10. Thermodynamics 3 Flashcards
What is oxidation?
Addition of oxygen to compounds OR number of hydrogen ATOMS is reduced
What is reduction?
Amount of oxygen in a compound is reduced OR number of hydrogen ATOMS is increased
What does oxidation and reduction mean in terms of the ionisation of metals?
Oxidation= Loss of electrons Reduction= Gain of electrons
Redox reactions= Involves movements of electrons between atoms or molecules
What does redox potential measure?
Measures the potential of a compound or ion to lose or gain an electron in a redox reaction
What does a positive redox potential mean?
The substance has a higher affinity for electrons than hydrogen ions= Higher potential to go to the ‘reduced’ side of equilibrium
What does a negative redox potential mean?
The substance has a lower affinity for electrons than hydrogen ions= Higher potential to go to the ‘oxidised’ side of equilibrium
Which redox half reaction is used as the standard?
What does E° mean?
What does E°’ mean?
The hydrogen/H+ redox half reaction is used as the standard
All components are present under standard state conditions, E is arbitrarily set to 0:
2H+ + 2e- ––––––––> H2 where E°= 0.00V
E°= Standard state conditions, assumes that [H+]= 1M (pH is at 0)
HALF REACTIONS: ALWAYS written with the electron ACCEPTOR on the left= ne– is always on LEFT
It goes: Electron acceptor + ne- —> Electron donor
E°’= pH is at 7
Which way do electrons flow in a half reaction?
Electrons flow from LOWER potential (Lower E°’ value) to the half reaction of HIGHER potential (Higher E°’ value)
Electrons flow from - to +
What is a reducing agent?
Donates electrons= Becomes oxidised
What is an oxidising agent?
Accepts electrons= Becomes reduced
Oxygen is the strongest oxidant agent in biochemical systems
When trying to use the standard electrochemical potentials of half reactions to work out the electrochemical potential for a process, which way do you add the equations?
ΔE°’ = E°’(electron acceptor) – E°’(electron donor)
Or
ΔE°’ = E°’(oxidising reagent) – E°’(reducing reagent) =
E°’(reduced product) – E°’(oxidised product)
What is the equation linking electrochemical potential (ΔG) to free energy (E) for each half reaction?
ΔG = –nFE
Where F= Number of coulombs per mole of electrons= 96,845 J V^-1 mol^-1
Where n= number of electrons transferred in the half reactions
Allows use of redox potentials as the equivalent of ΔG in determining whether redox processes occur spontaneously= IF ΔE°’ is +ve, then ΔG°’ is –ve, then it will occur spontaneously under standard conditions
What is the equation linking electrochemical potential (ΔG) to free energy (E) for COMPLETE reaction?
ΔG°’ = -nFΔE°’ = -nF { E°’(electron acceptor) – E°’(electron donor) }
What is the rule for whether the overall reaction is favoured when redox half reactions are combined?
The driver reaction (i.e. the one going as written) must have a more +ve (or less negative) ΔE value than the driven reaction (the one going backwards as written in the overall process)
-a process is thermodynamically favoured if ΔG is –ve BUT if ΔE is +ve
What happens if the reaction is under non-standard conditions, which equation do you use to work out the true ΔE’ and therefore the ΔG?
Nernst equation:
ΔE’ = ΔE°’ + ( RT / nF ) ln ( [electron acceptor] / [product] )
