[castromatic]

My brain is really trying to make this one work.

Carbon occurs in two forms, graphite and diamond. The enthalpy of the combustion
of graphite is -393.5 kJ/mol and that of diamond is -395.4 kJ/mol:

C(graphite) + O2{g) -----> C02{g)  DeltaH1 = -393.5 kJ
C(diamond) + O2(g) -----> CO2 (g) DeltaH2 = -395.4 kJ

Calculate DeltaH for the conversion of graphite to diamond
C(graphite) ----> C(diamond)

The answer in the workbook is positive 1.9 kilojoules.

But using Hess's equation:
reverse diamond so it's in the product side
-DeltaH2=395.4

395.4-(-393.5) = 788.9 kiloJoules! what???

if we don't reverse (which is the method I've been using for the 2 practice problems I solved)
-395.4-(-393.5) =-1.9 kilojoules.

How does it become positive?
Do I also need to reverse the first equation?
(395.4-393.5=1.9)
Or is it
-393.5-(-395.4)=1.9?
*delete me*

Also, saw in some manuals that you just add the two (395.4+(-353.5)) = 1.9

But that's not what I was taught!

Can someone educate me lol

[mjc123]

Don't try to blindly apply formulas you don't understand. Think about the situation. Draw the Hess's Law cycle for this scenario. What path is equivalent to graphite  diamond?

Be careful about using heats of combustion. You may be familiar with the equation
ΔH_rxn = ΣΔH_f(products) - ΣΔH_f(reactants)
But in the equation for the formation of X, X is the product. In the equation for the combustion of X, X is a reactant. How then would you write ΔH_rxn in terms of the enthalpies of combustion?

[Enthalpy]

Some reasons to think instead of applying formulas:

• You are a human, not a software.
• In 20 years, you'll have forgotten all formulas.
• Occasionally, conventions differ. This must not fool you.
• Some data sources contain sign mistakes.
• If you have 10% sign error probability on 10 individual steps, the result is random.

Here the heats of combustion are nearly identical, so the reactants have nearly the same heat of formation.