Gibbs free energy problem

[inaccensa]

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If acid dissociation constant of pyrophosphoric acid is 3*10^-2 & that of arsenous acid is 6.6*10^-10 at room temp,find gibbs free energy of each dissociation rxn and determine if its spontaneous and wt does it mean for change in enthaply n entropy respectively.

I solved the Q and I ended up with negative #'s. The correct ans is that its non-spontaneous. How would you use the Ka?

 

[RogueUnicorn]

G = - RT lnK

because ln K will be a negative (due to the negative exponent) you know G must be positive.

 

[inaccensa]

damn the natural logs!!

2.303 * log K
2.303 * log (3.2*10^-2)
2.303*-2 *log 3.2
-6 * smthing small.

So G =- (-6)RT
ha!! thanks

 

[inaccensa]

how do u guys normally solve natural logs

 

[RogueUnicorn]

ln x ~~ 2 log x

but it's rarely needed

edit: you can also think of this problem qualitatively - the fact that Ka is a negative exponent means that at equilibrium, the reactants are favored over the product - you can infer the non-sponateity from this.

 

[inaccensa]

ln x ~~ 2 log x

but it's rarely needed

edit: you can also think of this problem qualitatively - the fact that Ka is a negative exponent means that at equilibrium, the reactants are favored over the product - you can infer the non-sponateity from this.

makes sense thanks!