OCR AS Chemistry F332: Ionisation Enthalpies

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Ionisation Enthalpies
When electrons have been removed from an atom or molecule, it's been ionised.
The energy needed to remove the first electron is called the first ionisation enthalpy.
We define the first ionisation enthalpy of an element as the energy needed to remove one
electron from every atom in one mole of isolated gaseous atoms of the element .
The general equation for the first ionisation process is:
X (g) X+ (g) + e-
1 mole 1 mole 1 mole of electrons
of gaseous of gaseous
atoms 1+ ions
X represents the symbol for the element.
Important points about ionisation enthalpies:
You must use the gas state symbol (g) because ionisation enthalpies are measured
for gaseous atoms
Always refer to 1 mole of atoms rather than to a single atom
The lower the ionisation enthalpy, the easier it is to form an ion

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Factors Affecting Ionisation Enthalpy
Negative electrons are held in their shells by attraction to the positive nucleus.
To form a positive ion, energy must be supplied to an electron to overcome the attraction
from the nucleus.
Electrons in the outer shell are removed first since they experience the least nuclear
The outer-shell electrons are furthest away from the nucleus and require the least ionisation
energy.…read more

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Successive Ionisation Enthalpies
More than one electron can be removed from an atom.
So, as well as first ionisation enthalpies, there are second, third, fourth etc.
Successive ionisation energies are a measure of the energy required to remove each
electron in turn.…read more

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Trends in Ionisation Enthalpies in the Periodic Table
Ionisation Enthalpy across a Period
As you move across a period, the trend is for the
ionisation enthalpies to increase.
This is because it gets harder to remove outer electrons.
This is because the number of protons is increasing,
which means a stronger nuclear attraction.
Since al the outer-shell electrons are at roughly the same
energy level ­ there's generally little extra shielding
effect or extra distance to lessen the attraction from the
nucleus.…read more


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