each of step of a reaction.

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Calorimetry

• The term used to describe the quantitative
  determination of the change in heat energy which
  occurs during a chemical reaction.
• A calorimeter is used to measure this energy
  change.

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Standard enthalpy changes

• Both reactants and products are considered in
  their most stable state at 1 atmosphere pressure
  and a specified temperature (usually 298K).

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Standard Enthaply of Formation

• Refers to the enthalpy change which occurs when
  one mole of a substance is prepared from its
  elements in their standard state.

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• Examples
• C(s) O2(g) gt CO2(g)
• H2(g) ½O2(g) gt H2O(l)
• 2C(s) ½O2(g) 3H2(g) gt
  C2H5OH(l)
• Notes
• Only ONE MOLE of product on the RHS of the
  equation.
• Elements In their standard states have zero
  enthalpy of formation.
• Carbon is usually taken as the graphite
  allotrope.

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Using enthalpies of formation
?Hº ? ? Hf (products) ? ? Hf
(reactants)

• Example
• Calculate the standard enthalpy of reaction at
  298K for the complete combustion of methane.

Substance Enthalpy of formation (kJ mol-1)
Carbon dioxide Water Methane -394 -286 -74.9
-891.1 kJ mol l-1
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Bond Enthalpies
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Born-Haber cycles
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What are they?

• A thermochemical cycle applied to the formation
  of an ionic crystal.
• Used to calculate the enthalpy of lattice
  formation, which cannot be determined directly by
  experiment.
• Standard Enthalpy of Lattice Formation
  the enthalpy change which occurs when one mole of
  an ionic crystal is formed from the ions of their
  gaseous states, under standard conditions.

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Born-Haber cycle
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Apply Hesss Law
?HatmNa ?HatmCl ?H1st IE ?H1st EA
?Hlattice
?Hformation
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Born-Haber Cycles applying Hesss Law
Rearrange to find the lattice energy
?Hlattice
?Hformation - (?HatmNa ?HatmCl ?H1st IE
?H1st EA)
So Born-Haber cycles can be used to calculate a
measure of ionic bond strength based on
experimental data.
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Definitions

• Standard enthalpy of atomisation of an element
• The energy required to produce one mole of
  isolated gaseous atoms from the element in its
  standard state.
• Electron affinity
• The enthalpy change for the process of adding one
  mole of electrons to one mole of isolated atoms
  in the gaseous state.

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Enthalpy of solution

• Energy must be supplied to break up the ionic
  lattice ?Hlatt
• Energy is released when the free ions form bonds
  with water molecules ?Hhyd
• Hydration enthalpy
• The energy released when one mole of individual
  ions become hydrated.

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• Illustrate the hydration enthalpy for
• A sodium ion
• An sulphate ion

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