How to know which Electrode potential to use in DataB?

• nacacorps

  8 Sep 09, 21:24

  For example those H2O equations which look similar and Fe3+ Fe2+ and Fe, MnO4- etc...

  And why is it that some equation u can flip and make it E(ox) while some uu cant...im simply confused over all these... 

  someone please help me! thanks!!

   

  To further explain what i want to ask..:D

  For example...H2O appear in a few equations of the Electrode potential list...both on the left and right side...though with different H+ and e- and sometimes with O2. 

  How do u know which to use if u need one which have H2O.

  and also such as MnO4- which has 2 equations (i.e one with 8H+ and the other with 4H+)

  (SO4)2- equations also....

  they are just too confusing for me! :P

• yiha093

  8 Sep 09, 22:14

  dude, u gotta say is O or A lv sia, if not ppl canot answer ur qn acc. to ur standard  ?

• UltimaOnline

  8 Sep 09, 23:13

  'A' Level Notes. Redox potentials refer to reduction potentials (if a reduction half-equation is written) or oxidation potentials (if an oxidation half-equation is written). As long as the equation is given as reversible (ie. equilibrium double half arrow notation used), it means you can reduce one species (eg. Fe3+) to a reduced state (eg. Fe2+), and you can also oxidize the reduced state species (eg Fe2+) to the oxidized state species (eg. Fe3+). Hence, for such reversible equations, you can reverse the half equations and the standard potentials. When written as a reduction half-equation, the redox potential is called the (standard) reduction potential (if standard conditions). When written as an oxidation half-equation, the redox potential is called the (standard) oxidation potential (if standard conditions). The positive/negative sign of the redox potential is reversed when re-writing a reduction half-equation as an oxidation half-equation, or vice-versa. Whichever way (reduction or oxidation) you choose to write it, the more positive the standard redox potential, the more the position of equilibrium lies to the right. The more negative the standard redox potential, the more the position lies to the left. You can Google/Wikipedia the technical physics definition of redox/reduction/oxidation potentials (which is not required for 'A' level Chemistry). Technical physics definition aside, it is helpful for you to think of the meaning of "potential" as "tendency" or "inclination" or "propensity". Hence, the Reduction Potential numerical value (including positive/negative sign) describes the potential or tendency/inclination/propensity for a species to be reduced. The more positive the value, the more the position of equilibrium lies to the right (ie. the reactant species eg. Ag+ prefers to be reduced). (Note : due to formatting problems, the word equation is given instead) Eg. Silver cation + electron = (ie. left-to-right arrow) = Silver atom ( + 0.80V ) ( + 0.80V means Silver cation likes to be reduced to Silver atom) which can be also be written as : Silver atom = (ie. left-to-right arrow) = Silver cation + electron ( - 0.80V ) (Indeed, - 0.80V means Silver atom doesn't like to be oxidized to Silver cation) Likewise, the Oxidation Potential numerical value (including positive/negative sign) describes the potential or tendency/inclination/propensity for a species to be oxidized. The more positive the value, the more the position of equilibrium lies to the right. (ie. the reactant species prefers to be oxidized). (Note : due to formatting problems, the word equation is given instead) Eg. Potassium atom = (ie. left-to-right arrow) = Potassium cation + electron ( + 2.92V ) ( + 2.92V means Potassium atom loves to be oxidized to Potassium cation) which can be also be written as : Potassium cation + electron = (ie. left-to-right arrow) = Potassium atom ( - 2.92V ) ( - 2.92V means Potassium cation hates to be reduced to Potassium atom) Standard cell potential (which must be positive to be feasible, ie. for the current of a Galvanic/Voltaic cell to flow) is calculated as : Oxidation potential at Anode + Reduction potential at Cathode or Reduction potential at Cathode – Reduction potential at Anode As long as standard cell potential is positive, the reaction is feasible, and the current in the Galvanic/Voltaic cell will flow.
• nacacorps

  8 Sep 09, 23:58

  hi! sorry! im taking A level this year! and wow! people here are really fast in replying..! 

• nacacorps

  9 Sep 09, 00:12

  hey thanks UltimaOnline for the effort to help me understand! but i think my question havent been clear on my part....but still a very Appreciated THANK YOU!! :D

   

• UltimaOnline

  9 Sep 09, 00:27

  :)
• kivine

  28 Nov 09, 21:52

  just wanna ask.

   

  If i have a redox potential of

   

  A    -0.99V

  B  + 0.8V

  As noted, the more positive the potential, there's a tendency to be reduced. If i flipped A around , I will have a +ve value for A , i.e +0.99V. So A will be reduced compared to B? AM i correct in thinking that way?

• UltimaOnline

  28 Nov 09, 22:19

  When you say "redox potential, do you mean "reduction potential"? If you write your half equations as reduction equations, that you indeed mean reduction potential. Since A has a negative standard reduction potential -0.99V, it will have a positive standard oxidation potential +0.99V. Meaning it has the "potential" or propensity or tendency to be oxidized. And B has a positive standard reduction potential +0.8V. Meaning it has the "potential" or propensity or tendency to be reduced. So A will be oxidized at the anode (eg. A to A2+), and B will be reduced at the cathode (eg. B2+ to B).
• kivine

  29 Nov 09, 00:01

  thanks alot for the help

• UltimaOnline

  29 Nov 09, 00:46

  :)