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Enthalpy Change (ΔH)
- In AS/A-level Chemistry, the enthalpy change is the amount of heat taken in or given out at constant pressure during any physical or chemical change.
- The size of any enthalpy change depends on the pressure and the temperature as well as on the amount of substance used.
- The standard state of a substance at a given temperature (usually 298 K) is its pure, most stable, form at 100kPa.
- There remains the need to decide both an amount of substance and a temperature in order to fully specify any enthalpy change, since enthalpy changes can be quoted for any chosen amount of substance, or for any chosen temperature.
Ionisation Enthalpy
- It is the standard molar enthalpy change for the removal of an electron from a species in the gas phase to form a positive ion and an electron, both also in the gas phase.
Example:
Na (g) --> Na+(g) + e-(g) ΔHi = +496 kJ mol-1
The cation formed (Na+) may be further oxidised, according to the process:
Na+(g) --> Na2+(g) + e-(g) ΔHi = +4562 kJ mol-1
- This is called the second ionisation enthalpy.
- This is always larger than the first, because the removal of the second electron from an already positively charged species requires more energy (to overcome to electrostatic attraction) than the removal of the first from a neutral species.
Enthalpy of Atomisation (ΔHat )
- In AS/A-level Chemistry, it is the standard enthalpy change that accompanies the formation of one mole of gaseous atoms from the element in its standard state.
- For an atomic solid, such as an element, the standard enthalpy of atomisation is simply the standard enthalpy of sublimation of the solid.
- The enthalpy of atomisation is identical to the enthalpy of sublimation.
- Sublimation always requires an input of energy (endothermic process), so these enthalpies are all positive.
- In the case of bond fission, a diatomic molecule will produce two moles of atoms, so the enthalpy of atomisation is half the bond dissociation enthalpy.
Bond Dissociation Enthalpy (ΔHdiss)
- It is the standard molar enthalpy change that accompanies the breaking of a covalent bond in a gaseous molecule to form two gaseous free radicals.
- In order to indicate that each species formed by bond fission has one unpaired electron, it is considered correct, particularly in processes involving bond fission in a mass spectrometer or in radical chain reactions, to write a dot alongside the odd-electron species to indicate the unpaired electron.
- This species is called a free radical, e.g. the methyl radical (•CH3) or the chlorine atom (•Cl).
- In thermodynamic equations involving bond fission, it is quite common to omit dots representing unpaired electrons, since their presence is obvious from the equation as written.
Electron affinity, ΔHea
- In AS/A-level Chemistry, it is the standard enthalpy change when an electron is added to an isolated atom in the gas phase.
- E.g. Cl (g) + e-(g) --> Cl-(g) ΔHea = -364 kJ mol-1
- A chlorine atom in the gas phase has a strong affinity for an electron, so that the capture of an electron to form a gaseous chloride ion causes energy to be given out to the surroundings – an exothermic process.
That's all~