Under standard conditions for all concentrations, the following reaction is spontaneous at
25 ¢XC with EO = 0.16 V.
+ - 02 (g) + 4 H (aq) + 4 Br (aq) -+ 2 H20 (I) + 2 Br2 (1)
If [H+] is a?justed by adding a buffer of 0.10 M NaOCN and 0.10 M HOCN
(Ka = 3.5 x 10 ), what value will Ecell have, and will the reaction be spontaneous at this
[H+]?
Leo, I can't decipher the symbols. BTW, did you omit the exponent on Ka = 3.5 x 10?
To determine the value of Ecell and whether the reaction will be spontaneous at a given [H+], we need to follow these steps:
1. Write the half-reactions:
- 02 (g) + 4 H+ (aq) + 4 e- --> 2 H20 (I) (reduction half-reaction)
- 4 Br- (aq) --> 2 Br2 (l) + 4 e- (oxidation half-reaction)
2. Identify the relevant standard reduction potentials:
- E° for the reduction half-reaction: EO(reduction) = 0.16 V (given)
3. Calculate the overall cell potential, Ecell:
- Ecell = EO(reduction) - EO(oxidation)
- The standard reduction potential for the oxidation half-reaction (EO(oxidation)) can be found using a reference table or online database. You need to look up the standard reduction potential for the Br2/Br- half-reaction.
- Once you have both reduction potentials, you can find Ecell.
4. Determine whether the reaction is spontaneous:
- If Ecell is positive, the reaction is spontaneous.
- If Ecell is negative, the reaction is non-spontaneous.
5. Adjust [H+] using the buffer of 0.10 M NaOCN and 0.10 M HOCN:
- The buffer will maintain a nearly constant pH.
- Calculate the pH of the buffer solution using the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])
Where [A-] is the concentration of the conjugate base (NaOCN) and [HA] is the concentration of the weak acid (HOCN). The pKa value is given as Ka = 3.5 x 10.
6. Convert pH back to [H+]:
- Calculate [H+] using the equation: [H+] = 10^-pH
7. Substitute the adjusted [H+] into the Nernst equation:
- Ecell = E°cell - (0.0592/n) * log(Q)
- Where E°cell is the standard cell potential, n is the number of electrons transferred in the balanced equation, and Q is the reaction quotient.
- The reaction quotient Q is calculated using the concentrations of species involved in the reaction.
By following these steps, you should be able to determine the value of Ecell and whether the reaction is spontaneous at the given [H+].