IB Chemistry HL S2.4 From models to materials questions

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Question 1

Multiple choiceEasy1 mark

The electronegativity values, χ, of the elements in four binary compounds are shown.

ElementMgONaClCaFC
χ1.23.50.93.01.04.02.5

Which compound is predicted to have the greatest ionic contribution to its bonding?

Show answer and explanation

Answer: C [1]. A larger electronegativity difference indicates a greater ionic contribution. The differences are MgO: |3.5 − 1.2| = 2.3; NaCl: |3.0 − 0.9| = 2.1; CaF₂: |4.0 − 1.0| = 3.0; CF₄: |4.0 − 2.5| = 1.5. CaF₂ has the largest difference.

A: MgO has a substantial difference, but 2.3 is less than 3.0.
B: NaCl is ionic, but its difference of 2.1 is less than 3.0.
D: A high mean electronegativity alone does not indicate a greater covalent contribution; CF₄ is less ionic here because its electronegativity difference is smaller.

Remember: electronegativity difference indicates the ionic contribution to bonding.

Total: [1]

Question 2

Paper 2Hard11 marks

A 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:

CompoundRemained solid at 1500 °CElectrical conductivity in the test
MgOYesBelow the detection limit
SiCYesMeasurable
NiAlYesHigh
(a)

For MgO and SiC, calculate Δχ and the mean electronegativity, χ, to two decimal places.

[4]
(b)

Using your results from (a) and the electronegativities given for NiAl, state which bonding contribution is greatest for each compound: ionic, covalent or metallic.

[3]
(c)

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.

[4]

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