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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: minilunchbox on November 14, 2009, 10:04:18 pm

Title: Redox equations
Post by: minilunchbox on November 14, 2009, 10:04:18 pm: I'm having trouble balancing
$Cu(s) + HNO_3(aq) \to Cu^2^+(aq) + NO(g)$
and
$PbS(s) + H_2O_2(l) \rightarrow PbSO_4(s) + H_2O(l)$

I keep getting the wrong answer. Eep.
Title: Re: Redox equations
Post by: chem-nerd on November 14, 2009, 10:30:02 pm: Cu(s) -> Cu²⁺(aq) + 2e

HNO₃(aq) + 3H⁺(aq) + 3e -> NO(g) + 2H₂O(l)

3Cu(s) + 2HNO₃(aq) + 6H⁺(aq) -> 3Cu²⁺(aq) + 2NO(g) + 4H₂O(l)

PbS(s) + 4H₂O(l) -> PbSO₄(s) + 8H⁺(aq) + 8e

H₂O₂(l) + 2H⁺(aq) + 4e -> 2H₂O(l)

PbS(s) + 4H₂O(l) + 2H₂O₂(l) + 4H⁺(aq) -> PbSO₄(s) + 8H⁺(aq) + 4H₂O(l)

cancel out H⁺ and H₂O

PbS(s) + 2H₂O₂(l) -> PbSO₄(s) + 4H⁺(aq)
Title: Re: Redox equations
Post by: minilunchbox on November 14, 2009, 10:40:11 pm: The answers in the book says it's
$3Cu(s) + 2NO_3^-(aq) + 8H^+(aq) \rightarrow 3Cu^2^+(aq) + 2NO(g) + 4H_2O(l)$

and

$PbS(s) + 4H_2O_2(l) \rightarrow PbSO_4(s) + 4H_2O(l)$
Title: Re: Redox equations
Post by: chem-nerd on November 14, 2009, 10:54:15 pm: the first one is the same (they've just ionised the nitric acid)

and woops by me on the second one for having H⁺(aq) with no water present
Title: Re: Redox equations
Post by: nala on November 15, 2009, 11:15:31 pm: Just a couple of tips my chem teacher taught me when balancing redox equations (they make it much easier!):
-> firstly, balance the ions or metals involved (eg. Cu, Fe etc)
-> balance oxygen by adding the necessary number of water molecules to the needed side of the equation
-> balance hydrogen by adding H+
-> balance charges by adding electrons

:) this really helped me when I was struggling with redox, hope it helps!
Title: Re: Redox equations
Post by: minilunchbox on November 15, 2009, 11:28:34 pm: Quote from: nala on November 15, 2009, 11:15:31 pm
Just a couple of tips my chem teacher taught me when balancing redox equations (they make it much easier!):
-> firstly, balance the ions or metals involved (eg. Cu, Fe etc)
-> balance oxygen by adding the necessary number of water molecules to the needed side of the equation
-> balance hydrogen by adding H+
-> balance charges by adding electrons

:) this really helped me when I was struggling with redox, hope it helps!

KOHES method, yeah?
Title: Re: Redox equations
Post by: nala on November 15, 2009, 11:32:31 pm: I'm pretty sure that's it, it sounds familiar. I could just never remember what the letters stood for (I can never remember things like that :P), so I just remembered the dot points. :)
Title: Re: Redox equations
Post by: vcesuccess on November 22, 2009, 08:01:39 pm: Quote from: nala on November 15, 2009, 11:15:31 pm
Just a couple of tips my chem teacher taught me when balancing redox equations (they make it much easier!):
-> firstly, balance the ions or metals involved (eg. Cu, Fe etc)
-> balance oxygen by adding the necessary number of water molecules to the needed side of the equation
-> balance hydrogen by adding H+
-> balance charges by adding electrons

:) this really helped me when I was struggling with redox, hope it helps!
Thats correct. Follow that and you'll never be wrong. Perhap you could copy that in your notebook.