IB Chemistry HL 2.2 The Covalent Model Questions

Practise Lewis structures, molecular geometry, resonance, formal charge and hybridization, then apply intermolecular forces and Rf evidence in IB Chemistry 2.2.

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.
Use resonance to explain intermediate bonds and benzene’s equal C–C lengths, stability and substitution.
Draw expanded-octet Lewis structures and use formal charge to select or compare valid structures.
Relate sp, sp² and sp³ hybridization to electron domains, σ/π overlap and bond counts in molecules.
Identify ligand-to-metal coordinate bonds in transition-element complexes from donated lone pairs.

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 2.2.11 (HL)—Resonance structures 2.2.12 (HL)—Benzene (C₆H₆)2.2.13 (HL)—Expanded octets 2.2.14 (HL)—Formal charge 2.2.15 (HL)—Sigma (σ) and pi (π) bonds 2.2.16 (HL)—Hybridization

[Maximum number: 4]

Phosphine (IUPAC name phosphane) is a hydride of phosphorus, with the formula $\mathrm{PH}_{3}$ .

(a)

Draw a Lewis (electron dot) structure of phosphine.

[ 1 ]

(b)

State the hybridization of the phosphorus atom in phosphine.

[ 1 ]

(c)

Outline whether you expect the bonds in phosphine to be polar or non-polar, giving a brief reason.

Figure for Question (c) — IB Chemistry HL

Phosphine has a much greater molar mass than ammonia. Explain why phosphine has a significantly lower boiling point than ammonia.

[ 2 ]

[Maximum number: 12]

(a)

An organic compound, X, with a molar mass of approximately $88 \mathrm{~g} \mathrm{~mol}^{-1}$ contains 54.5 % carbon, 36.3 % oxygen and 9.2 % hydrogen by mass.

[ 5 ]

(i)

The carboxylic acid contains two different carbon-oxygen bonds. Identify which bond is stronger and which bond is longer.

Stronger bond:

Longer bond:

[ 2 ]

(ii)

Predict and explain the bond lengths and bond strengths of the carbon-oxygen bonds in $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COO}^{-}$ .

[ 3 ]

(b)

State the meaning of the term hybridization.

[ 1 ]

(c)

Describe the hybridization of the carbon atom in methane and explain how the concept of hybridization can be used to explain the shape of the methane molecule.

[ 3 ]

$\mathrm{sp}^{3}$ ;
isolated C atom electron configuration $1 \mathrm{~s}^{2} 2 \mathrm{~s}^{2} 2 \mathrm{p}^{2} /$ excited state C electron configuration is $1 \mathrm{~s}^{2} 2 \mathrm{~s}^{1} 2 \mathrm{p}^{3}$ ;
$2 \mathrm{~s}^{1} 2 \mathrm{p}^{3}$ electrons blend to form four identical hybrid orbitals;
hybrid orbitals lower in total energy than atomic orbitals;
repulsion of (identical hybrid) orbitals creates a tetrahedral shape;

(d)

Identify the hybridization of the carbon atoms in diamond and graphite and explain why graphite is an electrical conductor.

[ 3 ]

diamond:
$\mathrm{sp}^{3}$ ;
graphite:
$\mathrm{sp}^{2}$ ;
(p) electrons delocalized (around layer);

[Maximum number: 2]

Solid ionic compounds form crystal lattices.

Cobalt also forms chlorides with the formula $\mathrm{CoCl}_{2}$ .

Hydrated cobalt (II) ions, $\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}{ }^{2+}$ , are pink. Describe the interaction between the cobalt ion and a water molecule in terms of the type of bond and how this bond is formed.

Type of bond:
How the bond forms:
$\mathrm{CoCl}_{4}{ }^{2-}$ ions are blue.

Type of bond: coordinate/dative/covalent

How the bond forms: oxygen/water molecule /ligand donates $\mathrm{e}^{-}$ pair to cobalt«(II) ion»

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Covered syllabus points

Question 1

Question (a)

Question (b)

Question (c)

Question 2

Question (a)

Question (i)

Question (ii)

Question (b)

Question (c)

Question (d)

Question 3