ThermochemistryHard

Question

Tungsten carbide is very hard and is used to make cutting tools and rock drills. What is the enthalpy of formation (in kJ/ mol) of tungsten carbide? The enthalpy change for this reaction is difficult of measure directly, because the reaction occurs at 1400°C. However, the enthalpies of combustion of the elements and of tungsten carbide can be measured easily.

2W(s) + 3O2(g) → 2WO3(s): ΔH = −1680.6 kJ

C(Graphite) + O2(g) → CO2(g): ΔH = −393.5 kJ

2WC(s) + 5O2(g) → 2WO3(s) + 2CO2(g): ΔH = −2391.6 kJ

Options

A.−38.0
B.−76.0
C.−19.0
D.−1233.8

Solution

(a) 2W(s) + 3O2(g) → 2WO3(s); ΔH = –1680.6 KJ

(b) C(s) + O2(g) → CO2(g); ∆H = –393.5 KJ

(c) 2WC(s) + 5O2(g) → 2WO3(s) + 2CO2(g); ΔH = –2391.6 KJ

Required T.C.E. is W(s) + C(s) → WC(s); ΔH =?

From $\frac{1}{2}(a) + (b) - \frac{1}{2}(c);\Delta H = - 38.0\text{ KJ}$

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