Block 4: Redox Reactions Flashcards
(18 cards)
What is the basic theory behind redox reactions?
Electrons are transferred between species, where one substance is oxidised (loses electrons) & the other is reduced (gains electrons). The reduced species is an oxidising agent & the oxidised species is the reducing agent
What does standard (redox) potential represent?
The electrical current (e-) that could be generated by a redox reaction
Pe = -log[e-]
What do positive & negative standard potential values indicate?
Positive Pe = few electrons available, so species will donate electrons (oxidising conditions)
Negative Pe = many electrons available, so species will gain electrons (reducing conditions)
What does the Nernst Equation calculate & correct for?
It calculates Eh (electron potential in volts), correcting for species activity (concentration) & temperatures
How does the stability of water set the limit for possible redox conditions?
Oxidants strong enough to oxidise water (form O2) & reductants strong enough to reduce water (form H2) cannot persist in natural environments as they will spontaneously react with H2O & push the system back within water’s redox stability limits
How is iron transported through soil in natural redox conditions?
Rainwater (pH~5) absorbs atmospheric CO2 to form H2CO3, which generates additional formic & humic acids in the soil profile (pH~4). In waterlogged surface environments, ferric iron acts as the oxidant & is reduced to soluble ferrous iron, which dissolves into porewaters & is transported down profile. Deeper in the profile, porewater is buffered by clays (pH~5.5) & oxygen availability oxidises ferrous into ferric iron, which precipitates out as iron oxides
What redox conditions are associated with acidic & alkaline environments?
Acidic environments are generally oxidising as low electron availability promotes oxidation
Alkaline environments are generally reducing as high electron availability promotes reduction
How does waterlogging in soil affect oxygen availability and mineral precipitation?
Waterlogged soils have less pore space for oxygen, creating anaerobic conditions & a reducing environment
Why are anaerobic environments generally more reducing?
In the absence of O2 (a strong oxidant), weaker oxidants are used that accept electrons less readily. Electrons accumulate more easily, allowing strong reductants to accumulate
Why is oxygen quickly consumed in sediments or aquifers & what happens afterwards?
Oxygen produced in sediments & aquifers is quickly consumed by microbial activity in upper layers, causing the system to become anaerobic once oxygen is depleted
After oxygen is consumed, which species act as electron acceptors?
O2, NO3, Mn-oxide, Fe (III)-oxide, organic matter, sulphate, methane, N2, H2 formation
After organic matter is consumed, which species act as electron donors?
Organic matter, sulphide, Fe (II) , ammonium, Mn (II), N2, O2 formation
What processes cause the enrichment of Fe-II in soils?
Partial oxidation of pyrite or reduction of iron oxides
When does oxidation state increase & decrease?
Increase: electrons are donated, charge is more positive, species is oxidised/is the reductant
Decrease: electrons are accepted, charge is more negative, species is reduced/is the oxidant
How are oxidation states assigned?
Free atoms are always 0 unless charged (then charge = oxidation state)
Sum of oxidation states = molecular charge
Oxygen = -2, hydrogen = +1, fluorine = -1, alkali metals = +1, alkaline metals = -1, halogens = -1
What is the formal definition of oxidation number?
The charge an atom would have if all bonds were considered ionic, indicating the more positive & negative atoms in a molecule rather than representing actual charges
Why do oxides & hydroxides tend to degrade in reducing environments?
In reducing conditions, ferric iron is reduced to ferrous iron, which is more soluble in water & causes the oxide/hydroxide to dissolve
What electron activities do reducing & oxidising conditions correspond to?
Reducing conditions correspond to high electron activities, as concentrations are high so species will accept electrons
Oxidising conditions correspond to low electron activities, as concentrations are low so species will donate their electrons
