A-LevelChemistryGroup 179701/23 · May/June 2021 · Q113 Marks

The elements in Group 17 are known as the halogens.

(a)(i)State the trend in the relative strength of van der Waals' forces down Group 17.[1]
(a)(ii)State the physical state of each of the halogens under room conditions. chlorine bromine jodine[1]
(b)A solution of aqueous bromide ions, Br-(aq), is added to separate samples of Cl2(aq) and I2(aq). Describe what is observed in each reaction. Explain your answer in terms of the relative reactivity of these elements as oxidising agents. observation on addition to Cl2(aq) observation on addition to I2(aq) explanation[3]
(c)Bleach is made by reacting Cl2 with cold NaOH(aq). Write an equation for the reaction of Cl2 with cold NaOH.[1]
(d)(i)When ClO-(aq) is added to water, it behaves as a Brønsted-Lowry base. Define the term Brønsted-Lowry base.[1]
(d)(ii)Write an ionic equation for the reaction between ClO- and H2O.[1]
(e)The concentration of NaClO in bleach S is xg dm-3. NaClO reacts with H2O2(aq) as shown. H2O2(aq) + NaClO(aq) → H2O(l) + NaCl(aq) + O2(g) A 5.00 cm3 sample of S completely reacts with H2O2(aq). The volume of O2(g) produced is 24.0 cm3 under room conditions. Assume that only the NaClO in S reacts with H2O2(aq). Calculate x. Show your working. x = ................................................. g dm-3[3]
(f)Sodium chlorate(I), NaClO, oxidises dilute hydrochloric acid to form three products. The products which contain chlorine have chlorine species with oxidation number -1 or 0. No other species changes its oxidation number during the reaction. Use this information to complete the ionic equation. ........ClO- + ......HCl → ........ + ........ + ........[2]

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

[1] 1(a)(i) increases
[1] 1(a)(ii) chlorine gas bromine liquid iodine solid
[1] 1(b) M1 observation with Cl2(aq) (colourless / pale green to) orange / brown
[1] M2 observation with I2(aq) no visible change
[1] M3 explanation chlorine is a stronger oxidising agent (than bromine) AND iodine is a weaker oxidising agent
[1] 1(c) Cl2 + 2NaOH → NaCl + NaClO + H2O
[1] 1(d)(i) proton / H+ acceptor
[1] 1(d)(ii) ClO- + H2O → HClO + OH-
[1] 1(e) M1 Use volume O2 to express/find no mol O2 produced 24/24 000 mol O2 produced = 1 × 10-3 (mol)
[1] M2 Use 1:1 ratio to deduce no mol NaClO in 1000 cm³ sample M1 × 1000 / 5 = 0.2 (mol dm-³)
[1] M3 Use amount of NaClO and its molecular mass (74.5) to find the mass of NaClO added -24 / 24 000 × 1000 / 5 × 74.5 = 14.9 (g dm-3)
[1] 1(f) M1 products contain Cl- AND Cl2
[1] M2 correct balanced equation ClO- + 2HCl → Cl- + H2O + Cl2