change in enthalpy value

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Lilly Lin 1L
Posts: 14
Joined: Fri Sep 25, 2015 3:00 am

change in enthalpy value

Postby Lilly Lin 1L » Sun Jan 10, 2016 10:09 am

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

Shreya Banerjee 2F
Posts: 24
Joined: Fri Sep 25, 2015 3:00 am

Re: change in enthalpy value

Postby Shreya Banerjee 2F » Sun Jan 10, 2016 10:34 am

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

104422816
Posts: 17
Joined: Fri Sep 25, 2015 3:00 am

Re: change in enthalpy value

Postby 104422816 » Sun Jan 10, 2016 3:54 pm

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
Posts: 56
Joined: Fri Sep 25, 2015 3:00 am

Re: change in enthalpy value

Postby Alexander Vong 3I » Sun Jan 10, 2016 4:20 pm

104422816 wrote: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.


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