Chemical Reactions: Redox Reactions

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Lecture materials for the Introductory Chemistry course for Forensic Scientists, University of Lincoln, UK. See http://forensicchemistry.lincoln.ac.uk/ for more details.

Lecture materials for the Introductory Chemistry course for Forensic Scientists, University of Lincoln, UK. See http://forensicchemistry.lincoln.ac.uk/ for more details.

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  • 1. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License REDOX reactions
  • 2. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License REDOX reactions involve the transfer of electrons between two chemical substances: one willing to accept them (oxidant) and the other willing to donate them (reductor). The oxidant will get reduced while the reductor gets oxidised. What are REDOX reactions? Many reactions occurring naturally are REDOX reactions, for example combustions and enzymatic reactions. Many reactions of interest in Forensic Sciences are also REDOX reactions (explosions, drug metabolism, fire, ... ) In REDOX reactions the oxidation number of the oxidant is reduced while the oxidation number of the reductor is increased reductor oxidant +e -e
  • 3. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Acidic media Balancing redox equations +2 +5 +7 +3 Basic media
  • 4. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Cu(s) + HNO 3 (aq) -> Cu(NO 3 ) 2 (aq) + NO(g) + H 2 O(l) 3Cu(s) + 8HNO 3 (aq) ->   3Cu(NO 3 ) 2 (aq) + 2NO(g) + 4H 2 O(l) Oxidation: Cu (0) to Cu (+2) Reduction: N (+5) to N (+2) Examples NO 2 (g) + H 2 (g)   ->   NH 3 (g) + H 2 O(l) Oxidation: H (0) to H (+1) Reduction: N (+4) to N (+3) 2NO 2 (g) + 7H 2 (g)   -->   2NH 3 (g) + 4H 2 (l)
  • 5. Standard conditions (represented for the ̊ symbol) are pressure of 1 bar (≅1 atm), temperature of 298° K (or 25° C, or room temperature) and concentration of 1.0 M This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Nernst equation The transfer of electrons occurs at a specific point of energy, known as redox potential (E˚), in which electrons are transferred. This point is characteristic for each metal or substance and for the number of electrons exchanged. This potential is measured in volts (V). Fe 2+ (aq) + 2 e - ------> Fe (s), E° = -0.44 V Cd 2+ (aq) +2 e - ------> Cd (s), E° = -0.40 V Nernst equation is the equation we use in cases in which we do not have standard conditions. E˚ values can be found in tables E= E˚ - (RT/nF) ln Q
  • 6. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License
  • 7. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Free energy and the standard potential can also be related through the following equation: We can therefore predict whether an electrochemical process will be spontaneous or not depending on ∆G < O An application of the spontaneous electrochemical reactions can be found in the energy cells. By introducing two metal rods in solutions containing ions of the same metal, we can produce potential and a flow of electrons between the two metals that we can use to power instruments. The intensity of the electric current produced Will depend on the concentration of the different solutions in the electrochemical cells
  • 8. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Electrochemical cells
  • 9. This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.0 UK: England & Wales License Acknowledgements
    • JISC
    • HEA
    • Centre for Educational Research and Development
    • School of natural and applied sciences
    • School of Journalism
    • SirenFM
    • http:// tango.freedesktop.org