Greg Druschel

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GEO 195Introduction to Geochemistry

• Greg Druschel
• 321 Delehanty Hall
• Gregory.Druschel_at_uvm.edu

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Course Goals

• At the end of this course
• You will be able to identify a number of
  different potential reactions in any environment
  involving elements in solutions, solids, or
  gases, as affected by abiotic and biotic
  processes
• You will be able to utilize thermodynamic,
  kinetic, and transport calculations to determine
  if individual reactions are feasible/important
  under any given condition
• You will be able to appreciate both the dynamics
  and complexity of geochemistry yet utilize what
  you know to ascertain processes important in the
  stability, movement, and reactivity of elements
  in the earth and other planets

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What do you see now??
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What will you see after this class??
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b
Light ? photochemical rxns, phototrophic
organisms??
O2 diffusion
FeS2 3.5 O2 H2O ? Fe2 2 SO42- 2 H
Bacteria/ archea ? Fe oxidizers, S oxidizers
Fe2 O2 H ? Fe3OOH 2 H
H SO42- lt -- gt HSO4-
CH2O FeOOH ? Fe2 CO2
CH2O SO42- ? HS- CO2
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What is Geochemistry??

• Victor Goldschmidt defined the study of
  geochemistry as the laws governing the
  distribution of the chemical elements and their
  isotopes throughout the earth
• What does that mean?
• We are interested in understanding the different
  ways in which elements move ? whether in the
  core, mantle, crust, oceans, sediments, air,
  space, or other planets

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Review of Basic Chemistry

• A nucleus (protons and neutrons) is surrounded by
  electrons - has an atomic number ( of
  protons) of 16 and an atomic mass (neutrons
  protons) of 32. S0 has 16 electrons, while S2-
  has 18 and S6 has 10.
• Many elements have several different isotopes
  where the number of neutrons is different - has
  18 neutrons for instance.
• The atomic weight (32.07 grams per mole for S) is
  determined from the average percentage of each
  isotope.
• Isotopes are either stable or unstable unstable
  ?radioisotopes decay by emitted a, b, or g
  particles

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Atomic Weights

• The exact atomic weight of an element is the sum
  of all individual isotope masses corrected for
  their abundance
• S(32) 31.972072 95.02
• S(33) 32.971459 0.75
• S(34) 33.967868 4.21
• S(36) 35.967079 0.020
• Catch is We must not only identify all the
  stable isotopes, we have to decide what their
  average distribution is problems with that???

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Stoichiometry and Units

• Reactions are always balanced with respect to
  of individual atoms, charge, and isotopes
• Chemical concentrations in water, other liquids,
  solids, or gases can be represented in many ways
• A REACTION is always presented in terms of
  molecules ? Molarity (M, moles/liter soln) or
  molality (m, moles/kg solvent)

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Periodic Table (chemistry)

• Arranged in horizontal periods and vertical
  groups ? groups determined by of electrons in
  outer orbital
• Atoms in a group have similar chemistry (more
  similar outer electrons)
• In order of increasing atomic number ( of
  protons) though an element can have many
  charges/redox states (different of electrons)
  and isotopes (different of neutrons)

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Properties derived from outer e-

• Ionization potential ? energy required to remove
  the least tightly bound electron
• Electron affinity ? energy given up as an
  electron is added to an element
• Electronegativity ? quantifies the tendency of an
  element to attract a shared electron when bonded
  to another element.

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Electronegativities
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Shielding

• Nucleus of an atom contains protons() and
  neutrons ? positively charged center of the atom
• Largest volume of the atom contains the
  electrons, zipping around, held in place by the
  electrostatic attraction to the nucleus
• Outermost electrons determine bonding, more
  electrons in between the outermost e- and
  nucleus, the less net charge and attraction they
  feel

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• In general, first ionization potential, electron
  affinity, and electronegativities increase from
  left to right across the periodic table, and to a
  lesser degree from bottom to top.

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Atomic Radius

• Another function of shielding, size is critical
  in thining about substitution of ions, diffusion,
  and in coordination numbers

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Periodic Table (Geochemists)

• Yes, here is now a separate table organized to
  reflect ion affinity for oxygen and each other
  (some elements appear multiple times different
  valances)

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Electrons in an atom

• Bohr noted that electrons are arranged in
  distinct energy levels arranged in orbital shells
• deBroglie showed electrons exhibit wave-particle
  duality
• Heisenbergs Uncertainty Principle he showed
  that you cannot define the exact position
  (particle) and the energy (wave function) of an
  electron simultaneously.
• Schrodinger then showed that the structure of
  electrons around a nucleus could be explained by
  the probability of an electron of x energy by in
  some space y this results in describing
  electron distribution around a nucleus with
  atomic orbitals

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Atomic Orbitals
1 type of s orbital 3 types of p orbitals (x, y,
z) 5 types of d orbitals (4 clovers) 7 types of f
orbitals
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Orbitals and the periodic table

• Arrangement of periodic table allows quick
  determination of electrons in s, p, d, f orbitals

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Models

• Models are attempts to describe reality, that
  doesnt mean they necessarily have anything to do
  with reality
• Models describe some aspect(s) of a system
  governed by phenomena the model attempts to
  describe
• How does Geochemistry differ from Chemistry???
• COMPLEXITY

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Variables

• In any model, looking at a process involves
  something that can change, a variable
• Extensive variable depends on the amount present
  (mass, volume, ______)
• Intensive Variable property is not additive,
  divisible (temperature, ______)
• Models describing energy transfer fall under the
  study called thermodynamics
• Whats energy??

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Variables

• For models, variables are key, and how some
  process changes a variable is the key to these
  models
• ex. As we heat a pool of water how does the
  amount of mineral dissolved change, as our car
  burns gas, how does its position change
• Describing these changes is done through
  differential calculus

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Review of calculus principles

• Process (function) y driving changes in x
  yy(x), the derivative of this is dy/dx (or
  y(x)), is the slope of y with x
• By definition, if y changes an infinitesimally
  small amount, x will essentially not change
  dy/dk
• This derivative describes how the function y(x)
  changes in response to a variable

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Partial differentials

• Most models are a little more complex, reflecting
  the fact that functions (processes) are often
  controlled by more than 1 variable
• How fast Fe2 oxidizes to Fe3 is a process that
  is affected by temperature, pH, how much O2 is
  around, and how much Fe2 is present at any one
  time
• what does this function look like, how do we
  figure it out???

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• Total differential, dy, describing changes in y
  affected by changes in all variables (more than
  one, none held constant)
• Back to our example of Fe2, expressing the
  entire equation in terms of all variables

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Pictures of variable changes

• 2 variables that affect a process 2-axis x-y
  plot
• 3 variables that affect a process 3 axis ternary
  plot (when only 2 variables are independent know
  2, automatically have 3)

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Units review

• Mole 6.02214x1023 units make up 1 mole, 1
  mole of H 6.02214x1023 H ions, 10 mol FeOOH
  6.02214x1024 moles Fe, 6.02214x1024 moles O,
  6.02214x1024 moles OH. A mole of something is
  related to its mass by the gram formula weight ?
  Molecular weight of S 32.04 g, so 32.04 grams S
  has 6.02214x1023 S atoms.
• Molarity moles / liter solution
• Molality moles / kg solvent
• ppm 1 part in 1,000,00 (106) parts by mass or
  volume
• Conversion of these units is a critical skill!!

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Homework 1

• Chapter 1, Walthers, Problems 1-7, 11, 12