change in enthalpy value

[Lilly Lin 1L]

When do you have to change the sign on the deltaH for a reaction?

[Shreya Banerjee 2F]

You change the sign of the enthalpy value whenever you reverse the equation.

[104422816]

But how do you know what the sign of the forward reaction was to begin with? is that dependent on whether the reaction is endothermic or exothermic?

[Alexander Vong 3I]

But how do you know what the sign of the forward reaction was to begin with? is that dependent on whether the reaction is endothermic or exothermic?

Yes, an enthalpy value for a reaction depends on whether it's endo- or exo- thermic. For example, consider the reaction of methane with water vapor.

CH₄(g) + H₂O(g) --> CO₂(g) + 3H₂(g)

delta H = -318 kJ

The delta H value signifies that this reaction is exothermic. Reversed, it would be endothermic and the delta H value would be positive.