Enthalpy : Mean Bond Enthalpy [ Guide Needed ]?

Let's write out what we are given:

Si(s) + 2F2(g) ---> SiF4(g) ΔH = -1615kJ

Si(s) ---> Si(g) ΔH = +456kJ

1/2F2(g) ---> F(g) ΔH = +79 kJ

Here's the reaction we want:

SiF4(g) ---> Si(g) + 4F(g)

The mean bond enthalpy is the energy required to break a bond, in this case one Si-F bond.

Adjust the given reactions as follows:

1) flip

2) leave unchanged

3) multiply by 4

That results in this:

SiF4(g) ---> Si(s) + 2F2(g) ΔH = +1615kJ

Si(s) ---> Si(g) ΔH = +456kJ

2F2(g) ---> 4F(g) ΔH = +316 kJ

When you add the three equations, you add the enthalpies to get +2387 kJ. That is the enthalpy to disrupt four Si-F bonds, so divide by 4 to get +596.75 kJ, which rounds off to +597 kJ

Nice problem.