what is the standard enthalpy change for the reaction:

Question

Given the standard enthalpy changes for the following two reactions:

(1) 2Ni(s) + O₂(g)2NiO(s)......H° = -479.4 kJ

(2) 2Zn(s) + O₂(g)2ZnO(s)......H° = -696.6 kJ

what is the standard enthalpy change for the reaction:

(3) NiO(s) +  Zn(s)Ni(s) +  ZnO(s)......H° = ?

 kJ

Answer 1

Hi Ahmad, I'm sorry that old man Hess is giving you a hard time. To follow up on my last answer, with Hess's law, you must take into account the direction of the process both 1) as it is presented in the table 2) as it occurs in the problem you have to solve. If the direction in 1) differs from that of 2), then you gotta reverse the sign. And, again, stoichiometry is important. In the nickel oxidation reaction, the standard enthalpy change is -479. Since we only do half of it, we divide by 2 to get about -240 kJ/mol. However, in the question, the direction is reversed! So, when you set up your Hess's law equation to solve the problem, you would use POSITIVE 240 as the value for that half-reaction. Make sense? Thomas