IB Chemistry SL The Covalent Model

Review IB Chemistry the covalent model through Lewis structures, bond polarity, molecular shape, intermolecular forces and separation evidence.

Syllabus
First assessment 2025
Topic
2.2
Level
SL

Exam points

  • draw Lewis structures and use VSEPR to predict shapes, bond angles and lone-pair effects
  • deduce bond and molecular polarity from electronegativity, geometry and net dipoles

Covered syllabus points

2.2.1—Covalent bonding2.2.2—Bond types and strength2.2.3—Coordination bonds2.2.4—VSEPR model2.2.5—Bond polarity2.2.6—Molecular polarity2.2.7—Covalent network structures2.2.8—Intermolecular forces (IMF)2.2.9—IMF strength and properties2.2.10—Chromatography

2.2 The covalent model question 1

[Maximum number: 3]

Ethyne, C2H2\mathrm{C}_{2} \mathrm{H}_{2}, reacts with oxygen in welding torches.

Question (a)

(a)

Deduce the Lewis (electron dot) structure of ethyne.

[ 1 ]

Question (b)

(b)

Compare, giving a reason, the length of the bond between the carbon atoms in ethyne with that in ethane, C2H6\mathrm{C}_{2} \mathrm{H}_{6}.

[ 1 ]

Question (c)

(c)

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

[ 1 ]

2.2 The covalent model question 2

[Maximum number: 7]

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

Question (a)

(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 ]

Question (i)

(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 ]

Question (ii)

(ii)

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

[ 2 ]

Question (b)

(b)

Another compound of phosphorus is phosphine, PH3\mathrm{PH}_{3}, it forms a phosphonium ion, PH4+\mathrm{PH}_{4}^{+}.

State the types of bonds present in the phosphonium ion, PH4+\mathrm{PH}_{4}{ }^{+}, and explain how these bonds are formed.

[ 2 ]

Question (c)

(c)

Phosphorus trichloride, PCl3\mathrm{PCl}_{3}, is a common compound of phosphorus.

Deduce the Lewis formula of PCl3\mathrm{PCl}_{3}.

[ 1 ]
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