Why do you need to reverse the sign of the change of enthalpy (delta H) when you reverse an equation?

Question

if you can an example with the answer would be helpful.
Thank you soo much!

Answer 1

If the enthalpy change wasn't exactly equal but of opposite sign, then the First Law of Thermodynamics would be broken if you went backwards and forwards - you could create (or lose) energy.

In making one mole of liquid water from hydrogen gas and oxygen gas, 286kJ of heat energy are released. To break up one mole of liquid water into hydrogen and oxygen, 286kJ of energy are required. So the values are exactly equal, but one is negative (the formation) and the other is positive (the thermal decompostion).