For the reaction: 2H2(g) + O2(g) <—> 2H2O(g) H = -296 kJ

Question

a) How much heat is evolved when 150 g of hydrogen is burned in excess oxygen? 

b) How much heat is evolved when 125 g of water is produced? 



c) How much heat is evolved when 5 g of hydrogen reacts with 15 g of oxygen?

Answer 1

2H₂(g) + O₂(g) <==> 2H₂O(g) ∆H = -296 kJ

I'll do (a), and hopefully you can use the same/similar procedure to do (b) and (c).

(a). How much heat is evolved when 150 g of hydrogen is burned in excess oxygen?

Explanation: There are 296 kJ of heat evolved for every TWO moles of H₂ that reacts. Converting this to kJ per mole we get 296 kJ / 2 mol = 148 kJ / mole of H₂. We find that we have 75 moles of H₂, so we can easily calculate the heat evolved.

150 g H₂ x 1 mol H₂ / 2 g = 75 moles H₂

-296 kJ / 2 mol H₂ x 75 mol H₂ = 11,100 kJ of heat evolved

(b). Find moles of water is produced and apply the same reasoning as in (a).

(c). This is a little different in that you must find which reactant (H₂ or O₂) is limiting. Calculate moles of H₂O formed from 5 g H₂ and also from 15 g of O₂ (as you did above). Whichever gives the smaller answer would be limiting so multiply that by the kJ to get the answer.