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Date	May 2019	Marks available	2	Reference code	19M.2.sl.TZ2.4
Level	SL	Paper	2	Time zone	TZ2
Command term	Describe	Question number	4	Adapted from	N/A

Question

Rhenium, Re, was the last element with a stable isotope to be isolated.

One chloride of rhenium has the empirical formula ReCl₃.

Before its isolation, scientists predicted the existence of rhenium and some of its properties.

Suggest the basis of these predictions.

[2]

Describe how the relative reactivity of rhenium, compared to silver, zinc, and copper, can be established using pieces of rhenium and solutions of these metal sulfates.

[2]

State the name of this compound, applying IUPAC rules.

[1]

c(i).

Calculate the percentage, by mass, of rhenium in ReCl₃.

[2]

c(ii).

Markscheme

gap in the periodic table
OR
element with atomic number «75» unknown
OR
break/irregularity in periodic trends [✔]

«periodic table shows» regular/periodic trends «in properties» [✔]

place «pieces of» Re into each solution [✔]

if Re reacts/is coated with metal, that metal is less reactive «than Re» [✔]

Note: Accept other valid observations such as “colour of solution fades” or “solid/metal appears” for “reacts”.

rhenium(III) chloride
OR
rhenium trichloride [✔]

c(i).

«M_r ReCl₃ = 186.21 + (3 × 35.45) =» 292.56 [✔]

«100 × $\frac{{186.21}}{{292.56}}$ =» 63.648 «%» [✔]

c(ii).

Examiners report

This nature of science question generated a lot of discussion among teachers. Some in support of such questions and others concerned that it takes a lot of time for candidates to know how to answer. Some teachers thought it was unclear what the question was asking. It is pleasing that about a quarter of the candidates answered both parts successfully and many candidates gained one mark usually for “periodic trends”. However, some candidates only focused on one part of the question. Quite a few candidates discussed isotopes, probably thrown off by the stem. A teacher was concerned that since transition metals are not part of the SL syllabus that Re was a bad choice, however, the question did not really require any transition metal chemistry to be answered.

This question was a good discriminator between high-scoring and low-scoring candidates. It was well answered by more than half of the candidates who had obviously carried out such displacement reactions and interpreted the outcomes during the course. Some candidates did not state the obvious of dipping the metal into the sulfates.

More than half of the candidates named ReCl₃ correctly. Common mistakes included “rhenium chloride” and “trichlororhenium”.

c(i).

The majority of candidates calculated the percentage, by mass, of rhenium in ReCl₃ correctly. Some rounding errors were seen that students should be more careful with.

c(ii).

Syllabus sections

Core » Topic 9: Redox processes » 9.1 Oxidation and reduction

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Core » Topic 9: Redox processes

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