chemistry! ):

• jaydunkfull

  16 Sep 09, 02:07

  physical chemistry. topic: electrochemistry. please help. thanks! 1) regarding Nernst equation, E=E'-(RT/nF)ln q, whats the difference between E and E'? Is the value of E' = E'(red)-E'(ox)? 2) what is q in ln q in the above eqn? in my notes it says ln((concentration1)/(concentration2)). which concentration do i use? 3) An electrochemical cell was set up using the chemical reaction Zn(s) + Cu2+(aq) -> Zn2+(aq) + Cu(s) If the concentration of Zn2+ is maintained at 1.0M, what is the minimum concentration of Cu2+ for the above reaction to be spontaneous in the forward direction at 25 degree celsius?
• ThunderFbolt

  16 Sep 09, 08:32

  for 1 & 2, google and wiki are your friends. 

  http://en.wikipedia.org/wiki/Nernst_equation

• UltimaOnline

  16 Sep 09, 09:48

  for 3, apply Nernst equation (overall value equal to zero for equilibrium point between spontaneity and non-spontaneity) to find value q (ratio of molarities of the two species undergoing redox), hence the required molarity.
• youphoria

  16 Sep 09, 12:19

  just curious what level is this qn, i.e. is it university level? or a level h3 chem?

• UltimaOnline

  16 Sep 09, 14:07

  - just curious what level is this qn, i.e. is it university level? or a level h3 chem? - H3 Chem is Organic Chem and Analytical Chem, not Physical Chem (which ElectroChem belongs to). Nernst equation is studied at higher (University) levels, but this question may be asked at 'A' levels, as a so-called "data-based" question, in which the formula is given to you together with brief explanations. Similarly (as with Nernst equation), higher level concepts or formulae, such as Arrhenius equation, partition coefficients, stabillity constants, Grignard reagents, any of the higher level Organic Chem mechanisms (see http://en.wikipedia.org/wiki/List_of_organic_reactions ), any of these may be asked at 'A' levels (and is indeed commonly included in Prelim papers to stretch JC students), as long as the so-called "data-based" question provides the relevant information required by the student (to see if the student is able to competently apply new information to new problem situations). It's similar to at 'O' levels, if you give the formula pH = -log[H+], and ask an 'O' level student to calculate pH of resulting solution given initial volume and molarities of acid and alkali. Challenging at 'O' levels, but it can be done since formula is provided, and functions as a 'distinction' discriminating question.
• jaydunkfull

  16 Sep 09, 18:05

  but what are the values of E and E'?

  Originally posted by UltimaOnline:

  for 3, apply Nernst equation (overall value equal to zero for equilibrium point between spontaneity and non-spontaneity) to find value q (ratio of molarities of the two species undergoing redox), hence the required molarity.

  
  
• jaydunkfull

  16 Sep 09, 18:06

  hello. its uni level chem. physics and chem are killing me. esp quantum physics. ):

  Originally posted by youphoria:

  just curious what level is this qn, i.e. is it university level? or a level h3 chem?

  
  
• UltimaOnline

  16 Sep 09, 19:56

  E' ( it should be read as "E-standard", commonly misread as "E-nought" ) is simply the standard cell potential (ie. only valid under standard conditions), while E is the cell potential under any conditions, not necessarily standard. For the standard cell potential or E', calculate it by refering to a Data Booklet. For the E, assign it a value of zero (position of equilibrium). Then calculate the required ratio of molarities.
• jaydunkfull

  17 Sep 09, 00:02

  hi, but if i want to ensure that it is spontaneous, do i need to use the value of G? change in G=-nFE(cell)?
• UltimaOnline

  17 Sep 09, 00:46

  Gibbs free energy must be negative for the reaction to be spontaneous. Since n and F are positive quantities, it follows that the cell potential must be non-zero and positive, for Gibbs free energy to be negative (ie. spontaneous). When I suggested earlier to set cell potential to be zero (using the Nernst equation), it should be understood that you would then state that the molarity of Cu2+ (the species to be reduced at the cathode) should be GREATER than the calculated value (based on cell potential equals zero), to ensure that cell potential would be positive (and thus Gibbs free energy would be negative).