Electrochem Half-equations

[watto_22]

So I'm currently struggling to work out how you work out the half-equations to electrochem reactions.

obviously balancing something like ( Cu(s) --> Cu²⁺ (aq) + 2e^- ) is easy enough.

but for something slightly tougher, is there a specific way to go about deducing the equation for the oxidation of something like ethanoic acid:
CH₃COOH + 2H₂O --> 2CO₂ + 8H⁺ + 8e^-


and then finally how can you go about working out the half-equations from a more complex, complete equation such as:
Cd + Ni₂O₃ + 3H₂O  --> Cd(OH)₂ + 2Ni(OH)₂



my first thought with these was that there is some rule like there was for combustion, where you first consider the number of Carbon atoms in your reactants and then make this same number the coefficient of CO₂ molecules, then similarly for Hydrogen then finally Oxygen.
However, I can't find any easy way to go about finding, let alone balancing, the half-equations that stem from a complete equation.

Can anyone help?

[sahil26]

With ethanoic acid :

Firstly, identify what your main product is going to be. This is something you are just going to have to know. In the case of ethanol, it is CO2. This is the case with all alkanols so perhaps it is worth keeping in mind.

So you would have something like :

CH₃COOH ----> 2CO₂

Secondly - balance out the oxygen by adding water.

CH₃COOH + 2H₂O ----> 2CO₂

Then, add hydrogen ions to balance out the hydrogen and then add electrons to balance out the charges.

So at the end you get :

CH3COOH + 2H2O --> 2CO2 + 8H+ + 8e-

So in summary, the steps are always :
Balance out all elements except hydrogen and water.
Balance oxygen by adding water (unless it isn't available ...eg in the case of NiCd cells)
Balance Hydrogen adding by ions.
Balance charge by adding electrons.

[sahil26]

Cd + Ni2O3 + 3H2O  --> Cd(OH)2 + 2Ni(OH)2

This one is definitely more complicated. I think the key is that you identify which substance undergoes oxidation and which one undergoes reduction.

Look at the oxidation numbers and think about the reactants and products in the two half equations.