HardChemistryHigh School (11–12)The d- and f-Block Elements › Transition Elements (d-Block)ConceptualA-LevelBoard ExamIB
The third ionisation enthalpy of manganese is 3260 kJ/mol…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
The third ionisation enthalpy of manganese is . That of iron is . Iron has the larger nuclear charge, and yet it loses its third electron more easily. Explain this with the configurations of the ions.
Need a reminder? The key ideaShow
The nuclear charge rises by one at each step of a series, but a new electron screens the electrons by about , so rises slowly. Metal bonds are strong where many d electrons can bond. Enthalpy of atomisation, , is the energy to turn one mole of solid metal into gaseous atoms. The energy to reach is , and hydration returns part of it.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. The two third ionisation enthalpies are and .
Step 1. Manganese. is , a half-filled subshell. The third electron must come out of that stable arrangement, and leaves as . This costs a lot of energy.
Step 2. Iron. is . The sixth d electron is paired with another, so the repulsion helps it to leave. Removing it gives as , the stable half-filled subshell. This costs less.
Answer: the third electron of iron leaves to give a half-filled ion. The third electron of manganese must leave a half-filled ion. The stability of the arrangement raises one value and lowers the other.
Watch out. The nuclear charge helps in the opposite direction, but the configuration of the ion decides here.
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