Aquatic biogeochemistry – Flashcards
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| What is the Chemical Compoisition of natural waters? |
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| H2O,(H+), Major cations (Ca2+, Mg2+, K+, Na+), Major anions (SO4^2-, NO3-, Cl-, CO3^2-) Organic matter, trace metals, particles and biological materials. |
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| What do biogeochemical processes include? |
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| Chemical reactions and biological processes |
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| What are the major chemical reactions? |
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| Precipitation-dissolution, sorption-desorption, acid-base and redox reactions. |
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| Important biological processes are linked to uptake and release of a range of essential and non essential elements, but what are these processes? |
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| Photosynthesis and respiration. |
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| Chemical reactions may be at thermodynamic equilbrium or may be rate controlled by reaction kinetics but how are biological processes controlled? |
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| rate controlled. |
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| Biological processes may effect chemical reactions indirectly, but how? |
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| An example being a biologically induced pH change resulting in calcium carbonate precipitation-disolution change. |
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| What are the two internal transport processes that occur to chemical species in natural waters? describe these processes. |
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| advection and diffusion. advection occurs in the direction of flow and is equal to concentration multiplied by the mean velocity. diffusion occurs in the direction of decreasing concentration and is equal to the concentration gradient multiplied by the diffusion coefficent. |
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| What is a steady state condition? |
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| It is where the change is concentration divided by the change in time is equal to 0, meaning concentration of the system does not change with time. this does not imply chemical equilibrium it could be caused by and input-output balance. |
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| What is a non-steady state condition? |
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| where the change in concentration divided by the change in time is not equal to 0, meaning the concentration of the system varies over time. this is not a result of disequilbrium it can be a result of an inblance in input and output. |
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| what is residence time (tau)? |
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| the time a constituent of the system (water, solutes, particles) resides in that system and is equal to the total mass of constituent (kg) divided by rate of input or output(kg/time. water in minderwere has a tau of 1year whereas ocean water has a tau of 4000 years. |
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| How do we define; truely dissolved, colloidal and particulate? |
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| Truely dissolved species are <1nm, colloidal are 1nm to ~1 micro metres and particulaate are >1 micro metre. |
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| What makes colloids stable? |
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| They do not settle out under gravity in suspension meaning they are dispersed in surface waters along with other dissolved constituents. |
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| What is chemical speciation? |
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| Chemical speciation is where substances present in various forms have a different chemistry, for example Fe2+ in aqueous environment behaves differently to an Fe2-humic complex. this links into biological processes because Cu2+ can be uptaken by algae but it cannot be uptaken when in a humic complex. |
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| How do you separate subtances on a physical basis? |
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| Filtration, typically filter used size between 0.1 and 1 micro meter, thereofre filtrate will include collidal material. other factors that effect filitration are; (i) non-uniform pore sizes of filters, (ii) changing filter characteristics due to clogging, (iii) adsoption of dissolved and colloidal materail by filter apparatus and (iv) colloid aggregation. |
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| why does chemical speciation raise issues in analytical methods? |
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| Because analytical methods are not specific to speciation; meaning if a method detects Fe2+ it'll detect it in its aqueous form and its humic complex form. |
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| what analytical technique can be used to overcome chemical speciation problems? |
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| ion selective elctroeds: such as glass electrode to measure H+ with any number of water moleculs of hydration. |
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| What is the major group of dissolved inorganic solutes in natural waters and why are they such? |
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| ions due to, crustal abundances(abundance of certain elemesnt within the earths crust), the aqueous chemstries of these ions and their biological effects. |
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| what is the most important aspect of aqueous chemistry? |
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| the strength of the interaction between ions and water molcules, this depends on ratio of ion charge (z) to the ionic radius (r) which is known as the ionic potentila=z/r. |
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| What is the master variable that effects most aqueous chemical reactions? |
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| pH, H+ ion concentrations genreally <10^-mol/l |
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| How do we define chemical equilbrium? |
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| Chemical equilbrium is where a dynamic balance has occured: the rates of the opposing chemical reactions are equal so that in the absence of any external factors the concentration of each species reamins the same (this is not to say that the concentrations of each separate species is equal). |
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| Why can reactions give falase impression of being at equilibrium? |
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| by having slow reaction kinetics (weatering of aluminum-silicate rocks) |
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| How are equilbrium reactions quantified? |
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| by the equilibrium constant dervived from the law of mass action: K=[Y]^y[Z]^z/[A]^a[B]^b of the following reaction: aA+bB-->yY+zZ |
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| What is ideal and non-ideal behaviour. |
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| ideal behaviour of ions ignores electrostatic interactions that occur between the species(the error of this assumption increases as ionic strength increases) whereas non-ideal accounts for this interaction. |
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| what effects K values? |
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| Temperature and pressure. 10m of water=1 atmos of pressure, so deep oceans effect K a lot. |
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| why does [H2O] remain at ~55.5 moles dm^-3 in all aqueous sysmtems? |
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| due to water deionisation and the equilibriation of water. |
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| what is K for precipitation-dissolution reaction called and how is it expressed? |
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| it is the solubility product and expressed as: Ksp=[Ca2+][SO42-] |
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| how do we define supersaturated, saturated and undersaturated in relation to Ksp? |
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| supersaturated, [product]>Ksp. saturated, [product]=Ksp Undersaturated, [product] |
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| How do we define a bronsted acid and base? |
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| brosted acid is a proton donor, bronsted base is a proton acceptor. |
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| FACT |
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| protons are always present in their hydrated form and are indistinguishable in almost all respect: including their role in chemical reactions. |
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| What is amphoteric |
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| a molecule displaying both basic and acidic behaviour. |
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| what are polyprotic acids? |
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| acids that can donate more than one proton, examples being H2SO4 and H2CO3 |
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| how do we define weak and strong acids? |
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| using equilibrium constants of the proton transfer. strong acid K--->infiniy, weak acid K<10^-6.5 |
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| how is pH expressed? |
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| pH=-Log[H+] |
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| Many reactions are effected by pH, this is why pH is known as the master variable. describe how CO2 from the atmosphere or respiration sources effects pH. |
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| When CO2 comes into contact with water the following reaction occurs, Co2 (g)M-->CO2(aq) equilbriation is often not met. CO2(aq) then reacts further with water to form H2CO3(Carbonic acid). carbonic acid then undergoes two dssociation steps, H2CO3<-->H^+ + HCO3^- then HCO3^- <-->H^+ + CO3^2-. so this increases the level of H+ in the system and so decreases pH, increasing acidity. |
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| How does photosynthesis effect pH |
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| photosynthesis consumed CO2, if CO2 is consumed it cannot react with water and form carbonic acid which would dissociate into H+ and so the pH would increase due to photosynthesis. this tends to occur near surface waters. |
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| how does respiration effect pH |
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| respiration produces CO2 and so it would do the reverse of photosythesis and decreae pH. |
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| why is H2SO4, sulphuric acid, important? |
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| because it can be produced by weathering of sulphide minerals and would effect pH by lowering it. it can cause water to reach pH ~2 which is close to acid rain. |
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| what are humic subtances? |
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| the name given to a mixture of complex, acidic organic substances that form in all aquatic and terrstrial systems as a result of a degradation of organic matter. they have pKs in region of 3-3.5 so can produce acidic waters. |
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| what is a pH buffer? |
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| a chemical system that absorbs H+ or OH- without showing much change in pH. so it maintains pH by neutralising added acid or base. ideal buffer contains equal amounts of acid and base of a conjugate acid-base pair. |
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| The effect of the buffer is not infinite, what is this? |
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| as a strong acid or base is added some of its capacity is consumed. buffers are only effective as long as the ratio of acid to base remains within ~0.1-10. |
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| What does it mean when we say that most major ions are conserved in natural waters with respect to acidity? |
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| it means that their concentrations are not effected by pH which is due to their tendancy not to react with H+ or OH- |
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| what is acid neutralising capacity? |
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| the equivalent sum of all bases that are titrated with a strong acid to an equivalence point. often referred to as titration alaklinity. |
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| what is buffer intensity? |
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| -d[acid]/dpH. |
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| so in practise how is the pH of most natural waters fixed? |
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| by alkalnity ([HCO3^-]-+2[CO3^2-] and PCO2, with respect to K1 and K2. |
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| FACT |
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| alkalinity is positive at pH>5.4 and negative at pHs lower. |
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| how can we qualitatively describe alkalinity |
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| a measure of the ability of a solution to neutralize acids to equivalence point of carbonate or bicarbonate. |
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| What are most weathering reactions? |
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| acid-base reactions. |
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| What are the main volcanic volatiles? |
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| H20, CO2, CO, HCl, HF, N2, H2S, SO2 And H2 |
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| what is natural water pH linked to? |
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| dynamic balance between biogeochemical processes both acting globally and within aquatic system,s particulary through their effcots on CO2 partial pressure and alkalinity as well as to input-output balances of acids and bases. |
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| Where can typical weathering reactions occur? |
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| in terrestrial parts such as soils/soil waters, bedrock, groundwater, rivers and lakes. they affect alaklinity. |
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| what can the hydrolysis of silicates be broken down into? |
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| incongruent weathering reaction and congruent weather reaction. |
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| what is an incongruent weathering? |
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| reactions between orthoclase, protons and water to form a kaolinite and silicic acid. 2KALSi3O8+2H^+ + 9H2O-->Al2Si2O5(OH)4+ 2K^+ + 4H4SiO4. page 20 Lnotes. |
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| what is congruent weathering? |
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| where a forsterite reacts with protons to fomr silicic acid and Mg ions. Mg2SiO4 + 4H^+-->2Mg^2+ + H4SiO4 |
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| what do chemical weathering rates depend on and why? |
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| they depend on pH, ligand activities (humics) pCo2, ToC, particle size and mineralogy. because ion concentrations when controlled by mineral weathering are a function of reaction kinetics and ground water soil dynamics. |
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| what are the relative wearering rates? |
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| olivines>pyroxenes>Plagioclase feldpars>amphiboles and orthoclase>micas>quartz |
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| Why can equilbrium be achieve with regards to carbonate weathering? |
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| because it is faster than weathering of silicates. |
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| Carbonate reaction with a strong acid depends on pH what are the effects of this giving chemical equations? |
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| at low pH water, calcium ions and carbon dioxide form, at neutral pH carbonate and calcium ions form, low pH CaCO3+2H^+-->Ca^2+ +H2O + CO2, neutral pH: CaCO3+ H^+-->Ca^2+ +HCO3^- |
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| what is the chemical weathering reaction of carbonate with a weak acid such as H2CO3? |
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| CaCO3+H2CO3-->Ca^2+ + 2HCO3^-. forward reaction being limestone dissolution and backwards being carbonate formation. bicarbonate can be consumed through dissociation and then used in respiration/photosythensis thus photosynthesis decreases pCO2 meaning [H not only rises by precipitation of CaCo3 may be induced. opposite applies to repisration. |
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| What are surface ion exchange reactions? |
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| chemical weathering processes where ions bound to clay minerals or oxides "switch" places. can be cation exchange, =Mg^+ + H^+ --> =H^+ + Mg^2+, (= is clay mineral/oxide) or sulphate adsorption, =-(OH)2 + SO4^2- --> =-SO4 + 2OH^- |
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| What does the fact that exchange equilbrium achieved in <1 hr tell us about surface ion exchange reactions? |
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| that equilbrium usually exists. and that ion exchange capacities of catachments are highly variable and patchy. |
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| how do humic subtances form? what is their relation to chemical weathering? |
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| they form as the reslting of weathering or more correctly he microbial and chemical degradation of organic matter in terrestrial aquatic environments. they represent residula organic matter that is microbially and chemicall ressitant. they are acidic creating low pH conditions and form complexes with cations promoting the chemical weathering of minerals. |
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| what are metal oxides and how do they relate to chemical weathering? |
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| they survive weathering or are formed during weathering. they are important as they have a large exchange or adsorption capacities, often collodially dispersed. they include oxides of Si, Al, Fe(III) and Mn (IV). it is recognised that high Al concentrations in acidified catchment waters cause many of the ecological problems associated with acid waters. eg hamrmful effects to trout and salmon because Al^3+ is adsorbed to and through the gills. and has a link to toxicity mechanisms in plants and animals; can be linked to altzheimers. |
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| how is Al solubility related to pH |
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| at normal pH Al solubility is low, but as pH values decrease solubility increases. |
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| what is the camelford incident of 1988? |
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| occured near camelform cornwall, contamination fo drinking water with alum, Al2(SO4)3.14H2O, which is a strong electrolye that is used to aggregate colloidal suspensions during water treatment, most commonly mixtures of Hs and Fe oxides. key reactions are the complete dissociation of alum followed by: Al3+ +3H2O--> AL(OH)3+3H^+. the precipitation of Al(OH)3 entrains the colloids. in normal treatment the generated acid is either neutralized by natural alkalinity, in the case of hard waters or by added alkalinity, in the case of soft waters ie, H+ +HCO3--->H2CO3-->H2O+CO2. if alkalinity is inadequate the solution will become increasingly acid and hence highly corrosive. a variety of problems arose at camelform, associated with the acidity of metals dissolved by acidity, espeically Cu2+ and Pb2+ and high residiual [Altotal] stable at low pH |
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| what are ooxidation and redox reactions? |
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| chemical reactions that can be quantified in terms of equilibrium constants but involved electron transfer. they open up an area known as electrochemistry. in which example reactions can be studied directly by elecrical means eg measuring potentials or current. includes pt electrode to measure electric potential of a chemical system and selective techniqes such as ion selective electrodes and andoic stripping voltammetry. |
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| what can oxidation be thought as? |
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| an increase is positive valence; Fe2+ --> Fe3+ + e- or decrease is negative valence, 2Cl- -->Cl2 + 2e- oxidation can alternatively be described as a loss of electrons. reduction is the opposite |
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| what does a complete redox reaction involve? |
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| two complete half reactions, one being oxidation the other being reudction because free electrons do not exist in aqueous solution. |
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| what is an oxidising agent? |
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| a subtance that brings about oxidation, this subtances is itself chemically reduced. reducing agent is the reverse of this. oxidising agents are electron acceptors because it has induced the loss of electrons from another subtance. |
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| what is a standard potential of a half reaction or full redox reaction? |
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| defined as the electric potential at standard conditions-25 degrees, 1 atmos. realted to hydrogen couple which is arbitarily assigned a zero potential H2<-->2H^+ +2e^- E^o=0.00v |
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| what are potential more reducing than hydrogen couple assigned? |
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| a negative sign. |
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| what does a negative voltage indicate? |
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| that the reaction will proceed spontaneously from left to right, reverse for positive. 0 voltage means reaction is at equilibrium. |
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| how can tabulated standard potential of half reactions be used quantitatively? |
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| substracting one half reaction from another gives the desired reacants and products. subtracting the voltages also givens the overall voltage. |
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| what is the nernst equation? |
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| E=Eo+RT/nF ln K at 25 degrees E=Eo + 0.059/n log10 K |
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| what is a key redox reaction in nature? |
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| photosynthesis/respiration. |
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| what is the photosynthesis equation and its simplified version? |
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| 6CO2+6H2O-->6O2+C6H12O6 simplified: CO2+H2O-->CH2O+O2 |
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| what is the forward redox reaction of photosynthesis considered as? |
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| A redution of C from Iv to O and an oxidation of O from 2x (-II) to O. organic matter is a strong reducing agent and O2 is a strong oxidising agent. |
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| why does aerobic respiration predominate in presence of O2 |
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| dissolved O2 is the strongest of the commonly occurring oxidizing agents in natural waters, so that per unit mass consumption of organic matter the energy return is maximal. |
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| why do difficulties arise when studying sorption theory? |
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| because there are a number of different sorption processes that can be identified. |
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| What are the two main mechanisms of adsorption |
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| i) specific chemical binding that in principle can be described by an (intrinsic) equilibrium constant (ii) electrostatic attraction (and repulsion) |
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| what hydrophobic binding effects occur? what is the effect of ionic strength of EDL |
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| The particle (a clay perhaps, hydrophobic in nature such as halogenated hydrocarbons) is surrounded by a diffuse layer containing a balancing countercharge of ions(which moves with the particle). the particle has competitive binding sites, which may or may not be occupied by adsorbed ions thus contributing to surface charge. a zero net charge is maintained these two layers are refferd to has double layer theory. the diffuse layer known as the gouy layer, the fixed charge area (particulate) known as the Stern layer. Ions entering the diffuse layer will be subject to an electrical potential and hence attraction or repulsion, depending on sign of their charge. Diffuse layer moves with particle but its size, and hence sphere of influence, decreases with increasing ionic strength (I) where: I=1/2 SUM ci*zi, c=molar conc of each ion, z=charge |
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| why do particles genreally have electrically charged surfaces? |
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| due to the presence of surface functional groups that undergo chemical reactions with solution species this includes: a) proton dissociation (metal oxides) =MeOH2-->=MeOH + H^+ ---> =MeO^- + 2H^+ left occuring at lower pH and right occuring at higher pH(due to amphoteric functional groups which act as base or acid) |
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| What is the zero proton condition ZPC? |
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| it relates to electrically charged particulates containg functional groups which are amphioteric it is the pH where the concentration of the left side is equal to that of the right. the opposing negative and positive charged forms. [=MeOH2^+] equal to [=MeO^-] |
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| Why is there a tendancy for all particles to have an adsorbed HS coating? |
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| due to hydrophobic binding imparting a surface -ve charge. |
