OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 16

7 marks · Medium difficulty · Structured Questions

Draw a dot-and-cross diagram for the [AlH4]- ion, predict shapes and bond angles for NH4+ and NH2- ions, and explain differences in boiling points of hydrides based on intermolecular forces.

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Question

Question text

16 This question is about ions and compounds containing hydrogen.

(a) Lithium aluminium hydride, LiAl H , contains the Al H – ion.

Draw a ‘dot-and-cross’ diagram to show the bonding in an Al H – ion.

Show outer electrons only.

[1]

(b) Nitrogen forms NH + and NH – ions.

Predict the name of the shape of, and H–N–H bond angle in, NH + and NH –.

Ion Name of shape H–N–H bond angle

NH +

NH –

[2]

The boiling points of their hydrides are shown below.

Element Hydride Boiling point / °C

N NH3 –33

P PH3 –88

As AsH3 –55

(i) Explain why the boiling point of PH3 is lower than the boiling point of NH3.

… [2]

(ii) Explain why the boiling point of PH3 is lower than the boiling point of AsH3.

… [2]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

16 (a) (i) 1 IGNORE no brackets, no charge or wrong charge

H Circles not needed

H Al H ALLOW different sign for ‘extra’ electron, e.g.

H H

H Al H

DO NOT ALLOW 4 dots and 4 crosses

(b) NH +: tetrahedral AND 109.5(º) 2 ALLOW 109−110(º)

NH –: non-linear AND 104.5(º) ALLOW 104−105(º)

ALLOW bent, v-shaped, angular

IGNORE planar, ‘not straight’

NH3 has hydrogen bonding

PH3 does not have hydrogen bonding Assume that comparison is with PH3

Hydrogen bonding is stronger DO NOT ALLOW response that implies covalent or

OR ionic bonds breaking

More energy to overcome hydrogen bonding

(ii) 2 ORA throughout

AsH3 / As has more electrons (than PH3 / P) ALLOW larger electron cloud

in AsH3, ALLOW ‘forces’ OR ‘bonds’ for ‘interactions’

stronger/more induced dipole–dipole interactions ALLOW instantaneous/temporary–induced dipole

OR stronger/more London forces (than PH3) interactions

OR more energy required to overcome induced ALLOW dispersion forces

dipole–dipole interactions

IGNORE van der Waals’ / vdW

IGNORE permanent dipole–dipole

DO NOT ALLOW response that implies covalent or

ionic bonds breaking

Total 7

How to answer it

Ions and Compounds Containing Hydrogen

What this question tests

This question assesses your understanding of chemical bonding, electron pair repulsion theory (VSEPR) for predicting shapes and bond angles of ions, and intermolecular forces (specifically hydrogen bonding vs. induced dipole-dipole interactions) to explain physical property trends like boiling points.

Part (a)

Dot-and-Cross Diagram for AlH₄⁻

Drawing covalent bonding in complex hydride ions

✅ Correct Answer

Centre Al surrounded by 4 single covalent bonds to H atoms. Use dots for Al outer electrons and crosses for H electrons (or vice versa), plus an extra electron from the negative charge. Enclose in square brackets with a [-] charge outside.

💡 Key Knowledge

Aluminium is in Group 13 with 3 outer electrons. The 1− charge adds a 4th electron, meaning Al contributes 4 electrons. Each H contributes 1 electron, giving 8 bonding electrons in total (4 pairs) arranged tetrahedrally.

❌ Common Errors

Students often draw incorrect electron configurations by forgetting to include the extra electron for the 1− charge, or incorrectly drawing 4 dots and 4 crosses without accounting for sharing.

Mark: 1 mark

Part (b)

Shapes and Bond Angles of NH₄⁺ and NH₂⁻

Applying VSEPR theory to species with different lone pairs

✅ Correct Answer

NH₄⁺: Shape = Tetrahedral | Bond angle = 109.5°
NH₂⁻: Shape = Non-linear (or bent / v-shaped / angular) | Bond angle = 104.5°

💡 Key Knowledge

• NH₄⁺ has 4 bonding pairs and 0 lone pairs around N, leading to equal repulsion.
• NH₂⁻ has 2 bonding pairs and 2 lone pairs. Lone pair–lone pair repulsion is greater than bond pair–bond pair repulsion, compressing the bond angle by roughly 2.5° per lone pair.

🧠 Exam Technique

Both conditions (correct shape AND correct angle) must be met to score the mark for each ion. Make sure to memorise standard angles: tetrahedral (109.5°), trigonal pyramidal (107°), non-linear (104.5°).

Mark: 2 marks (1 per row)

Part (c)(i)

Boiling Point Comparison: NH₃ vs PH₃

Explaining the anomaly of hydrogen bonding

✅ Correct Answer

NH₃ has hydrogen bonding (whereas PH₃ does not). Hydrogen bonding is stronger, so more energy is required to overcome these intermolecular forces in NH₃.

💡 Key Knowledge

Hydrogen bonding occurs when H is covalently bonded to highly electronegative atoms with lone pairs (N, O, or F). Phosphorus is not sufficiently electronegative to form hydrogen bonds.

❌ Common Errors

CRITICAL ERROR: Stating that "covalent bonds break" during boiling. Boiling only overcomes intermolecular forces, not covalent bonds within the molecules!

Mark: 2 marks

Part (c)(ii)

Boiling Point Comparison: PH₃ vs AsH₃

Trends down Group 15 hydride boiling points

✅ Correct Answer

AsH₃ has more electrons than PH₃, leading to stronger / more induced dipole-dipole interactions (London forces). Therefore, more energy is required to overcome them in AsH₃.

🧠 Exam Technique

Use precise terminology mandated by examiners: write "induced dipole-dipole interactions" or "London forces". Avoid outdated terms like "van der Waals' forces" or "vdw" as examiners heavily penalise ambiguity.

❌ Common Errors

Failing to link the increase in boiling point to the number of electrons or size of the electron cloud. Simply saying "AsH₃ is a bigger molecule" is often insufficient without referencing electrons or dipole interactions.

Mark: 2 marks

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · 3.1 The periodic table · 2.2 Electrons, bonding and structure

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.