'A' Level Chemistry Problem Analysis: Electrolyte Color Change In Salt Bridge |
|---|
|
2 beakers, A and B, contain a bar of zinc and iron each along with indicators phenolphthalein (detects alkaline solution) and potassium hexacyanoferrate (detects presence of Fe2+: will turn blue, otherwise colorless). The metals are linked by a wire and a voltage is observed. A salt bridge connects the 2 beakers together, with the salt bridge being partially submerged in both beakers' indicators. A colour change was observed at the cathodic electrolyte. Suggest and explain two possible scenarios (including identities of electrolytes) that would allow this. Thought process: One possible scenario, would have the cathodic electrolyte as an aqueous Fe3+ solution. Assuming unimolarities (ie. standard conditions), the standard reduction potential of the cathodic half equation Fe3+ + e- --> Fe2+ is +0.77V; the standard oxidation potential of the anodic half equation Zn --> Zn2+ + 2e- is +0.76V; hence a current would certainly flow and the standard cell potential would be +1.53V.  In such a scenario, the colour change at the cathodic electrolyte would be due to Prussian Red reacting with Fe2+(aq) to generate Prussian Blue. Another possible scenario would have the cathodic electrolyte as an aqueous acidic solution. Assuming unimolarities (ie. standard conditions), the standard reduction potential of the cathodic half equation 2H+ + 2e- --> H2 is +0.00V; the standard oxidation potential of the anodic half equation Zn --> Zn2+ + 2e- is +0.76V; hence a current would certainly flow and the standard cell potential would be +0.76V. Notice that H+ ions came from the solvent water, generating OH- ions as a consequence. In such a scenario, the colour change at the cathodic electrolyte would be due to phenolphthalein reacting with the OH- ions; specifically, di-deprotonation and structural change from a non-conjugated, tetrahedral center and absorbing UV frequencies (hence colourless) to a highly conjugated, trigonal planar center and absorbing blue-green frequencies (hence pink/fuchsia). ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The above content is contributed by Mr Heng, owner and 'A' Level Chemistry tutor at Bedok Funland JC. He also goes by the handle UltimaOnline on various online popular homework forums. |
|
YOU MAY WISH TO READ: 'A' Level Chemistry Problem Analysis: Determining if Ink Smudges Are Ionic or Covalent In Nature 'A' Level Chemistry Problem Analysis: Adding Aqueous Bromine To 4-aminophenol |
