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»

[Maximum number: 1]

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

Draw a Lewis (electron dot) structure of phosphine.

[Maximum number: 2]

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.

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

Stronger bond:

Longer bond:

[Maximum number: 3]

Urea, $\left(\mathrm{H}_{2} \mathrm{~N}\right)_{2} \mathrm{CO}$ , is excreted by mammals and can be used as a fertilizer.

The structural formula of urea is shown.

Predict the electron domain and molecular geometries at the nitrogen and carbon atoms, applying the VSEPR theory.

	Electron geometry	Molecular geometry
Nitrogen	tetrahedral	trigonal pyramidal
Carbon	trigonal planar	trigonal planar

[Maximum number: 2]

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

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

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

[Maximum number: 1]

Organic compounds have many industrial applications.

The chloroalkene with the formula $\mathrm{C}_{4} \mathrm{H}_{7} \mathrm{Cl}$ can exist as several stereoisomers.

Outline why the cis-isomer is polar.

[Maximum number: 4]

Ethanol has many industrial uses.

Identify three allotropes of carbon and describe their structures.

Allotropes:

diamond
graphite
fullerene
graphene;

Structures:
Diamond:
tetrahedral arrangement of (carbon) atoms/each carbon bonded to four others / $\mathrm{sp}^{3}$ and 3D/covalent network structure;
Graphite:
each carbon bonded to three others (in a trigonal planar arrangement) / $\mathrm{sp}^{2}$ and 2D / layers of (carbon) atoms;
Fullerene:
each (carbon) atom bonded to three others (in a trigonal arrangement) / $\mathrm{sp}^{2}$ and joined in a ball/cage/sphere/connected hexagons and pentagons;
Graphene:
each carbon bonded to three others (in a trigonal arrangement) / $\mathrm{sp}^{2}$ and 2D structure;

[Maximum number: 2]

Urea, $\left(\mathrm{H}_{2} \mathrm{~N}\right)_{2} \mathrm{CO}$ , is excreted by mammals and can be used as a fertilizer.

Sketch two different hydrogen bonding interactions between ammonia and water.

[Maximum number: 1]

Urea, $\left(\mathrm{H}_{2} \mathrm{~N}\right)_{2} \mathrm{CO}$ , is excreted by mammals and can be used as a fertilizer.

Suggest one reason why urea is a solid and ammonia a gas at room temperature.

[Maximum number: 6]

A student wanted to determine what colour inks were mixed to produce black ink and their relative polarities using paper chromatography. The student decided to use two different solvent systems for the separation, both a mixture of propan-2-ol and water. System 1 was 20 % propan-2-ol and system 2 was 80 % propan-2-ol.

Figure for Question 12 — IB Chemistry HL

(a)

Calculate the retardation factor, $R_{\mathrm{F}}$ , of the red ink in both chromatograms.

Table for Question (a) — IB Chemistry HL

[ 2 ]

Substances	$R_{\mathrm{F}}$ values
Red ink separated with 20\% propan-2-ol	0.566
Red ink separated with 80\% propan-2-ol	0.451

Accept 0.530-0.580 for M1.

Accept 0.430-0.480 for M2.

(b)

Deduce, with a reason, which coloured ink is more polar, red or blue.

[ 2 ]

(c)

Explain how the separation of inks is achieved using paper chromatography.

[ 2 ]

[Maximum number: 1]

Urea, $\left(\mathrm{H}_{2} \mathrm{~N}\right)_{2} \mathrm{CO}$ , is excreted by mammals and can be used as a fertilizer.

The C-N bonds in urea are shorter than might be expected for a single C-N bond. Suggest, in terms of electrons, how this could occur.

[Maximum number: 2]

Benzene is an aromatic hydrocarbon.

Discuss the physical evidence for the structure of benzene.

[Maximum number: 3]

Phosphoryl chloride, $\mathrm{POCl}_{3}$ , is a dehydrating agent.

(a)

$\mathrm{POCl}_{3}$ can be prepared by the reaction of phosphorus pentachloride, $\mathrm{PCl}_{5}$ , with tetraphosphorus decaoxide, $\mathrm{P}_{4} \mathrm{O}_{10}$ .

[ 3 ]

(i)

Deduce the Lewis (electron dot) structure of $\mathrm{PCl}_{5}$ .

[ 1 ]

(ii)

Predict the shape of this molecule, using the valence shell electron pair repulsion theory (VSEPR).

[ 1 ]

$\mathrm{PCl}_{5}$ has a trigonal bipyramidal shape.

(iii)

Identify all the different bond angles in $\mathrm{PCl}_{5}$ .

[ 1 ]

$120^{\circ}$ and $90^{\circ}/180^{\circ}$ ;

[Maximum number: 1]

Sulfur trioxide is an important compound in industry.

Sulfur trioxide has more than one possible Lewis (electron dot) structure.

State how chemists decide which Lewis (electron dot) structure is more stable.

[Maximum number: 2]

Describe the formation of $\sigma$ and $\pi$ bonds in an alkene.

[Maximum number: 1]

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

State the hybridization of the phosphorus atom in phosphine.

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Covered syllabus points

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