IB Chemistry SL 2.2 The Covalent Model Questions

Practise covalent structures, molecular shapes, bond polarity and intermolecular-force explanations, then interpret chromatograms using Rf and phase attractions.

Exam points

Draw Lewis structures from valence-electron counts, showing shared pairs, lone pairs and any ionic charge.
Compare single, double and triple bonds by order, length and strength using bond-enthalpy evidence.
Identify a coordinate bond from the atom that donates both electrons to the shared pair.
Use Lewis structures to predict electron geometry, molecular shape, bond angles and lone-pair effects.
Assign δ+ and δ− from electronegativity, then use geometry to decide whether molecular dipoles cancel.
Explain conductivity, melting behaviour or strength by comparing molecular and giant covalent structures.
Identify London, dipole–dipole and hydrogen-bonding forces; draw valid intermolecular hydrogen bonds.
Explain boiling-point and solubility trends by comparing intermolecular attractions.
Calculate Rf from spot and solvent-front distances; use phase affinity to explain separation.

Covered syllabus points

2.2.1—Covalent bonding 2.2.2—Bond types and strength 2.2.3—Coordination bonds 2.2.4—VSEPR model 2.2.5—Bond polarity 2.2.6—Molecular polarity 2.2.7—Covalent network structures 2.2.8—Intermolecular forces (IMF)2.2.9—IMF strength and properties 2.2.10—Chromatography

[Maximum number: 6]

Ethyne, $\mathrm{C}_{2} \mathrm{H}_{2}$ , reacts with oxygen in welding torches.

(a)

Deduce the Lewis (electron dot) structure of ethyne.

[ 1 ]

$\mathrm{H}: \mathrm{C}:: \mathrm{C}: \mathrm{H} / \mathrm{H}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}$

Marking guidance:

Accept any valid combination of lines, dots and crosses.

(b)

Compare, giving a reason, the length of the bond between the carbon atoms in ethyne with that in ethane, $\mathrm{C}_{2} \mathrm{H}_{6}$ .

[ 1 ]

(c)

Identify the type of interaction that must be overcome when liquid ethyne vaporizes.

[ 1 ]

(d)

Product $\mathbf{B}, \mathrm{CH}_{3} \mathrm{CHO}$ , can also be synthesized from ethanol.

[ 3 ]

(i)

Explain why product B is water soluble.

[ 3 ]

[Maximum number: 7]

Phosphorus is an element that is an essential part of the biological molecules involved in both respiration and photosynthesis.

(a)

7.75 g of phosphorus was combusted in a limited supply of oxygen, the mass of the oxide produced was 13.75 g .

[ 4 ]

(i)

This oxide of phosphorus has a melting point of 296.9 K .

Outline, with reference to structure and bonding, the reasons why the melting point of this oxide is low.

[ 2 ]

(ii)

Describe the nature of the bond between oxygen and phosphorus. Use sections 9 and 17 of the data booklet.

[ 2 ]

$\text { " } 3.4-2.2=1.2,(3.4+2.2) / 2=2.8 \text { " }$

Electronegativity difference is 1.2 .

AND

Average electronegativity is 2.8

polar covalent

(b)

Another compound of phosphorus is phosphine, $\mathrm{PH}_{3}$ , it forms a phosphonium ion, $\mathrm{PH}_{4}^{+}$ .

State the types of bonds present in the phosphonium ion, $\mathrm{PH}_{4}{ }^{+}$ , and explain how these bonds are formed.

[ 2 ]

(c)

Phosphorus trichloride, $\mathrm{PCl}_{3}$ , is a common compound of phosphorus.

Deduce the Lewis formula of $\mathrm{PCl}_{3}$ .

[ 1 ]

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Covered syllabus points

Question 1

Question (a)

Question (b)

Question (c)

Question (d)

Question (i)

Question 2

Question (a)

Question (i)

Question (ii)

Question (b)

Question (c)