Enthalpy Concept Check

[Nicole Nakama 1A]

This is more specifically to our current Achieve Assignment.

Carbon disulfide can be prepared from coke (an impure form of carbon) and elemental sulfur.

4C(s)+S8(s)⟶4CS2(l) ΔH∘=+358.8 kJ

Calculate the amount of heat absorbed when 1.33 mol S8 is reacted at constant pressure.


Originally, I used the third method of finding standard reaction enthalpy and got a different answer. The feedback said to simply multiply the new number of moles of S8 by the standard enthalpy of the reaction due to molar ratios. Why is the third method not applicable in this case?

[Abeni Liu 2B]

I think this has to do with the fact that the standard enthalpy of formation for an element in its most stable form is 0. Since C and S are in its most stable forms, you can just use the ratio to solve the problem. In addition, the methods are used to calculate standard enthalpy of formation, which is already given in this case so you wouldn't need the third method.

[Katherine Phan 1J]

As already mentioned, you wouldn't need to use the third method because the reactants C and S are already elements in their most stable form and thus have a standard enthalpy of formation of 0. Instead, since each mol of reaction requires 1 mol of S8 and absorbs 358.8 kJ, the heat absorbed and S8 are in a 1:1 mole ratio, which is why you would just multiply 358.8 by 1.33