What is the change in enthalpy when 1.50 moles of H2 are reacted with an excess O₂ of according to the following balanced chemical reaction: 2H₂(g)+O₂(g)→2H₂O(l) ∆H=-572 kj

Answer 1

In the equation given, you are reacting 2 moles of H₂ with oxygen to produce water. This reaction with 2 moles gives -572 kJ. We are being asked how much energy is given for only 1.5 moles. Because we have less than the usual 2 moles, you should expect your answer to be less than the -572. You can set this up as a ratio to solve.

1.5 moles = x kJ

2 moles -572 kJ

Solve for x

0.75 moles • -572 kJ = -429 kJ

Sometimes, I don't always know when to set up a ratio like that. Instead, you can think of it as only having 75% of the total value. 1.5 moles divided by 2 moles is 0.75 = 75%. 75% of -572 is -429.