Comprehensive Geochemistry and Solution Chemistry Key Concepts

Using 1 atm as total pressure, present day CO2 is commonly taken as what?

PCO2 (very important)

PCO2 approx =

4.2 * 10-4 atm = 420 ppm (v)

Log atm commonly used =

-3.37

All reactions must be

charge balanced

Law of mass action (important)

Molarity =

moles of solute/liters of solution

Molality =

moles solute/kilograms solvent (H2O)

parts per million (ppm) =

(mass of solute/mass of sample) * 1,000,000

Formality =

moles/kg solution

Normality (N) =

number of moles equivalents/1 L of solution

Mole fraction is a unit of concentration equal to

the number of moles of a component divided by the total number of moles of a solution. It is a ratio so it has no unit.

Best practice in high-salinity systems?

use mg/kg and compute ppm from that, not from mg/L (Mmol/kg is often better)

Thermodynamics is a branch of science that deals with...

energy levels and transfers of energy between states of matter

Thermodynamics of what is of most importance in geochemistry?

chemical reactions

Kinetics →

some reactions may not be able to occur due to certain limitations

Equilibrium thermodynamics provides...

a good approximation of the state of the real world, can indicate reaction direction, provides basis for calculating rates of natural processes, and a system at equilibrium has a state of minimum energy that reactions move toward by releasing energy

A system at equilibrium will _____ after being perturbed?

return to that state after being perturbed

Energy states:

unstable, metastable, stable

Diamonds are ___________ at Earth's surface, because they are ________ stable at high pressure, deeper underground

metastable, most

___________ is most stable form of Carbon on Earth's surface

Graphite

Mixtures are _____ stable than non-mixtures !! They lower overall energy

more

Enthalpy:

the amount of heat

delta_H < 0, heat is given off

exothermic

delta_H > 0, heat is absorbed

endothermic

delta_H can be calculated from

standard enthalpies of formation

Product will be ___________ , reactant will be _____________

positive, negative

Standard state conditions:

298.15 K (or 25oC) and 1 atm (or 1 bar)

Entropy (S):

system property which increases spontaneous reactions in isolated systems with constants U and V

We will mostly always use Free Energy to determine if a reaction will "go" or "not go"

A reaction is thermodynamically favorable in the forward direction when

delta_G < 0

Two basic ways a reaction can be thermodynamically favorable:

(1) delta_H < 0 (exothermic, releases heat), which lowers delta_G, (2) delta_S > 0 (entropy increases) and term -T delta_S becomes more negative as T increases

"Spontaneous" in chemistry means that a reaction is predicted to ________, there is no amount of time associated with it

occur

Stoichiometric coefficient:

𝒗 (-1 if reactant, +1 if product) (product over reactant)

The equilibrium constant (K):

the ratio of the product of products (raised to their stoichiometric power) divided by the product of the reactant (raised to their stoichiometric power) is a constant

Reactions can be predicted to "stop" when the equilibrium constant is satisfied

For the reaction aA + bB + ... ←→ cC + dD + ...

K = (acCadD)/(aaAabB)

delta_rG:

negative (goes from reactant to product), positive (goes from product to reactant)

mu_i =

mu_io + RT ln(a)

Standard state (unit activity):

mu_io = delta_Gof

Find log K using

standard state (delta_G), entropy (delta_S), and enthalpy (delta_H)

Q is a __________ term, K is a __________ term

measured, calculated

Van't Hoff Equation:

-RT ln K = delta_Ho - T delta_So, Derived form: ln (K2/K1) = (delta_Ho/R)(1/T1 - 1/T20)

The Activity (a) of an ion in solution is related to its Molar Concentration (c) and its Activity Coefficient (gamma), following the expression:

a = gamma * C

How to calculate the Activity Coefficient (gamma):

log (gamma) = (0.5 (Z)2 sqrt(I))/(1 + sqrt(I)), Z = oxidation number of the ion, I = ionic strength of a solution

Ionic strength:

I = ½ sum c * (z)2

Chemical equilibrium condition:

mu_g = mu_aq

pH of most mineral-bearing waters is 6 to 9

pH and composition of natural waters is regulated by reactions of acids and bases

Strong acids have a substantial tendency to donate a proton, this depends on the nature of the acid as well as the base accepting the proton (often water)

Ocean acidification:

destruction of phytoplankton and coral ecosystems, shellfish larva viability

Metal sulfide mineral oxidation:

the threat of mine tailing's oxidation and groundwater quality - locally and globally

Acid precipitation (rain)

Bronsted-Lowry definitions:

an acid is a substance that donates a proton (H+), and a base is a substance that accepts a proton (H+), Example: HA → H+ + A-

Proton donor vs acceptor:

Acid: proton donor, Base: proton acceptor

The conjugate base of an acid differs by

exactly one proton

The Equilibrium Constant (every reaction we do will have Kw in it)

Kw = [H+][OH-] = 1.0 * 1014 (at 25oC)

Autodissociation of water:

[OH-] = Kw / [H+]

pH values:

pH < 7 → acidic, pH = 7 → neutral, pH > 7 → basic

Acid Dissociation Constant:

Ka = [H+][A-] / [HA]

Log form:

pKa = -log Ka

Ka meaning:

large Ka → strong acid (dissociates completely) (pKa < ~2), small Ka → weak acid (partially dissociated) (pKa > ~2), Ka relates to the tendency of HA to release H+

Acid strength vs conjugate bases:

Strong acids: weak conjugate bases, Weak acids: strong conjugate bases

For conjugate acid-base pairs:

Ka * Kb = Kw (In pK form: pKa + pKb = 14.00 (at 25C))

When pH = pKa, the acid is exactly...

50% dissociated !!! (At this point, [HA] = [A+])

The Proton Condition is a powerful method for...

solving acid-base equilibria, that tracks which species have gained or lost protons relative to a reference level

Proton condition for carbonic acid:

[H+] = [OH-] + [HCO3-] + 2[CO3-2]

Water is not part of the proton condition...

it gives up its life for [H+] and [OH-]

Equilibrium constants: acids: HA = H+ + A-, HCl + H2O = H3O+ + Cl-, HCl = H+ + Cl-, bases: B + H2O = BH+ + OH-, NH3 + H2O = NH4+ + OH-

Proton condition:

an equation that expresses the balance of all species that donate or accept protons in a solution, written in terms of [H+] and the acid-base species present. It comes from combining charge balance with the relevant acid-base equilibria

Monoprotic acid HA in water:

The proton condition can be written as: [H+] = [A-] + [OH-]

Four principal steps:

1. List all species present, 2. List all independent equations (equilibria, mass balances, proton balance (or electroneutrality equation)), 3. Combine equations and solve for proton condition, 4. Solve for other species

ENE:

Electrical Neutrality

Ionization fractions:

alpha_0 : [HA]/C = 1 / (Ka/[H+] + 1), As (H+) increases, [HAc] = C, and alpha_1 : [A-]/C = 1 / (1 +[H+]/Ka), As (H+) decreases, [A-] = C

Really important antibacterial agent and pH buffer:

pK = aH + aNH3 / a+NH4

Proton condition aka proton balance equation:

defines the equilibrium state of a mixture by equating the sum of concentration terms for species that have gained protons to the sum of concentration terms for species that have lost protons, relative to a designated reference level

Equivalence point:

for an acid-base titration, that means all of the acid's reactive protons have been exactly neutralized by base (or vice versa)

Henderson-Hasselbach equation:

pH = pKa + log [A-]/[HA]

used to find a ph of a buffer

When pK is equal to pH, equivalence point

the one that was increasing levels out and the one that was level begins to decrease

pH = pK

buffer capacity

When H+ crosses the conjugate base...

that is the Proton Condition

Buffer Intensity:

amount of strong acid or base required to cause a specific small shift in pH

The buffer intensity for any point on the titration curve is...

inversely proportional to the slope of the titration curve

Alkalinity:

acid neutralizing capacity of a fluid - conservative property, versus pH (conservative properties don't change with temperature, and can be mixed together)

Ct:

total dissolved inorganic carbon (DIC) - conservative property

Mineral acidity:

base neutralizing capacity of a fluid - conservative property

Mineral acidity =

negative alkalinity

Electroneutrality:

[H+] = [OH-] + [HCO3-] + 2[CO32-]

CO2 added to water will...

lower pH, but the alkalinity will not change

Does the alkalinity of a natural water (isolated from its surroundings) increase, decrease, or stay constant when small quantity of the following is added:

HCl: decrease the alkalinity (because of the chloride), NaOH: increase (because of the sodium), Na2CO3: increase (because of the sodium), NaHCO3: increase (because of the sodium), CO2: constant, AlCl3: decrease (because of the aluminum, precipitates and forms H+)

Mixing calculation revisited:

0 = V1(-10-2)+V2(210-3)V1+V2

Respiration: causes ______ CO2

higher

Photosynthesis causes _______ CO2

lower

Atmospheric pH is typically

7.7

Upwelling on the west coast of the US, CO2 is more soluble in colder water, making the water...

more acidic

Calcite and aragonite become _______ as depth increases, because of the increase in pressure.

more soluble

K1 =

10^(-6.35)

K2 =

10^(-10.33)

Kw =

10^(-14)

KSO =

solubility product constant

ENE =

electrical neutrality equation

MBE =

mass balance equation