Solving Hess's Law Problem: ΔH for Mg2+ + H2O --> MgO + 2H+

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Discussion Overview

The discussion revolves around calculating the enthalpy change (ΔH) for the reaction Mg2+(aq) + H2O(l) --> MgO(s) + 2H+(aq) using Hess's Law. Participants explore the relationship between given reactions and how to manipulate them to find the desired ΔH value.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • One participant expresses confusion about whether to use ΔH = Ʃreactants - Ʃproducts or to manipulate the equations, indicating a lack of clarity on the approach.
  • Another participant suggests comparing the equations reagent by reagent, although the specifics of this approach are not elaborated.
  • A participant describes their thought process of dividing the first equation by 3 and flipping it to derive the second equation, questioning the validity of their method.
  • One participant confirms that the described manipulation is indeed what was expected, framing it as a direct application of Hess's Law.

Areas of Agreement / Disagreement

There appears to be a general agreement on the method of manipulating the equations to find ΔH, but some participants remain uncertain about the process and its correctness.

Contextual Notes

Participants have not fully resolved their understanding of Hess's Law applications, and there are indications of missing clarity on how to approach the problem without a target equation.

Who May Find This Useful

This discussion may be useful for students learning about Hess's Law and enthalpy changes in chemical reactions, particularly those struggling with the manipulation of equations in thermodynamics.

Jbreezy
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Homework Statement



ΔH for the rxn 3MgO(s) + 6H+(aq) --> 3Mg2+(aq) + 3H20(l) = 409.5 kj/mol
What is ΔH for Mg2+(aq) + H20(l) --> Mg0(s) + 2H+(aq)






The Attempt at a Solution


We just went over this in class I'm confused as ever. I don't know if it is a
ΔH = Ʃreactants - Ʃproducts
Or if I had to somehow manipulate the equations which is what we just went over . I don't really understand so if you point me in the right direction I would appreciate it. I'm confused because all the worksheets where we did the latter choice I mentioned we were given a target equation. Here I don't see one. Explain your thinking please.
Thanks ,
J
 
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Compare both equations reagent by reagent.
 
Can you elaborate? I don't understand. Thank you.
 
Hi
I looked at it again. I thought how can I get from the first equation to the second
ΔH for the rxn 3MgO(s) + 6H+(aq) --> 3Mg2+(aq) + 3H20(l) = 409.5 kj/mol
What is ΔH for Mg2+(aq) + H20(l) --> Mg0(s) + 2H+(aq)
If I divide the first equation by 3
I get MgO(s) + 2H+(aq) --> Mg2+(aq) + H20(l) = 136.5 kj/mol
Then I can flip this and I will get
Mg2+(aq) + H20(l) --> Mg0(s) + 2H+(aq) = -136.5 kj/ mol
Which is the first second equation. So I just transformed the first into the second. I'm not sure if this is even proper I just kind of went with it lol. What does someone think?
 
That's exactly what you were expected to do. Note that it is a direct application of the Hess law.
 
Thanks dude we just went over it and it didn't sink in yet. Appreciate your help.
 

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