What is the amount of heat in kJ associated

Question

According to the following thermochemical equation, what is the amount of heat in kJ associated if using 4.1 g of HF? Assume excess SiO₂. (molar mass of HF: 20.01g/mol). Report your answer to the first decimal place.

SiF₄(g) + 2H₂O(l) →SiO₂(s) + 4HF(g)   ΔHrxn = +184 kJ

Answer 1

Hello, Aly,

The balanced equation tells us that when 1 mole of SiSiF₄(g) is combined with 2 moles of H₂O(l), we'll get 1 mole of SiO₂(s) and 4 moles of HF(g) and it will release 184kJ as a result.  But I'll a little confused about the question, since you then state that we are "using HF and that the SiO₂ is in excess." Does the question pertain to the reverse reaction? If so, the reaction would require heat, and ΔHrxn would become -184 kJ (it absorbs energy).

If that is the case, then the question becomes one of calculating how much heat is released if you have less than 4 moles of HF, looking at the original equation. In fact, you only have 4.1 g/(20.01 g/mole) = 0.20 moles. The amount of heat required for this smaller reaction is thus (0.20/4)*(-184 kJ).

I hope that helps,

Bob



ΔHrxn = +184 kJ