Electrochem q

[emericana]

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Not sure if any of you have the TPR gen chem book (not hyperlearning) but if you do, this is on page 246.

Pretty simple question, but still I do not understand.

"For the reaction below, which of the following statements is true?

2Au + 3Fe2+ --> 2Au3+ +3Fe

A) The rxn is spontaneous, because its cell voltage is positive
D) The rxn is not spontaneous because its cell voltage is negative
(I ommitted B and C because they are blatently incorrect choices)
"

The answer is D.

The solution provided with the problem says

2 (Au -- > Au 3+ + 3 e-) E = -1.5v
3 (Fe2+ + 2e- --> Fe) E = -.44v

However, on the previous page there is a chart. I am going to attach this chart to this post.

If you look on this chart, it says that for Au --> Au3+ + 3e- the voltage is 1.5v not -1.5v.

Why is the voltage negative and not positive? The chart says it is positive which would make the reaction spontaneous (+1.5v-.44v).

 

[ModernAlchemist]

The value for the reaction in the table is the reduction potential, while the value listed in the solution is the oxidation potential. Look at if each species is gaining or losing electrons. If losing, the value you are looking for is the oxidation potential, while most tables for electrochemical cells list values as reduction potentials. To find the oxidation potential for the species listed, take the negative value of it.

PS: you know you've been studying for too long when your explanations sound like prep book explanations haha

Not sure if any of you have the TPR gen chem book (not hyperlearning) but if you do, this is on page 246.

Pretty simple question, but still I do not understand.

"For the reaction below, which of the following statements is true?

2Au + 3Fe2+ --> 2Au3+ +3Fe

A) The rxn is spontaneous, because its cell voltage is positive
D) The rxn is not spontaneous because its cell voltage is negative
(I ommitted B and C because they are blatently incorrect choices)
"

The answer is D.

The solution provided with the problem says

2 (Au -- > Au 3+ + 3 e-) E = -1.5v
3 (Fe2+ + 2e- --> Fe) E = -.44v

However, on the previous page there is a chart. I am going to attach this chart to this post.

If you look on this chart, it says that for Au --> Au3+ + 3e- the voltage is 1.5v not -1.5v.

Why is the voltage negative and not positive? The chart says it is positive which would make the reaction spontaneous (+1.5v-.44v).

 

[emericana]

The value for the reaction in the table is the reduction potential, while the value listed in the solution is the oxidation potential. Look at if each species is gaining or losing electrons. If losing, the value you are looking for is the oxidation potential, while most tables for electrochemical cells list values as reduction potentials. To find the oxidation potential for the species listed, take the negative value of it.

PS: you know you've been studying for too long when your explanations sound like prep book explanations haha

Round two.

It straight up says here that Pb 2+ has a more positive reduction potential than Fe 2+ when the sign of E is reversed.

How is this true?
How is +.13V (Pb2+) a more positive reduction potential than +.45V?

 

[TwoPaddles]

If you see that the products (or reactants) are switched from the list, you switch the signs also.
BUT
If you multiply the products or reactants by an integer to balance out the number of electrons, DO NOT multiply the E value.