DP Chemistry (first assessment 2025)

Question 21M.2.SL.TZ2.2

Date	May 2021	Marks available	[Maximum mark: 17]	Reference code	21M.2.SL.TZ2.2
Level	SL	Paper	2	Time zone	TZ2
Command term	Deduce, Describe, Explain, Justify, State, State and explain, Suggest	Question number	2	Adapted from	N/A

[Maximum mark: 17]

21M.2.SL.TZ2.2

The properties of elements can be predicted from their position in the periodic table.

(a(i))

Explain why Si has a smaller atomic radius than Al.

[2]

Markscheme

nuclear charge/number of protons/Z/Z_eff increases «causing a stronger pull on the outer electrons» ✓

same number of shells/«outer» energy level/shielding ✓

(a(ii))

Explain the decrease in radius from Na to Na⁺.

[2]

Markscheme

Na⁺ has one less energy level/shell
OR
Na⁺ has 2 energy levels/shells AND Na has 3 ✓

less shielding «in Na⁺ so valence electrons attracted more strongly to nucleus»
OR
effective nuclear charge/Z_eff greater «in Na⁺ so valence electrons attracted more strongly to nucleus» ✓

Accept “more protons than electrons «in Na⁺»” OR “less electron-electron repulsion «in Na⁺»” for M2.

(b(i))

State the condensed electron configurations for Cr and Cr3⁺.

[2]

Markscheme

Cr:
[Ar] 4s¹3d⁵ ✓

Cr³⁺:
[Ar] 3d³ ✓

Accept “[Ar] 3d⁵4s¹”.

Accept “[Ar] 3d³4s⁰”.

Award [1 max] for two correct full electron configurations “1s²2s²2p⁶3s²3p⁶4s¹3d⁵ AND 1s²2s²2p⁶3s²3p⁶3d³”.

Award [1 max] for 4s¹3d⁵ AND 3d³.

(b(ii))

Describe metallic bonding and how it contributes to electrical conductivity.

[3]

Markscheme

electrostatic attraction ✓

between «a lattice of» cations/positive «metal» ions AND «a sea of» delocalized electrons ✓

mobile electrons responsible for conductivity
OR
electrons move when a voltage/potential difference/electric field is applied ✓

Do not accept “nuclei” for “cations/positive ions” in M2.

Accept “mobile/free” for “delocalized” electrons in M2.

Accept “electrons move when connected to a cell/battery/power supply” OR “electrons move when connected in a circuit” for M3.

(c)

Deduce the Lewis (electron dot) structure and molecular geometry of sulfur dichloride, SCl₂.

[2]

Markscheme

(d)

Suggest, giving reasons, the relative volatilities of SCl₂ and H₂O.

[3]

Markscheme

H₂O forms hydrogen bonding «while SCl₂ does not» ✓

SCl₂ «much» stronger London/dispersion/«instantaneous» induced dipole-induced dipole forces ✓

Alternative 1:
H₂O less volatile AND hydrogen bonding stronger «than dipole–dipole and dispersion forces» ✓

Alternative 2:
SCl₂ less volatile AND effect of dispersion forces «could be» greater than hydrogen bonding ✓\

Ignore reference to Van der Waals.

Accept “SCl₂ has «much» larger molar mass/electron density” for M2.

(e)

Consider the following equilibrium reaction:

2SO₂(g) + O₂(g) $⇌$ 2SO₃(g)

State and explain how the equilibrium would be affected by increasing the volume of the reaction container at a constant temperature.

[3]

Markscheme

pressure decrease «due to larger volume» ✓

reactant side has more moles/molecules «of gas» ✓

reaction shifts left/towards reactants ✓

Award M3 only if M1 OR M2 is awarded.

Syllabus sections

Structure 3. Classification of matter » Structure 3.1—The periodic table: Classification of elements » Structure 3.1.3—Periodicity refers to trends in properties of elements across a period and down a group.

Structure 2. Models of bonding and structure » Structure 2.1—The ionic model » Structure 2.1.1—When metal atoms lose electrons, they form positive ions called cations. When non-metal atoms gain electrons, they form negative ions called anions. Predict the charge of an ion from the electron configuration of the atom.

Structure 1. Models of the particulate nature of matter » Structure 1.3—Electron configurations » Structure 1.3.5—Each orbital has a defined energy state for a given electron configuration and chemical environment, and can hold two electrons of opposite spin. Sublevels contain a fixed number of orbitals, regions of space where there is a high probability of finding an electron. Apply the Aufbau principle, Hund’s rule and the Pauli exclusion principle to deduce electron configurations for atoms and ions up to Z = 36.

Structure 2. Models of bonding and structure » Structure 2.3—The metallic model » Structure 2.3.1—A metallic bond is the electrostatic attraction between a lattice of cations and delocalized electrons. Explain the electrical conductivity, thermal conductivity and malleability of metals.

Structure 2. Models of bonding and structure » Structure 2.2—The covalent model » Structure 2.2.1—A covalent bond is formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei. The octet rule refers to the tendency of atoms to gain a valence shell with a total of 8 electrons. Deduce the Lewis formula of molecules and ions for up to four electron pairs on each atom.

Structure 2. Models of bonding and structure » Structure 2.2—The covalent model » Structure 2.2.4—The valence shell electron pair repulsion (VSEPR) model enables the shapes of molecules to be predicted from the repulsion of electron domains around a central atom. Predict the electron domain geometry and the molecular geometry for species with up to four electron domains.

Structure 2. Models of bonding and structure » Structure 2.2—The covalent model » Structure 2.2.6—Molecular polarity depends on both bond polarity and molecular geometry. Deduce the net dipole moment of a molecule or ion by considering bond polarity and molecular geometry.

Structure 2. Models of bonding and structure » Structure 2.2—The covalent model » Structure 2.2.9—Given comparable molar mass, the relative strengths of intermolecular forces are generally: London (dispersion) forces < dipole–dipole forces < hydrogen bonding. Explain the physical properties of covalent substances to include volatility, electrical conductivity and solubility in terms of their structure.

Reactivity 2. How much, how fast and how far? » Reactivity 2.3—How far? The extent of chemical change » Reactivity 2.3.4—Le Châtelier’s principle enables the prediction of the qualitative effects of changes in concentration, temperature and pressure to a system at equilibrium. Apply Le Ch.telier’s principle to predict and explain responses to changes of systems at equilibrium.

Question 21M.2.SL.TZ2.2

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