Basic Background Review: Acid Base, Redox, and Stable Isotopes

Transcription

1 Basic Background Review: Acid Base, Redox, and Stable Isotopes

2 1) Basic Physical Chemistry Thermodynamics Potential for reaction to proceed Expected state at equilibrium Kinetics Reaction rates Reaction inhibition, catalysis

3 What makes reactions go? Maximum entropy, minimum free energy (= equilibrium state of system) Gibbs Free Energy for a chemical substance: G=H TS ΔH largely function of strengths of chemical bonds in products vs, reactant (more strong bonds in product than reactants (negative ΔH) contributes to spontaneous reaction) ΔS measure of relative degree of disorder in product and reactant greater disorder in product than reactant (positive ΔS) contributes to spontaneous reaction `

4 What makes reactions go? If T of product and reactants are equal, ΔG=ΔH TΔS For a chemical reaction, ΔG=Gproducts Greactants Reaction has thermodynamic potential to react if ΔG is negative; larger ΔG, more likely spontaneous reaction. `

5 How fast will they go? RATE LAW, reactants A, B, C da/dt = ka m B n C p Where: k is rate constant, units depend on concentration units and exponents m, n, p define order of reaction with respect to each reactant order of overall reaction is m + n + p

6 How fast will they go? Rate constant can be temperature dependent Arrhenius Equation K = A exp( E a /RT) E a is ACTIVATION ENERGY Catalyst for a reaction lowers the activitation energy

7 How fast will they go? Reaction Mechanisms Multi step series of simple reactions leading to final product are common in natural systems Typically have rate limiting step determining kinetics Catalyst is something which lowers the activation energy (and increase rate) without being changed itself

8 Example: ozone depletion Oxygen only mechanism Chapman Model O2 +hν O + O J1 O + O2 + M O3 + M k2 O3 +hν O + O2 J3 O + O3 2O2 k4 Balance of first and fourth reactions determines amount of O3, solve at steady state, consistently overestimates concentration of ozone in stratosphere Therefore, must be other ozonedepleting reaction

9 ozone depletion series of catalytic ozone destroying reactions with NO/NO 2 as catalyst: NO + O 3 NO 2 + O 2 k 5 NO 2 + O NO + O 2 k 6 O 3 + O 2O 2 NET REACTION Each NO, NO 2 molecule goes through cycle hundreds to thousands of times before being rained out

10 Biogeochemical Systems and Physical Chemistry Non equilibrium states are typical Biological input of energy Typically boundaries Often very slow kinetics Always consider possible role of catalysts Environmental gradients typical Temperature, pressure, Ph, Eh

11 2) Acids and bases ph = log 10 [H + ]

12 ACID BASE REACTIONS Involve exchange of protons in liquid phase Bases dissociate to give [OH ] (are bitter, slippery, and also turn litmus paper blue!) Acids dissociate to give [H + ] (are sour, non slippery, turn litmus paper red!) Lots of complexity: Strong vs. weak, hard vs. soft, etc.. Dissociation constant of water H 2 O H + + OH, log K = ph of a neutral solution of water = 7 Acid base balances within Earth system generally involve elements of relatively high abundance

13 ACID BASE REACTIONS C, N, S are especially important in Earth System Acid base Chemistry All undergo ~ same oxidation rx to acids R+ H 2 0 => HxR0x Eg: H 2 CO 3, H 2 S0 4, H 2 NO 3

14 3) Redox Reactions

15 Redox and Life As unstable chemical entities fueled by a continual diet of decomposing compounds, sustained by a constant energy flux from oxidation reactions, we are motivated by more than pure academic curiosity to study redox chemistry; it is a simple (or not so simple) matter of life and death. Anonymous (to me)

16 Redox: super basics OXIDATION STATE OF AN ELEMENT Example: C( IV) methane C(0) C, CH 2 O C(+IV) CO 2 and other dissolved inorganic carbon species In any compound, assume O( II) and H(+I)

17 Redox: super basics Oxidation reduction reactions or redox reactions = exchange of electrons LEO the lion says GERRRR* Loss of electrons is oxidation Gain of electrons is reductant Reductant=electron donor Oxidant=electron acceptor Reducing force=electron supply Oxidizing force=electron drain *anonymous, strikes again..

18 Super important: redox of C, N, O, S, plus Fe, Mn Because life is a matter of stochiometry as well as one of energetics, the cycles of these elements are then tightly coupled in two different ways.* Anonymous; speaks again

19 Except for P: Phosphorous has no redox chemistry.

20 EQUILIBRIUM CONSTANTS FOR REDOX REACTIONS Redox reactions generally expressed as individual half redox reactions (Really aren t free electrons in solution in way there are H +) Half reactions written by convention as reductions BY CONVENTION, H + + e = ½ H 2 (g), ΔG 0 = 0. K = 1 COMPLETE REDOX REACTION Ox 1 + Red 2 = Red 1 + Ox 2 Transfer of n electrons from Red 2 to Ox 1

21 Complex possible notations, all same idea: FOUR DIFFERENT, EQUIVALENT SCALES TO DEFINE THERMODYNAMIC CHARACTERISTICS OF REDOX REACTIONS pe = log 10 [e 1 ], nondimensional scale like ph for electron activity E 0 H, redox potential (volts) K, equilibrium constant K=[products]/[reactants] ΔG 0, free energy change ΔG 0 = RTlnK

22 BIOGEOCHEMICAL CYCLES AND REDOX Photosynthesis Photochemical reactions Oxidation of organic matter Reduced compounds from Earth s interior (e.g., hydrothermal vents)

23 A most fundamental biogeochemistry example: Organic matter reduction/oxidation 1) Photosynthesis captures sunlight, (increasing the free energy of the planet) CO 2 + H 2 O CH 2 O + O 2 = a reduction of carbon C(+2) C(0) Delta G = 686 kcal/ mol

24 POM sink CH 2 O 2) Organic matter oxidation, electron acceptor series

25 Redox + Acid/Base: mechanistic control of most biogeochemical cycle reactions ACID BASE: In natural systems, some master system (often dissolved inorganic carbon systematics) sets the overall ph. Then can use ph as master variable to figure out degree of dissociation of other acids/bases. (ph diagrams) REDOX: In natural systems, some master system (typically either the presence of dissolved oxygen or dissolved sulfide), sets the overall redox state. Then can use a master variable to describe the state of the system, figure out the poise of other redox couples. (pe ph diagrams or E h ph diagrams)

26 4) Stable Isotopes

27 I. Introduction A. What are stable isotopes? Recall: Stable isotopes vary in mass, but chemistry is unchanged (ie outer shell electron configuration) Eg: Stable isotopes of Oxygen: Isotope # Protons #Neutrons Atomic Wt Atom% 16 O O O

28 Distribution and Occurrence Main Organic Stable Isotopes 1. All the major bioactive elements (except P) have multiple stable isotopes. 2. Within this group, the light isotope (L) is consistently more abundant than the heavy (H) counterpart(s). 3. It is very small (ppt) differences in (H/L) that constitute the basis of using stable isotope signatures as geochemical source and process indicators Isotope Atom% 1 H H (D) C C N N O O O S S S S 0.014

29 Why do useful differences occur in Nature? Reactions with kinetic component, always end up enriching in LIGHT isotope. Note: Natural processes: often may have both a passive (diffusion) and active (biochemical reaction) component. BOTH parts will fractionate for light isotope. In fact, most natural fractionations = chain of processes

30 A bit more background: Isotopes of different mass have slightly different bond strengths. This affects their distribution because: 1) Lighter Isotopes tend to move faster rates of diffusive transport are different! (eg: 12 CO2(g) ~1% faster than 13 CO2(g)) 2) Lesser effect: equilibrium distributions in a reversible reaction, where heavier isotope is typically concentrated in more strongly bonded form: 13 CO2(g) + H 12 CO3 (aq) = 12 CO2(g) + H 13 CO3 (aq) favored (1)

31 RESULT: Virtually ANY and EVERY chemical process (both biological and abiotic) fractionates stable isotopes to some degree Eg: Photosynthesis, Respiration, Biochemcial reaction steps, Evaporation, Gas dissolution, Precipitation, etc etc etc. OFTEN these fractionations are very predictable thus Stable Isotopes represent an imbedded tag in every molecule for 1) Source and 2) process history

32 Stable Isotope signatures of organic matter are among most powerful off all Organic geochemical tools! A few examples of uses/ processes: C 12/13: ultimate origin of fixed carbon N 15/14: Trophic level processing O & H Hydrologic cycles: eg: PaleoRainfall S: ancient sulfer cycles. * C and N are most common (and easy) isotopes measured mainstays. Main Organic Stable Isotopes Isotope Atom% 1 H H (D) C C N N O O O S S S S 0.014

33 Some Terms and definitions: fractionation = The amount of change in light vs heavy (Measured in del notation) created by a given process. isotope effect (typically means kinetic isotope effect ) = fractionation occurs in the process of interest. 12 C 12 C C C An Isotope Effect (a phenomenon) Fractionation (observable quantity)

34 How isotope ratios are expressed: 1) Del (δ) notation: measure of relative ratio of heavy vs light isotopes a) expressed as ( per mil, or part per thousand) relative to a (semi arbitrary) standard. (why relative to a standard?) b) Written : δ 13 C of sedimentary bulk OC was 27.5 or δ 15 N of the bulk plankton tow from 100m was + 8 Formal Definition: H = (H/L)spl - (H/L)std x1000 (H/L)std where H= heavy isotope and L= light isotope

35 Reference Standards: Selected: To reflect the major source reservoir that processes are fractionating. EG: Ocean water, Atmospheric N, Carbon in deep space Primary Standards Isotope Ratios Ratios x 10-6 Standard mean ocean water 2 H/ 1 H O/ 16 O O/ 16 O 373 PeeDee belemnite (PDB) 13 C/ 12 C Air 15 N/ 14 N Canyon Diablo meteorite 32 S/ 34 S 22.22

36 How to make stable isotope measurements? Basic Problem: Differences are VERY SMALL! Recall: the per mil is not a direct measure, but is a relative measure IE actual carbon Std Ratio 12 C/ 13 C = already a BIG excess Boils down to measuring absolute differences in a small populations of atoms of the Heavy Isotope. (Analogy: being able to measure relative differences in distance of a cm or two, accurately and reproducibly of the scale of here to Paris)

37 How its Done: 1) As with CHN measures: Oxidize OM fully to CO2, NOx, etc. now, only have one kind of molecule to worry about detecting. 2) Now, need to measure RATIO of 13 CO 2 vs 12 CO 2, or the ratio of masses 44 vs 45 at exquisite sensitivity! Special kind of Mass Spec: Isotope Ratio Mass Spectrometer or IRMS

38 sample Roast (as w/ CHN) 1050 deg EA IRMS setup: CO 2 CO 2 CO 2 CO 2 CO 2 CO 2 Molecular Leak Ion Source Ratio Mass Spectrometer 60 o Detectors Signal Processor Ionizing Electron Beam Magnet Analyzer Tube Precision attained: typically to better than 0.1!!!!

39 END