8.61

[Connie2I]

For 8.61, could someone tell me why the second equation N2 (g) + 3H2 (g) --> 2NH3 (g) is supposed to be switched, which would give the value enthalpy of +92.22 kJ?

Since the question is asking for the reaction enthalpy value of H2 (g) + Br2 (l) --> 2HBr (g) the H2 in both equations are both on the reactants side already, so I'm not sure why you should switch the products and reactants.

[Kelly Kiremidjian 1C]

hi,

I was confused about this too but a TA explained it to me. Since there is no ammonia (NH3) in the desired reaction, the ammonia from the reactions with known enthalpies must be cancelled out. To do this, the ammonia must be on different sides of the two reactions it appears in.
Since H2 appears on the reactant side of two of the reactant equations given, switching the 2nd equation actually allows for the right number of moles of H2 to be present in the desired reaction.

Hope this helps

[Clarisse Wikstrom 1H]

Just remember that the goal is to get the desired reaction. Sometimes, you have to flip the internal reactions in order to get the correct reactants and products for the desired reaction.

[Jessica Benitez 1K]

Whenever you need to switch the equation remember that you need to multiply the enthalpy value given by negative one which changes its sign.