Answers to questions on Materials science introduction

Answers to Materials science introduction questions.

1. (a) Copper is much lower in the electrochemical series (E^⦵ = + 0.34 V) compared to tin (E^⦵ = − 0.14 V) so is less reactive than tin, i.e. less readily oxidised to its ions.

(b) (i) + 2

(ii) It will decompose to copper(II) oxide when heated, which can then be reduced with carbon (or another reducing agent such as carbon monoxide or hydrogen).
Cu₂CO₃(OH)₂(s) → 2CuO(s) + CO₂(g) + H₂O(l)
2CuO(s) + C(s) → 2Cu(s) + CO₂(g)

(c) Copper is a relatively soft metal, so even when copper was known most tools were still made of stone until bronze, which is much harder than copper, was discovered and in common use.

(d) Iron (E^⦵ = − 0.45 V) is even higher in the electrochemical series than tin, so its ions are harder to reduce to the elemental state than tin and copper. The reduction process requires a higher temperature, so it was not discovered until later.

2 (a) For MgCl₂ the difference is 1.9 and the average is 2.25 – this places MgCl₂ very much inside the ionic section of the triangle. For BeCl₂ the difference is 1.6 and the average is 2.4. This places BeCl₂ very close to the border between ionic and polar covalent

(b) Magnesium chloride has a high degree of ionic bonding so it has a high boiling point. Beryllium chloride has a large amount of covalent character so the forces of intermolecular attraction will be much lower and hence the lower boiling point will be much lower.

(c) For BeF₂ the difference is 2.4 and the average is 2.8. This puts BeF₂ into the ionic region but less so than MgCl₂ so it is likely to be somewhat lower than 1412 ^oC but considerably higher than 482 ^oC. (The actual value is 1169 ^oC).

3. For UF₆ the difference is 2.6 and the average is 2.7. This puts UF₆ just outside the triangular bonding diagram so no prediction can really be made on the basis of the diagram. It is closest to the ionic part so one might expect it to be ionic. This is actually true for uranium(III) fluoride, which boils at 2300 ^oC but the triangular bonding diagram takes no account of different oxidation states. In fact uranium forms many fluorides, e.g. UF₃, UF₄, UF₅, UF₆, U₄F₁₇ and U₄F₁₈. The higher the oxidation state the more likely the compound is to be covalent. Higher fluorides of other elements such as SF₆ and IF₇ are covalent and have low boiling points so it is not unreasonable for UF₆ to have a relatively low boiling point, so the statement made in C.7 has no real support.

Answers to Materials science introduction questions