RI Prelims 1999 P2 Section A Q 7
Flourine forms useful compounds called hexaflourides.
(a) Uranium forms 2 types of gaseous uranium hexaflouride, 235UF6 and 238UF6, based on the 2 isotopes of uranium.
(i) What are isoptopes?
(ii) Which isotope of uranium is radioactive? [out of syllabus]
(iii) Which gas, 235UF6 or 238UF6, diffuses faster? Explain your answer.
(b) Sodium aluminium hexaflouride or cryolite is used in the extraction of aluminium by electrolysis.
(i) Explain the role of cryolite in the extraction of aluminium. [out of syllabus]
(ii) Why is it not possible to extract aluminium by electrolysing a solution of an aluminium salt, like aluminium chloride, using carbon electrodes?
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Answer:
(a)
(i) They are elements with the same chemical properties and number of protons, but with different number of neutrons.
(ii) Uranium-235, 23592U [out of syllabus]
(iii) 235UF6 diffuses faster. This is because it has a smaller molecular mass than 238UF6.
(b)
(i) The mixture of cryolite and aluminium oxide has a lower melting point than pure aluminium oxide. [out of syllabus]
(ii) Aluminium is very reactive; it is higher in the reactivity series than hydrogen. Thus, hydrogen ions will be preferably discharged, not aluminium.
