GS-2 Physical Sciences Question 37

[mashbour8199]

Hi there!

The equation for H rxn is:

delta H rxn = product - reactants

and since these are the reaction pathways we see:

P(red) ---> CO2 (H= -2900 kJ/mol)
CO2 ---> P(white) (H= +3020 kJ/mol) (here, the sign is reversed from the one in the chart because we are looking at the formation of an oxide backwards)

Then shouldn't the final eqn be:

delta H rxn = 3020 - (-2900) = 5920?

That's the way I saw it, any suggestions/tips please? Thanks!

[enachbe18198]

There's a typo here... the answer choice says +120 kJ/mol but in the explanation I see -120 kJ/mol. If I understand this correctly, I think the answer should be negative...

-3020 + 2900 = -120 .. can someone please clarify?

[mintzer_ja7368]

I think that the answer is +120 because:

Pred ---> Pox

Rxn = -2900

Pox ---> Pwhite

Rxn = 3200

And according to Hess's law, enthalpy changes of reactions are additive.

Thus Delta rxn = 3200 + -2900 = +120 kj/mol

[enachbe18198]

Okay I see why I made my mistake.. thanks!

[boshenliu34457]