Hess's Law

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

Hess's Law

Postby Brandon Tao 1K » Sun Feb 02, 2020 11:44 pm

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

Postby Tanmay Singhal 1H » Sun Feb 02, 2020 11:53 pm

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

Postby Althea Zhao 1B » Mon Feb 03, 2020 12:41 am

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
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Joined: Thu Jul 11, 2019 12:16 am

Re: Hess's Law

Postby Gerald Bernal1I » Mon Feb 03, 2020 12:45 am

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

Postby Yiyang Jen Wang 4G » Mon Feb 03, 2020 1:12 am

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
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Joined: Sat Jul 20, 2019 12:15 am

Re: Hess's Law

Postby Gabriel Ordonez 2K » Mon Feb 03, 2020 10:10 am

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
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Joined: Thu Jul 11, 2019 12:16 am

Re: Hess's Law

Postby Robin Cadd 1D » Mon Feb 03, 2020 10:13 am

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

Postby BCaballero_4F » Mon Feb 03, 2020 9:34 pm

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


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