• For chromium we need to consider the following standard electrode potential values
• We will use zinc and hydrogen peroxide as oxidising agents
• The half equations are arranged from high negative EΘ?at the top to high positive?EΘ?at the bottom
  • The best reducing agent is the top right species (Zn (s))
  • The best oxidising agent is the bottom left species (Cr2O72-(aq))

Oxidation from +3 to +6

• The two half equations we need to consider are 3 and 4
• Chromiums oxidation number changes from +6 to +3 in half equation 3
• The EΘ?value for half equation 3 is more negative than the EΘ?for half equation 4
  • Cr(OH)3?(aq) is the best reducing agent
  • H2O2?(aq) is the best oxidising agent
• We can obtain the overall equation by reversing half equation 3 and combining it with equation 4
  • When adding half equations remember to multiply them so each has the same number of electrons

2Cr(OH)3?(aq)?+ 4OH-?(aq) + 3H2O2?(aq) → 2CrO42-?(aq)?+ 8H2O (l)

• This reaction is carried out in alkaline conditions due to the presence of OH-?ions in the equation

Reduction from +6 to +3

• The two half equations we need to consider are 1 and 5
• Chromiums oxidation number changes from +6 to +3 in half equation 3
• The EΘ?value for half equation 1 is more negative than the EΘ?for half equation 5
  • Zn is the best reducing agent
  • Cr2O72-?is the best oxidising agent
• We can obtain the overall equation by reversing half equation 1 and combining it with equation 5
  • When adding half equations remember to multiply them so each has the same number of electrons

Cr2O72-?(aq)?+ 14H+?(aq) + 3Zn (s) →?2Cr3+?(aq)?+ 7H2O (l) + 3Zn2+?(aq)

• This reaction is carried out under acidic conditions due to presence of H+?in the equation

Reduction from +3 to +2

• The Cr3+?ion can be further reduced by zinc
• The two half equations we need to consider are 1 and 2
• Chromiums oxidation number changes from +3 to +2 in half equation 3
• The EΘ?value for half equation 1 is more negative than the EΘ?for half equation 2
  • Zn (s) is the best reducing agent
  • Cr3+?(aq)?is the best oxidising agent
• We can obtain the overall equation by reversing half equation 1 and combining it with equation 2
  • When adding half equations remember to multiply them so each has the same number of electrons

2Cr3+?(aq)?+ Zn??(s)?→?2Cr2+?(aq)?+ Zn2+?(aq)

• As this reaction is a further step from the previous reduction this reaction is also carried out under acidic conditions

• The chromate CrO42-?and dichromate Cr2O72-?ions can be converted from one to the other by the following equilibrium reaction

2CrO42-?(aq) + 2H+?(aq)?? Cr2O72-?(aq) + H2O (l)

• Chromate(VI) ions are stable in alkaline solution, but in acidic conditions the dichromate(VI) ion is more stable
• Addition of acid will push the equilibrium to the dichromate
  • This results in a colour change from yellow to orange
• Addition of alkali will remove the H+?ions and push the equilibrium to the chromate
• This is not a redox reaction as both the chromate and dichromate ions have an oxidation number of +6
  • This is an acid base reaction