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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: mandy on September 21, 2009, 08:11:22 pm

Title: REDOX
Post by: mandy on September 21, 2009, 08:11:22 pm

Balance the following redox eqns by seperating them into two half eqns, balancing each eqn, and then combining the pair into a balanced complete redox equation.
a. H₂O₂(aq) + PbS(s) ------------>  PbSO₄(s) + H₂O(l)

There's a part a - f, but I think if I have some guidance on the first bit, I can do the rest :(
Anyone know what to do ?

Title: Re: REDOX
Post by: chem-nerd on September 21, 2009, 08:23:00 pm

your two half equations would start with:
H2O2       -> H2O

PbS      ->   PbSO4

to balance each one, use the KOHeS method
K balance the key element (usually the element other than O or H)
O balance oxygen atoms by adding water
H balance hydrogen atoms by adding H+
e balance electrons
S ensure you have correct states

Title: Re: REDOX
Post by: mandy on September 21, 2009, 08:38:53 pm

when i made the half eqns, i got :

H₂O₂ + 2H⁺ + 2e^-----> H₂O + H₂O

and

PbS + 4H₂O -----> PbSO₄ + 8H⁺ + 8e^-

So i multiplied the top eqn by 4, and then combined them to get :

4H₂O₂ + 8H⁺ + PbS + 4H₂O  -------> 8H₂O + PbSO₄ + 8H⁺

this is completely wrong :(

Title: Re: REDOX
Post by: chem-nerd on September 21, 2009, 08:41:09 pm

don't forget to cancel out the H2O and H+ that appear on both sides

Title: Re: REDOX
Post by: mandy on September 21, 2009, 08:42:25 pm

but there's 4H2O on one side, and 8 on the other :S

Title: Re: REDOX
Post by: chem-nerd on September 21, 2009, 08:47:14 pm

so cancel 4 from both sides :)

Title: Re: REDOX
Post by: mandy on September 21, 2009, 08:51:50 pm

oh thank you :DDDD

I have another qn,

List the following in order of increasing reducing strength:
Mg, I^-, Ag, F^-,Ni, Sn, Li, Cu

Do I just base this on the electrochemical series ?

Title: Re: REDOX
Post by: mandy on September 21, 2009, 09:39:07 pm

Quote from: mandy on September 21, 2009, 08:11:22 pm

Balance the following redox eqns by seperating them into two half eqns, balancing each eqn, and then combining the pair into a balanced complete redox equation.
a. H₂O₂(aq) + PbS(s) ------------>  PbSO₄(s) + H₂O(l)

Similar to this question, but this is part c.

SO₂^- + MnO₄^- ------> SO₃^2- + Mn²⁺

Nothing works :@