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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: science123 on April 24, 2009, 08:02:32 AM

Title: Calculate Gibbs energy of Formation , given delta G values ?
Post by: science123 on April 24, 2009, 08:02:32 AM
Given the following data,

H2(g) + 1/2 O2(g) => H2O(g) DGo = -236.0

2 H(g) + O(g) => H2O(g) DGo = -920.8

2 H(g) + 2 O(g) => H2O2(g) DGo = -1,068.0

2 O(g) => O2(g) DGo = -496.6

H2O2(l) => H2O2(g) DGo = 43.1

Determine DGfo for H2O2(l)

Correct : -178.0

I don't know which equations to combine and flip and how do we find the Gf of H2O2 (gas) . I was thinking of someway to find gf of H2O2 gas and subract from Grxn to get Gf of H2O2 liquid. Thanks!
Title: Re: Calculate Gibbs energy of Formation , given delta G values ?
Post by: sjb on April 24, 2009, 08:06:23 AM
What's the definition of  :delta: Gf?
Title: Re: Calculate Gibbs energy of Formation , given delta G values ?
Post by: science123 on April 24, 2009, 08:10:05 AM
Quote from: sjb on April 24, 2009, 08:06:23 AM
What's the definition of  :delta: Gf?

Change of Gibbs free energy that accompanies the formation of 1 mole of that substance from it's elements in standard state.
Title: Re: Calculate Gibbs energy of Formation , given delta G values ?
Post by: sjb on April 24, 2009, 08:12:06 AM
Quote from: science123 on April 24, 2009, 08:10:05 AM
Quote from: sjb on April 24, 2009, 08:06:23 AM
What's the definition of  :delta: Gf?

Change of Gibbs free energy that accompanies the formation of 1 mole of that substance from it's elements in standard state.

Right, so what does that mean, in terms of your reaction?
Title: Re: Calculate Gibbs energy of Formation , given delta G values ?
Post by: science123 on April 24, 2009, 08:20:49 AM
Delta Go = sum of Delta Gf of products - sum of Delta Gf of reactants
Am i right?
Title: Re: Calculate Gibbs energy of Formation , given delta G values ?
Post by: sjb on April 24, 2009, 08:26:21 AM
Essentially, yes. You're after  :delta: Gf for H2O2(l), which is for the reaction H2(g) + O2(g)  :rarrow: H2O2(l). Can you find a path using the data you have that allows you to calculate this  H2(g) + O2(g) :rarrow: A  :rarrow: B  :rarrow: C ...  :rarrow: H2O2(l)?
Title: Re: Calculate Gibbs energy of Formation , given delta G values ?
Post by: science123 on April 24, 2009, 08:40:52 AM
That's that part i am not getting. And btw why does eq. 2 has hydrogen in atomic form?
Title: Re: Calculate Gibbs energy of Formation , given delta G values ?
Post by: sjb on April 24, 2009, 08:54:02 AM
You won't necessarily have to use them all (I've not worked it through, so could be wrong here).

For instance, from

Quote from: science123 on April 24, 2009, 08:02:32 AM
H2(g) + 1/2 O2(g)  :rarrow: H2O(g)  :delta: Go = -236.0

2 H(g) + O(g)  :rarrow: H2O(g)  :delta: Go = -920.8

H2(g) + 1/2 O2(g)  :rarrow: H2O(g)  :rarrow: 2 H(g) + O(g)

you can calculate  :delta: Gf for 2 H(g) + O(g), by adding equation 1 to the reverse of equation 2; giving 684.8. (Don't forget your units!)