A-LevelChemistryChemical periodicity9701/21 · Oct/Nov 2019 · Q213 Marks

2 (a) Complete the table to give details of the type of bonding and structure shown by some of the oxides of Period 3 elements. [Figure 2.1]

(a)Complete the table to give details of the type of bonding and structure shown by some of the oxides of Period 3 elements. Na2O | MgO | Al2O3 | SiO2 | SO3 boiling point/°C | 1275 | 3670 | 2977 | 2950 | 45 nature of oxide | basic | basic | amphoteric | acidic | acidic bonding structure[2]
(b)(i)Explain why the boiling point of SiO2 is much higher than the boiling point of SO3.[3]
(b)(ii)Al2O3 is an amphoteric oxide. Explain what is meant by the term amphoteric. Use chemical equations to illustrate your answer.[3]
(b)(iii)State what you would observe when a small sample of Na2O is placed in water.[1]
(c)(i)Selenium is a Group 16 element which shows similar chemical reactions to sulfur. Selenium reacts with fluorine to form SeF6 molecules. Predict the shape of a molecule of SeF6.[1]
(c)(ii)The most stable oxide of selenium is SeO2. Gaseous SeO2 reacts to form a solid polymer, as shown. In the reaction one Se=O is replaced by two Se–O to form a polymer. [Figure 2.2] The bond enthalpy of Se=O is 514 kJ mol–1. Use these data to calculate the bond enthalpy, in kJ mol–1, of Se–O. bond enthalpy of Se–O =[2]
(c)(iii)SeO2 shows similar chemical reactions to SO2. Suggest an equation to show the reaction of SeO2 with aqueous sodium hydroxide, NaOH.[1]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

[2] 2(a) Na2O: ionic, giant; MgO: ionic, giant; Al2O3: ionic, giant; SiO2: covalent, giant/macro-molecular; SO3: covalent, simple/molecular. (Award one mark for each correct row)
[3] 2(b)(i) M1 SiO2 has a network of strong bonds / SiO2 has many strong bonds M2 SO3 has weak intermolecular forces OR weak VdW forces (between molecules) M3 high(er) / more energy required to break bonds than overcome forces (between molecules)
[3] 2(b)(ii) M1: reacts with both acid and base / alkali M2: use any equation with Al2O3 and an acid, e.g. Al2O3 + 6HCl → 2AlCl3 + 3H2O M3: use any equation with Al2O3 and a base / alkali, e.g. Al2O3 + 2NaOH + 3H2O → 2NaAl(OH)4
[1] 2(b)(iii) solid dissolves / disappears OR gets warm / hot
[1] 2(c)(i) octahedral
[2] 2(c)(ii) M1: use of the correct expression in terms of specific bond energies. (514 - xEse-o = -346) M2: use of correct stoichiometry AND correct processing of expression given in M1. Provided the values 514 and 346 are used. (514 - 2Ese-o = -346) = (+)430 (kJ mol-1)
[1] 2(c)(iii) SeO2 + 2NaOH → Na2SeO3 + H2O

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