Question 1
Paper 2Hard11 marksA furnace-lining manufacturer is comparing three binary compounds for a component that must remain solid at 1500 °C and limit electrical conduction. Use the electronegativity values in the Data Booklet for Mg, O, Si and C; for the third compound, use χ(Ni) = 1.9 and χ(Al) = 1.5.
In the qualitative bonding triangle, increasing the electronegativity difference, Δχ, moves a compound towards ionic bonding. When Δχ is small, a higher mean electronegativity favours covalent character, while a lower mean electronegativity favours metallic character.
Tests on the three samples gave the following results:
| Compound | Remained solid at 1500 °C | Electrical conductivity in the test |
|---|---|---|
| MgO | Yes | Below the detection limit |
| SiC | Yes | Measurable |
| NiAl | Yes | High |
For MgO and SiC, calculate Δχ and the mean electronegativity, χ, to two decimal places.
Using your results from (a) and the electronegativities given for NiAl, state which bonding contribution is greatest for each compound: ionic, covalent or metallic.
An engineer claims: “SiC must be a better electrical insulator than MgO because its smaller electronegativity difference places it closer to covalent bonding; covalent-dominant compounds do not conduct electricity.” Evaluate this claim and recommend one of the three compounds for the furnace lining, using the bonding model and the test results.