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# F325 -balancing

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1. hi can someone please show me how to balance this

Zn gives Zn(2+) + e-
Cr2o7(2-) + H+ + e- gives Cr(2+) + H20
the brackets are the charges

ok i know how to balance the first one
you do
Zn gives Zn(+2) + 2e-

then second part i balanced everything except the electron how do u balance the electron they got 8 ? how?
2. and should there have to be a 2 in front of cr2o7(2-)?
3. Ok, this is how you form and balance half equations:

First off, learn the basic first step. Dichromate ions are reduced to Chromium ions:

Oxygen always comes out as water:

We need more hydrogens on the LHS to balance the equation so add H+:

Now balance the electrons:

4. (Original post by Dreamweaver)
Ok, this is how you form and balance half equations:

First off, learn the basic first step. Dichromate ions are reduced to Chromium ions:

Oxygen always comes out as water:

We need more hydrogens on the LHS to balance the equation so add H+:

Now balance the electrons:

its wrong the electrons has to be 8?
5. (Original post by otrivine)
its wrong the electrons has to be 8?
No it's not wrong. If you add up the charges you will find it is:

+6 -> +6

Therefore the charges are balanced.
6. (Original post by Dreamweaver)
No it's not wrong. If you add up the charges you will find it is:

+6 -> +6

Therefore the charges are balanced.

mark scheme question 6)b) check
7. (Original post by otrivine)

mark scheme question 6)b) check
na what he is saying is right you have 8 electrons for the second equation then u multiply the zinc equation by 4 hence u get 8 electrons i dunno where u got 6, i understand the over equation needs 6 electrons plus 2- from the cr2072- you get 4+ on both sides hence an equal and balanced equation.

check it if u dont believe me.
8. (Original post by otrivine)

mark scheme question 6)b) check

(Original post by kurdishboy94)
na what he is saying is right you have 8 electrons for the second equation then u multiply the zinc equation by 4 hence u get 8 electrons i dunno where u got 6, i understand the over equation needs 6 electrons plus 2- from the cr2072- you get 4+ on both sides hence an equal and balanced equation.

check it if u dont believe me.
Both equations are correct. On the MS, the Cr ion has a 2+ charge but in the book you will find examples where Cr is 3+. This is because Cr can have different oxidation states. You need to be careful and always look at the question.

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