Chapter 20

20.1 Oxidation states

20.2 half reactions

20.3 Voltaic Cells Spontaneous redox reactions Spontaneous redox reactions Anode (-ve): oxidation occurs Anode (-ve): oxidation occurs Cathode (+ve): reduction occurs Cathode (+ve): reduction occurs Electrons flow from anode to cathode Electrons flow from anode to cathode Salt bridge allows ions to migrate Salt bridge allows ions to migrate – Anions flow towards anode – Cations flow towards cathode

20.4 Cell EMF (standard conditions)

20.5 Free Energy & Redox Free energy =  G° Free energy =  G°  G° = potential to be spontaneous  G° = potential to be spontaneous – Measured in joules (energy) – Negative = spontaneous  G° = -nFE°  G° = -nFE° – n = number of electrons transferred – F = Faraday’s constant = 96485J/V mole (C/mole) –  G ° = -nFE ° = -RTlnK  G ° is extensive (changes with n)  G ° is extensive (changes with n) E° is intensive E° is intensive

20.6 EMF non-standard conditions Nernst Equation: E = E ° – (RT/nF)LnQ Nernst Equation: E = E ° – (RT/nF)LnQ – E = E ° – 298K – During reaction Q↑ so E↓ until cell discharges Concentration cell Concentration cell – E ° cell = 0 for 2 cells with the same species but – E = E ° – (0.0592/n)LogQ Q depends on concentration Q depends on concentration

20.9 Electrolysis Electrolytic cell uses a battery to force a non-spontaneous reaction Electrolytic cell uses a battery to force a non-spontaneous reaction Quantitative electrolysis: Quantitative electrolysis: – Direct relationship: mole e-  mole product – 1 mole e- = 1F = 96485C (C = J/V & C/s = Amp) Electrical work:  G = w max = -nFE Electrical work:  G = w max = -nFE – Voltaic cell w max is –ve (work done by system) – Electrolysis w max is +ve (work done on system) – Work = energy/time. 1W = 1J/s – 1kW hr = 3.6 x 10 6 J