15 & 16) Redox 1 and 2

what do round pyrite grains indicate

no oxygen in water, because pyrite would have oxidized into sulfur

means that no oxygen in water if they got ground up into spheres, and not cubes or a different compound

details on redox processes

major influence on groundwater quality particularly for O2

fate of pollutants in groundwater is often controlled by redox processes

redox are thermodynamically favourable, but most of the time it’s determined by bacteria reacting

oxidation is __

reduction is __

losing = oxidation

gaining = reduction

reaction details: 2Fe2+ + MnO2 + 4H+ <-> 2Fe3+ + Mn2+ + 2H2O

iron is oxidized (2+ to 3+)

Mn is being reduced: in MnO2 Mn is 4+ (oxygen is always -2), then going to 2+, reducing

steps for writing redox reactions

balance elements, except H and O for each half reaction

balance O by balancing H2O

balance H by adding H+

balance charge by adding electrons

subtract half reactions to cancel free electrons and get overall reactions

flip around one reaction and add together to make full reaction

details on theory of redox

expression for change in gibbs free energy relates to produces and reactants of redox

RT/nf is not just moles because electrons have charge

whats wrong with this plot

for white circles and squares, they are consistently above the computed values

if this was actually what’s happening, the calculations would equal reality

the theory does not ever match the reality, so we’re not measuring something meaningful

oxygen is meaningful, we can measure whether water is in eqlm with atmosphere

what is pe

theoretical value, represents the relative tendency of a solution to accept or transfer electrons

analogous to pH (tendency to accept or donate H+)

pe= -log{e-}
{e-} is not expected to exist, can’t really have electrons floating around in solution, but its a way for us to see if reaction will go forward or backward, the drive for a reaction

high pe is what?

oxidizing conditions

pCO2 is always ___ in soils than atmosphere because why

higher
microbes running the reaction

redox diagrams details

at pH zero, pe should be zero for reduction of water to hydrogen

and slightly above 20 at pH 0

slopes are determinable

higher pe is oxidized, so C in upper section is +4
lower pe is reduced, so C in lower section is -4

details on stability of water redox

redox processes require water to allow electron transfer between species;

only reactions stable in water are of interest

H20 <-> 2H+ + 2e- + 1/2O2 is the oxidation limit

½ H2 <-> H+ + e- is the reduction limit

what to do for stability limit equations

lower: choose the value of logK where H2 bubbles out of solution, the easiest value to choose is 1 because log1=0 even though this is not accurate

higher: water cannot exist above the top line, it’s definitely accurate

details on the formation of methane

CH3COOH is acetic acid, typical in freshwater environments

using hydrogen is typical in marine environment

how to do this

write out full K equation with coeffs as exponents
cancel out water and compounds { }=1
take log of both sides writing out exponents as coeffs and adding and subtracting for multiply and division

logK is given
log{e-} = pe
log{H+} = pH

find slope and organize like this:
pe = slope*pH + logK

explain this

KH = {CO2 (aq)} / pCO2(g)

KH is constant

pCO2 is exchange with atmosphere (constant)

So {CO2(aq)} is constant

what is ionic strength

measure of total ions in solution, considering molar concentration and charge