EasyChemistryHigh School (11–12)The p-Block Elements › Groups 13 and 14: Boron and Carbon FamiliesNumericalA-LevelIB
The first three ionisation enthalpies are 577, 1816 and 2744 kJ/mol for aluminium, and 579, 1979…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
The first three ionisation enthalpies are , and for aluminium, and , and for gallium. Find the energy that changes one mole of gaseous atoms into one mole of gaseous ions for each element. Then find how much more gallium needs than aluminium, and the mean energy for each electron removed from aluminium.
Need a reminder? The key ideaShow
To make a gaseous ion, the three electrons leave one after the other. The energy is the sum . A weighted mean of isotope masses is . Aluminium dissolves in alkali: , so 2 mol of Al give 3 mol of H.
The worked solution
Have you tried it?
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.
Plan. The electrons leave one at a time, so the three enthalpies add.
Step 1. Aluminium. .
Step 2. Gallium. .
Step 3. Difference. .
Step 4. Mean for aluminium. for each electron.
Answer: , , and .
Check. Gallium costs more, as it should. The 3d electrons shield the nucleus poorly, so the outer electrons of gallium are held more strongly than those of aluminium.
Source. The ionisation enthalpies are the values printed in the NCERT table for group 13, which agree with the CRC Handbook of Chemistry and Physics.
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