Lecture 3
More General Chemistry Review
Pease read chapter 1 and 2 of McSween et al. for this
week
• Classes of chemical reactions
• Mathematically describing chemical reactions
GG325 L3, F2012
Chemical Reactions
GG325 L3, F2012
Chemical reactions
Let's review 6 geochemically pertinent chemical reaction types. l, v, s and aq refer
to "liquid", "vapor or gas", "solid" and "dissolved aqueous"phases, respectively.
a. Phase change
H2O(s) ↔ H2O(l) ↔H2O(g)
These two reactions drive the hydrologic cycle.
CaCO3 (s) (aragonite) ↔CaCO3 (s) (calcite)
(important for formation/preservation of carbonate sediments globally)
GG325 L3, F2012
Chemical reactions
b. Bond Reorganization
The Redfield equation of Photosynthesis/Respiration (most plant matter is made
and destroyed by organisms using something very close to this):
106CO2 +16NO3- + HPO42- + 122H2O +18H+
↑↓ƒ„
C106H263O110N16P + 138O2
or (CH2O)106(NH3)16(H3PO4) + 138O2
the "Urey" rxn is important for understanding rock weathering and global controls
on atmospheric CO2:
CaSiO3(s) + CO2(g) ↔ CaCO3(s) + SiO2(s)
GG325 L3, F2012
Chemical reactions
c. Dissolution/precipitation & Dissolution/gas release
CaSO4(s) ↔ Ca2+(aq) + SO42-(aq)
NaCl(s) ↔ Na+(aq) + Cl-(aq)
CO2 (g) ↔ CO2 (aq)
GG325 L3, F2012
Chemical reactions
d. Oxidation/Reduction - "Redox": electron transfer
MnO2(s) + 4H+ + 2Fe2+(aq) ↔ Mn2+(aq) + 2H2O + 2Fe3+(aq)
An important reaction all over the hydrosphere/geosphere
Photosynthesis/Respiration is in this category too
e. ion substitution - many different sub classes of this process will be important
this semester.
Na+ (aq) + [clay mineral]-K+ ↔ K+ (aq) + [clay mineral]-Na+
cation exchange between a clay mineral and ions in water
GG325 L3, F2012
Chemical reactions
f. Complexation/Chelation
Fe2+(aq) + 6H2O ↔ Fe(H2O)62+ (aq)
hydration of Fe2+ in water
aqueous metal complex with dissolved organic carbon
EDTA
An common multidentate ligand
GG325 L3, F2012
complexation/chelation
Complexes involve ligands and host ions.
Hydration is a specific type of complexation reaction where the
ligands are all water:
Fe2+ + 6H2O ↔ Fe(H2O)62+
Schematic depiction of water using it’s lone pairs
of electrons to stabilize an Fe2+ ion in solution.
The hydrate itself involves 5 other water molecules.
GG325 L3, F2012
ligands:
Other common natural ligands are Cl- (chloride) and :NH3 (ammonia).
These, along with water, are known as unidentate.
Unidentate ligands offer electrons from a single site to a complex.
In an aqueous Fe+3 solution with both Cl- and :NH3, many complexes
are possible involving these two ligands and H2O.
The charge on the complex remains unchanged relative to Fe+3 with
H2O and NH3 ligands, but each Cl- ligand brings one negative charge.
All of the following complexes are possible in this solution:
[FeCl6]-3 [FeCl3(NH3)3]0 [FeCl2(NH3)4]+ [Fe(NH3)6]+3
As you might expect, the relative proportions of these complexes
will vary with pH since NH3 + H+ ↔ NH4+ (ammonium is not a good ligand)
At low pH [FeCl6]-3 would be favored
GG325 L3, F2012
ligands:
Chelation is a type of complexation that involves multidentate ligands.
A chelate is a complex involving such a ligand.
A multidentate ligand is one that has more than one electron pair to donate to a
cation. The simplest type is a bidentate ligand.
A bidentate ligand has two active binding sites for a cation.
e.g., ethylene diamine, :NH2-CH2-CH2-H2N:
and oxalic acid/oxylate anion, which has the following forms in solution:
OO OO OO
-
-
HO-C-C-OH <----> HO-C-C-O <----> HO-C-C-O
OO O O
- -
<----> - O-C-C-O - <----> O-C-C-O
Carbon-oxygen double bond delocalized e- in π bond
GG325 L3, F2012
ligands: “stick figure” notation for a 5
carbon-hydrogen (CH2) 5 linear
chain. This ligand molecule is --
Bidendate ligands can bind in two ways: NH2-CH2-CH2-CH2-CH2-CH2-NH2
(diamino pentane)
Cl Cl
Cl N N Cl
M M
Cl N Cl N
Cl Cl
cis trans
next to across
Here, M is a metal or other cation and N is where the donor electron pairs
are in the ligand (squiggly line indicates linked carbon atoms).
a "small" bidentate ligand such as ethylene diamine (NH2-CH2=CH2-NH2) can
usually only bind cis for geometric reasons.
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Mathematically describing chemical reactions
how do we quantify reactions using chemical equations?
All reactions can be described by algebraic equations of reactants and
products. They must be material and charge "balanced" to be used
quantitatively.
For a generic chemical reaction involving reactants A and B and
products C and D, their reaction is mathematically balanced using
stoichiometric coefficients a, b, c, d so that the values of these
coefficients equalize the matter and electrical charge on both sides of
the equation.
aA +bB ↔ cC + dD
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Equilibrium
Chemical reactions involve reorganization of atoms and
molecules to form new materials. It is useful to have a numeric
description of reactions that will help us predict if and how
much it will occur under various circumstances.
For almost any environmental condition there is a unique mix
of reactants and products in an assemblage of molecules
that doesn't change with time because an energetically
favorable balance is achieved.
This condition is chemical equilibrium, wherein the relative
proportions of reactants and products are expressed with an
equilibrium constant (K): K= Cc Dd
Aa Bb
GG325 L3, F2012
K= Cc Dd
Aa Bb
The quantities A, B, C and D are element activities, which
are approximately equal to molar concentration (in liquids) or
pressure (in the gas phase). We will discuss situations this
semester where activity ≠ concentration.
By standard practice activities of solids and pure H2O(l) in
chemical equations always equal 1 in the formulation of K.
There is only one unique value of K for a given reaction at a
certain value of pressure, P, and temperature, T.
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Non-Equilibrium
If we find a system that has concentrations of reactants or products that differ
from those predicted by K it is not in equilibrium. At non-equilibrium conditions,
the reaction coefficient Q is defined as
Q= Cc Dd
Aa Bb
Q < K: deficit of products.
reaction proceeds in the direction written to correct this.
Q > K: excess of products.
reaction will proceed in the reverse direction.
Unfortunately, the value of Q says almost nothing about the rate at which the
system will return to equilibrium. This is the domain of another field of chemistry
known as Kinetics (which we will discuss some during the semester).
GG325 L3, F2012
Using equilibrium constants
What do the absolute values of K mean?
Consider two solubility reactions (solid ↔ dissolved (aq) solute). For this type of
reaction K is denoted with the subscript "sp" = Ksp.
NaCl(s) ↔ Na+(aq) + Cl-(aq) has a large Ksp - Products are favored, therefore halite
is very soluble in water.
BaSO4(s) ↔ Ba2+(aq) + SO42-(aq) has a small Ksp - Reactants are favored, so
barite is sparingly soluble.
Chemical equations and Equilibrium constants can be combined
algebraically.
(1) A + B ↔ AB K1= [AB]
[A] [B]
(2) AC ↔ A + C K2= [A] [C]
[AC]
------------------------------------------------------------------------------
(1 + 2) AC + B ↔ AB + C K3= [AB] [C] K3=K1•K2
[AC] [B]
GG325 L3, F2012
Example: Combining Equilibria in the aqueous CO2 system
Let's combine the system of inorganic reactions governing the amount
of dissolved CO2 in natural waters (which bears directly on the amount
of CO2 in our atmosphere, geosphere and biosphere)
CO2 has 4 forms in water, which are related by a series of chemical;
reactions shown schematically in the figure below:
a. dissolved gaseous carbon dioxide CO2(aq)
b. carbonic acid H2CO3(aq)
c. bicarbonate anion HCO3-(aq)
d. carbonate anion CO32-(aq)
Note that CaCO3 formation (by
inorganic precipitation or biogenic
precipitation) is the primary upper
limit control on dissolved carbon
dioxide concentration
GG325 L3, F2012
The 4 forms of CO2 in water are related by 5 chemical
reactions: (disregarding for now photosynthesis and respiration)
1. CO2(g) ↔ CO2(aq)
This is a gas solubility reaction (sometimes called a Henry's law reaction. K is
given the subscript "H": KH
2. CO2 (aq) + H2O ↔ H2CO3(aq)
This is a bond reorganization reaction. It is also a type of hydration (reaction with
water). K = Keq (no "special" nomenclature)
3. H2CO3 (aq) ↔ HCO3-(aq) + H+
An acid dissociation reaction. Commonly given equilibrium constant notations of
Ka if it is a monoprotic acid, or Ka1 for the first acid dissociation of a polyprotic acid
4. HCO3- (aq) ↔ CO32- (aq) + H +
Another acid dissociation reaction. This is the second dissociation of dioprotic
carbonic acid, so we call it or Ka2.
5. CaCO3 (s) ↔ Ca2+ (aq) + CO32- (aq)
This is a solubility/dissolution reaction. K is Ksp .
GG325 L3, F2012
Many texts use the simplifying assumption that reactions "2" and
"3" (on the prior slide) can be combined. This is because so little
H2CO3(aq) is found in natural waters.
Because of this last point, the reaction sequence is shortened to
1. CO2(g) ↔ CO2(aq) KH
2. CO2(aq) + H2O ↔ HCO3-(aq) + H+ K'a1
3. HCO3-(aq) ↔ CO32-(aq) + H+ Ka2
4. CaCO3(s) ↔ Ca2+(aq) + CO32-(aq) Ksp
Let's mathematically combine them to determine an expression for
the CaCO3 solubility in a natural water (in terms of [Ca2+(aq)]).
The water is open to gas exchange with the atmosphere and we
assume there are no other reactions affecting Ca2+, H+ or Σ CO2
(aq) (carbon dioxide in all of its aqueous forms).
GG325 L3, F2012
Working backward from reaction 4: Ksp = [Ca2+]·[CO32-]
we can rearrange to: [Ca2+] = Ksp/[CO32-] A.
from reaction 3: Ka2= [H+]·[CO32-]/[HCO3-]
we can rearrange to: [CO32-] = Ka2 [HCO3-]/[H+]
substituting into A. gives: [Ca2+]= Ksp·[H+]/Ka2·[HCO3-] B.
the same type of rearrangement of equation 2 in terms of [HCO3-] yields:
[HCO3-] = K'a1·[CO2(aq)]/[H+]
substituting into B. gives: [Ca2+]= Ksp·[H+]·[H+]/Ka2·K'a1·[CO2(aq)] C.
Finally rearrangement of eqution1 KH = [CO2(aq)]/PCO2
gives: PCO2 = KH·[CO2(aq)] D.
substituting into C. gives: [Ca2+(aq)] = Ksp · [H+]2
KH · K'a1· Ka2 · PCO2
What does this equation tell us? [Ca2+(aq)] will depend on the amount of CO2 present
in the atmosphere and the pH (= -log[H+]) of the water. We will discuss the absolute
values of these equilibrium constants and the predictions that can be made with the
expressions on this page later in the semester.
GG325 L3, F2012
