With the following information, use Hess's Law to find the heat of the reaction.

Question

Given the following data:

MnO(s) + H₂(g)  Mn(s) + H₂O(l), H = 100 kJ

MnO₂(s) + 2 H₂(g)  Mn(s) + 2 H₂O(l), H = -52 kJ

use Hess's law to calculate the heat of the reaction

MnO₂(s) + H₂(g)  MnO(s) + H₂O(l)

Answer 1

in order to find the heat of the reaction for the third reaction, you have to manipulate the first two reactions so that when you add the first two reactions together they will equal the third reaction.

if we reverse the first reaction we get:

Mn+H2O-->MnO + H2

adding this to the second reaction gives:

Mn+H2O + MnO2 +2H2-->MnO +H2 +Mn +2H2O

since there is Mn on both the left and right sides those cross out. There is an H2O on the left side and 2 on the right therefore we can simplify that to only one H2O on the right side. there are two H2 on the left and one on the right which simplifies to just one on the left. All of this leaves us with:

MnO2 +H2 --> MnO + H2O which is exactly the third reaction we are looking for!

because we reversed the first reaction we must also reverse the sign of the dH so it is -100kJ

we didnt change the second reaction at all so that stays as -52kJ

to solve for the heat of reaction for the third reaction we just add these two together: (-100)+(-52)=-152kJ