Potassium dichromate redox

For this redox equation:

Cr2O7(2-) + 6 e(-) + 14 H+ -> 2 Cr(3+) + 7H2O

I'm wondering why their are 6 electrons?

Let me answer with a question: what other number of electrons do you propose, and what is the logic behind the other number?

Reply 2

daniel1005

OP

4

Original post by Borek

Let me answer with a question: what other number of electrons do you propose, and what is the logic behind the other number?

Well the oxidation state of Cr in Cr2O7(2-) is 6 and the oxidation state of Cr in Cr3+ is obviously 3. So therefore there is a reduction of 3 so there is three electrons. That was my first way of doing it.

With that not been right I tried balancing the charges. So there is 14 +'s and a 2- which equals 12, so that would mean 12 electrons to balance. But that's incorrect.

Reply 3

Dead End Street

2

Original post by daniel1005

Well the oxidation state of Cr in Cr2O7(2-) is 6 and the oxidation state of Cr in Cr3+ is obviously 3. So therefore there is a reduction of 3 so there is three electrons. That was my first way of doing it.

With that not been right I tried balancing the charges. So there is 14 +'s and a 2- which equals 12, so that would mean 12 electrons to balance. But that's incorrect.

Product is 2Cr3+, not just Cr3+. 2 x 3 electrons = 6.

Reply 4

daniel1005

OP

4

Original post by Dead End Street

Product is 2Cr3+, not just Cr3+. 2 x 3 electrons = 6.

So for any reaction should:

Sum of reactants charges minus sum of products charges = 0

Reply 5

Borek

4

Original post by daniel1005

So for any reaction should:

Sum of reactants charges minus sum of products charges = 0

Charge is a conserved quantity, so charge of the products must be identical to the charge of the reactants.

Well the oxidation state of Cr in Cr2O7(2-) is 6 and the oxidation state of Cr in Cr3+ is obviously 3. So therefore there is a reduction of 3 so there is three electrons. That was my first way of doing it.

Dead End Street already addressed it, but I will reiterate: oxidation number goes from +6 to +3, so yes, it means reduction consuming three electrons PER REDUCED ATOM. But there are two Cr atoms both in the dichromate anion, and in the products, and each of them requires 3 electrons, so there are 6 electrons consumed per each dichromate anion.