Thursday
November 27, 2014

Homework Help: AP Chem

Posted by derek on Monday, November 1, 2010 at 1:04am.

Pretend you are a nuclear scientist and try to answer this bonus question. Uranium -235 (235U) is used as a fuel in nuclear power plants. However, natural uranium contains mostly U -238 (238U) and very little uranium -235. Therefore, it is necessary that the natural uranium be enriched in uranium -235 before it can be used. To do this, the natural uranium is first converted to uranium (IV) oxide, UO2, and then to the gaseous compound, uranium (VI) fluoride, UF6, as shown by the equations below.

(1) U(s) + O2(g) -> UO2(s)

(2) UO2(s) + 4 HF(g) -> UF4(s) + 2H2O(g)

(3) UF4(s) + F2(g) -> UF6(g)

The molecules of UF6 that contain the uranium -235 atoms are somewhat lighter in weight than those with the uranium -238 atoms. The difference in mass of the two uranium isotopes enables the molecules of UF6 to be separated by a gaseous diffusion process.

How much energy (in kilojoules) would be involved in producing 100 tons of UF6(g) from UO2(s) (1 ton = 9.08  105 g)? Use the standard heats (enthalpies) of formation below:

COMPOUND HEAT OF FORMATION (ΔHof)
UO2(s) - 1085 kJ/mol
UF4(s) - 1914 kJ/mol
UF6(g) - 2147 kJ/mol
H2O(g) -241.8 kJ/mol
HF(g) -271.1 kJ/mol

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