4D1

[JChen_2I]

Carbon disulfide can be prepared from coke and elemental sulfur:
4C(s)+S8(s) --> 4CS2(l) deltaH=+358.8kJ
(b) Calculate the heat absorbed in the reaction of 197g of carbon with an excess of sulfur.

Can someone explain why we have to divide by 4 moles of carbon to get the answer? And should we normally assume deltaH is giving in kJ/mol?

[Benjamin Feng 1B]

You need to divide by 4 since there are 4 moles in the product side, which means that when you form 4 moles of the product, 358.8 kJ is released. That means when 1 mole is formed, only 358.8/4 kJ of energy is released. You want the coefficient of the product you are interested in to be 1. Delta H is given in joules, assuming that the reaction occurs in units of moles.

[Amy Xiao 1I]

The way the solutions manual solves it is that it first converts the 197 g of Carbon to mols -> 16.40 mol C. It also divides the energy absorbed by 4 moles of Carbon by 4 so that you will know how much 1 mol Carbon absorbs. After that, you can just multiply that energy by however many moles of carbon you need to use.