Geochemistry Exam 2

Isotopes

an element with a different number of neutrons than expected; varying atomic mass, isotopes always have more neutrons than protons

Relative Abundance

comparison of the light and heavy stable isotopes in a sample to a known standard

light isotope

less neutrons

heavy isotope

more neutrons

VSMOW

vienna standard mean ocean water; pure water with no salt or other chemicals in it, used as a standard comparison for hydrogen and oxygen isotopes

PDB

Pee Dee Belenmite; geologic formation in North/South Carolina consisting of marine deposit fossils of belenmitella americana, used as a standard comparison for carbon isotopes

Fractionation

heavy/light, rare/abundant, = R, always comparing to fixed standard (VSMOW + PDB)

Depleted

lighter, has more of the lighter isotope than standard

Enriched

heavier, has more of the heavy isotope than standard

Change per mil

fractionization; (Rsample/Rstandard - 1) x 1000

Causes of Fractionation

covalent bonds, ionic bonds, oxidation states, crystallization, temperature

How do covalent bonds affect fractionation?

heavier bonds form more stable bonds and lighter isotope’s bonds are easier to break; more apparent at low temps

How do ionic bonds affect fractionation?

minimal effect

How do oxidation states affect fractionation?

heavier isotopes prefer to bond to higher oxidation state elements

How does crystallization affect fractionation

heavier isotopes form a solid first, leaving liquid behind that is depleted (more light isotopes)

How does temperature affect fractionation?

For calcite, as temps decrease more calcite/aragonite precipitates. Heavier isotopes precipitate out first, leaving behind depleted solution so as temps decrease, relative abundance increases

Kinetic fractionation

when an isotope reacts, diffuses, or evaporates faster than another due to the process and/or catalyst; results in lighter isotope accumulating in the product

What reacts faster?

lighter molecules

What carbon do plants prefer?

prefer lighter 12C because it reacts faster. CO2 is converted to lighter C in C6H12O2 (sugar) during photosynthesis

Rayleigh Fractionation

global scale evaporation, condensation, precipitation; isotopes fractionate when material phase changes

Uses for isotopes

can help us date things, see how climate is changing, see what an organism’s diet was like, find out origin of an organism

Daughter isotopes

called radiogenic because they form from radioactive decay. what the original isotope breaks down into

Alpha radiation radioactive decay

lose a particle with 2 protons, 2 neutrons, and a +2 charge. U→Th + He (a) + j + E

Beta radioactive decay

lose one electron in a series. happens in series with alpha decay. Th→Pa + -ie-(B) + j + v

Positron radioactive decay

emission of a positively charged electron (formed when a proton becomes neutral + releases a positive particle) Al→ MG + j + v

Gamma radiation

released in alpha, beta, and positron decay. stabilizes the nucleus to a lower energy/more stable state

Fission radioactive decay

nucleus splits into 2 or more nuclei. produces an alpha particle

Decay curve

exponential growth of daughter isotope, exponential decay of parent isotope

half-life

amount of time it takes for a substance to decay to half its original mass. Pt = Po x e^-kt. t1/2 = -0.69/y

Zero order rate

linear decay of substance. constant rate, typically seen in dissolution of some salts. y=mx+b

First Order Rate

exponential decay of isotope, rate increases over time (linear). typically seen in population growth, radioactive decay, hillslope. Pt = Po x e^-kt

Daughter isotope accumulation

D = Po(e^yt - 1) 

Isotopes and their uses

Carbon 14 = 100-30,000 years (clothing, more modern stuff)

Potassium 40 = 100,000-4.5 billion (rocks)

Rubidium 87 = 10 million - 4.5 billion (rocks)

Uranium 238/235 = 10 million - 4.6 billion (rocks)

Ka

dissociation constant or equilibrium constant of each H+ dissociation reaction. Ka = [A] [H+] / [HA]

pKa

pKa = -log (Ka). quantitative measure of the strength of an acid

strong vs weak acid

strong acids have smaller pKa/Ka while weak acids have larger ones

why is the pH of water 1-14?

Because kw= [H+] [OH-] = 1×10^-14 or the concentration of each is 10^-7. when [H+] = [OH-] then pH is 7

why are some polyprotic acids (multiple hydrogens) weak?

they don’t completely dissociate immediately, well-buffered

dissociation of strong vs weak acid

strong acids completely dissociate while weak ones don’t

Dissociation of polyprotic acids

at different pHs, different species of the acids are dominant in solution. Bjerrum plot

what is H2CO3*

it encompasses all of the reactions that determine how much CO2 is gaseous and how much is aqueous

Alkalinity definitions

1. the potential of a liquid to neutralize acid

2. ability of a solution to accept acidity and not change pH

3. amount of acid needed to convert anions in solution into uncharged species

4. Amount of acid needed to titrate a solution to the pH of the CO2 equivalence point

Alkalinity lab definition

defined by titration, a process that adds acid slowly to a solution while pH is measured after each addition, discussed in meq

Calculating alkalinity

put data into USGS calculate and get alkalinity in mg/L HCO3 -

In karst waters, alkalinity roughly equals [HCO3-] so amount of acid = alkalinity

buffer capacity

measure of buffering in a solution (resistance to change)

buffer zones on bjerrum

1. right before H2CO3

2. second downward curve/hump

3. final downward curve/hump

How do we get stuff in water?

chemical reactions add stuff to H2O. can be from mineral dissolution+precipitation, mineral weathering, and reactions within solution between species or solids

Big Picture Controls on Water Chemistry

1. water cycle

2. dissolved vs particulate ratio 

3. mineral solubility and reaction types

4. water measurements/parameters (ionic strength, likeliness to react, etc)

5. stability of species in water

TDS - Total Dissolved Solids

measure of all dissolved components in water (organics, inorganics, ions, colloids, etc)

NAPLS - non acqueous phase liquids

things in water that aren’t dissolved

DNAPLS

dense non acqueous phase liquids, sink to the bottom of the H2O column (diesel fuel)

LNAPLS

light non acqueous phase liquids, float on top of water, common organic contaminants (gasoline)

colloids

small solids that are in water but not dissolved, have surface chemistry reactions

sizes of undissolved stuff in water

nanostructures (atoms, ions, proteins, viruses) → colloids (atmospheric dust, spore, soot, latex paint) → particulates (pollen, fog, sand, bacteria, red blood cells)

Ionic Potential

z/r; ration of electric charge to radius of the ion, tells us how strongly the ion will be attracted to ions of opposite charge or repelled by ions of the same charge.

high z/r

associated with strong bonding, high charge, small radius. strong bonds with oxygen and repels other cations (PNS), less soluble

low z/r

associated with weaker bonding, more soluble

Mineral Solubility

depends on pH, not all minerals have same solubility patterns

DIC (dissolved inorganic carbon)

DIC = CO2 (g) + H2CO3* + HCO3- + CO32-

Important Equations

1. CO2 + H2O → H2CO3*

2. H2CO3* →H+ + HCO3-

3. HCO3- → H+ + CO32-

4. H2O → H+ + OH-

5. CaCO3 → CA2+ + CO32-

6. CaMG(CO3)2 →Ca2+ + MG2+ + 2 CO32-

pH/solubility of CaCO3 graph

above curve/line = precipitation occurs (oversaturation), on curve/line = equilibrium, below curve/line = dissolution (undersaturated)

Saturation index

represents saturation (how full water is) numerically; what is measured in solution vs what we expect to be there

Partial Pressure

Px = n (gas A)/ n (total) x 1 atm

Saturation Index + Equilibrium

at equilibrium SI = 0

oversaturated (precipitate) SI = positive 

undersaturated (dissolved) SI = negative

How do you form a cave?

Need dissolvable rock (limestone or dolostone) and weak acid (carbonic acid formed from CO2 dissolved in water)

Calcite solubility

more soluble in cold water and at higher pH

Ionic Strength

element/charge of z; higher charged elements have a strong impact on IS

Electrical Conductivity

related to TDS and IS; more ions in water means better conductivity

Hardness

“soap test”= ions prevent lathering of soap; soft water = lots of suds, less ions; hard water = no suds, more ions. 50,000 x ([CA] + [Mg])

Karst

a special type of aquifer with very large pore spaces

How does structure affect cave development?

more synclines/anticlines allows for more aggressive water runoff into rock, causing dissolution; narrower/inconsistent pathways

How does water control cave development?

aggressive recharge of water through sinkholes creates large, tubular passages