2 Q's on Hess's Law

I'm a bit lost on Hess's Law. I understand it completely but these two questions are driving me crazy :-

2. When 10.0g of HI is completely decomposed under standard conditions, 860J of heat is absorbed. Calculate the enthalpy change in kJmol-1 of the following reactions:
HI(g)  1/2H2(g) + 1/2I2(g)
H2(g) + I2(g)  2HI(g)

3. When 12.0g each of carbon, hydrogen and ethyl ethanoate (molar mass 88gmol-1 ) are each burned completely in oxygen, the quantities of heat given out are respectively 394.2kJ, 1715.5 kJ and 306kJ. Calculate the standard enthalpy of combustion of these three substances.

Can anyone guide me on how to answer them step by step?
Thanks!

Reply 1

ooze

10g of HI = X moles.

The first equation is just the decompostition which you are told requires 860J for 10g so (860J / X moles) / 1000 = enthalpy change in kj per mole.

The second equation is the reverse and uses twice as much of everything, so twice as much energy. So the enthalpy change is twice the first, and remember it will be negative.

Enthalpy of combustion is just the enthalpy change during combustion so it's the same thing really. Write out balanced equations, work out number of moles of the substance and use the energy given out to calculate energy per mole. They will all be negative.