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Find the answers: Edexcel GCSE maths unofficial mark scheme 05-05-2016
1. hi i am really stuck on balacing the equations like
FeO4 2- + h+ gives Fe3+ + 02 + H20

how would you balance it please someone show me the step really confused
2. You'd need to conserve charge and also ensure you have the same atoms on each side.

As it stands you have a net charge of -1 on the LHS and a net charge of +3 on the RHS.
You also have four oxygens and one hydrogen on the LHS and three oxygens and two hydrogen atoms on the RHS.

I started by trying to balance the charge - 5 lots of H+ would work if we had 5/2 lots of water on the RHS.
(Note we haven't dealt with the oxygen yet.)

Double everything and you end up with

2FeO42- + 10H+ --> 2Fe3+ + 2O2 + 5H2O

Charge is conserved but we still need to deal with oxygen:
LHS we have 8 oxygens, RHS we have 9 oxygens.

If we change the no. of O2 everything else will still be balanced, so replace 2 with 1.5 before the O2.

Double everything (again) to end up with whole numbers.

The trick here was to notice that manipulating the number before O2 retained the same charge and left all the other atoms as they were.
3. (Original post by Killjoy-)
You'd need to conserve charge and also ensure you have the same atoms on each side.

As it stands you have a net charge of -1 on the LHS and a net charge of +3 on the RHS.
You also have four oxygens and one hydrogen on the LHS and three oxygens and two hydrogen atoms on the RHS.

I started by trying to balance the charge - 5 lots of H+ would work if we had 5/2 lots of water on the RHS.
(Note we haven't dealt with the oxygen yet.)

Double everything and you end up with

2FeO42- + 10H+ --> 2Fe3+ + 2O2 + 5H2O

Charge is conserved but we still need to deal with oxygen:
LHS we have 8 oxygens, RHS we have 9 oxygens.

If we change the no. of O2 everything else will still be balanced, so replace 2 with 1.5 before the O2.

Double everything (again) to end up with whole numbers.

The trick here was to notice that manipulating the number before O2 retained the same charge and left all the other atoms as they were.
Does this rule work for everything
4. (Original post by otrivine)
Does this rule work for everything
I do these by inspection. Usually they aren't too difficult so it isn't a problem.

If you knew the half-equations it would be simple - just eliminate the electrons!

I think it helps to start by balancing a species that appears in only one reactant and product, but I doubt my reasoning would work for every question.

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