How hard can thermochem be? Past years’ qns for thought!

As the title implies,

this post is about past years’ A level questions on thermochem!

Alright! Thermochem might seem very mundane and boring, but in fact, if  you look at the questions, CAMBRIDGE can really set some pretty exciting questions that can take you aback and make you wonder if thermochem questions are as standard as they are!

J99/2/4

Anhydrous magnesium sulphate is sold by pharmacists as a treatment for boils, where are infected areas of the skin. The paste is applied to a boil to draw out water and the infected matter.

(i) suggest why anhydrous magnesium sulphate is able to draw water out of the treated boil.

answer : Mg2+, being a relatively small cation,

has a high charge density.

Hence, it has a more exothermic Hydration Energy,

which is greater than the Lattice Energy of MgSO4

therefore, it dissolves readily in water, and draws water out of the boil.

Notes : Notice how in this question, an interesting story and scenario is used and how it is cunningly worded to give only a slight hint that they are testing you on this very familiar equation :

Enthalpy change of Solution -> Lattice Energy + Hydration energy

(b) Suggest a liquid which could be added to the anhydrous magnesium sulphate to make a paste.

answer : Alcohol, which can function as an antiseptic.

Notes : Alcohol, just our friend CH3CH2OH from organic chem but used in a surprise inorganic question, can actually dissolve ionic solids like MGSO4 because it is a polar solvent!

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