## Hess's Law

Moderators: Chem_Mod, Chem_Admin

Brandon Tao 1K
Posts: 100
Joined: Sat Aug 17, 2019 12:15 am

### Hess's Law

Can someone explain Hess's Law because I have tried to look it up and still do not understand

Tanmay Singhal 1H
Posts: 143
Joined: Sat Jul 20, 2019 12:16 am

### Re: Hess's Law

you are given equations with their delta h. you add the equations to make the final equation you want and then you add up all of the individual delta H's

Althea Zhao 1B
Posts: 101
Joined: Mon Jun 17, 2019 7:24 am

### Re: Hess's Law

Hess's law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for each step of the reaction. So, you just balance the equations given and add their delta H's.

Gerald Bernal1I
Posts: 102
Joined: Thu Jul 11, 2019 12:16 am

### Re: Hess's Law

Hess's law uses the step equations or reactions along with the energy transferred in each reaction to give you the overall delta H. It shows that enthalpy is a state function since it does not matter how many steps in between give the overall reaction since it will add up to the enthalpy of the overall reaction.

Yiyang Jen Wang 4G
Posts: 76
Joined: Wed Nov 21, 2018 12:18 am

### Re: Hess's Law

Ex: Delta H for CS2(l) + 3 O2(g) → CO2(g) + 2 SO2(g)?
Given:
C(s) + O2(g) → CO2(g); ΔHf = -393.5 kJ/mol
S(s) + O2(g) → SO2(g); ΔHf = -296.8 kJ/mol
C(s) + 2 S(s) → CS2(l); ΔHf = 87.9 kJ/mol
Solve:
C(s) + O2(g) → CO2(g), ΔHf = -393.5 kJ/mol
2 S(s) + 2 O2(g) → 2 SO2(g), ΔHf = 2(-326.8 kJ/mol)
CS2(l) → C(s) + 2 S(s), ΔHf = -87.9 kJ/mol
ΔH = -393.5 kJ/mol + 2(-296.8 kJ/mol) + (-87.9 kJ/mol)(When all three reactions above are added, the extra two sulfur and one extra carbon atoms are canceled out)
ΔH = -393.5 kJ/mol - 593.6 kJ/mol - 87.9 kJ/mol
ΔH = -1075.0 kJ/mol

Gabriel Ordonez 2K
Posts: 113
Joined: Sat Jul 20, 2019 12:15 am

### Re: Hess's Law

All Hess’s Law is that you balance all of the chemical equations and cancel when possible to ultimately add all of their delta H’s.

Robin Cadd 1D
Posts: 113
Joined: Thu Jul 11, 2019 12:16 am

### Re: Hess's Law

In addition to what everyone else has posted, it's also important to know why Hess's Law works. Because enthalpy is a state function, you don't have to consider the processes that it took to go from the reaction's final to initial state.

BCaballero_4F
Posts: 94
Joined: Wed Nov 14, 2018 12:22 am

### Re: Hess's Law

You basically alter the reactions to get the reaction you want, by multiplying or moving reactants to products and products to reactants

Return to “Reaction Enthalpies (e.g., Using Hess’s Law, Bond Enthalpies, Standard Enthalpies of Formation)”

### Who is online

Users browsing this forum: No registered users and 0 guests