Environmental Chemistry C Solutions

7/29/2019 Environmental Chemistry C Solutions

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EnvironmentalChemistryCQuestionsScienceOlympiad2009GeneralKnowledgeQuestions

1. Whyisnaturalrainslightlyacidic?When CO2 (g) in the earths atmosphere dissolves in water, it produces carbonic

acid (H2CO3).

e.g., CO2 (g) + H2O H2CO3

2. Consideringthatplantroots,soilmicrobes,andotherorganismsareinthesoilreleaseCO2(g)aspartoftherespirationprocess,andthatthereisgasexchangebetweentheearthsatmosphere

andsoilair,wouldyouexpecttheconcentrationofCO2(g)insoilairtobehigher,lower,orequal

thanthatoftheearthsatmosphere?

higher

3. Explainyourreasoning.High biological activity in soil consumes O2 (g) and produces a lot of CO2(g). Soil

pores are small and tortuous, so it is difficult for the excess CO2 in the soil pores

to diffuse to the surface and be released to the earths atmosphere. The situation

is analogous to having a lot of people smashed in a very small room with one small

window.

TheUSparkservicehasaskedtorecommendoneoftwo lakesforrecreationaldevelopment thatwill

include swimming boating and fishing. Both lakes and their surroundings are beautiful, sojust by

lookingat themeither lakewouldbesuitable.Readinggeochemistryreportsyou learnthatBlueLake

formed over a limestone formation so that the water in the lake is at equilibrium with calcium

carbonate. Silk Lake, on the other hand, formed over natron deposits so that the lakewater is in

equilibriumwithsodiumcarbonate.

Mineral Solubilityinwater pHofwater inequilibrium

with mineral at 25C and

PCO2=0.00038atm

CaCO3 6.2mgL1 8.3

Na2CO3 106mgL1 10.1


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4. Whichlakewaterwouldhavethehighestconcentrationoftotaldissolvedcarbonateandbicarbonate? Silk Lake

5. Whichlakewouldyourecommendforrecreationaldevelopment?Blue Lake

6. Explainyourreasoning:The elevated pH of the water in Silk Lake that is due to the higher solubility of

Na2CO3 over CaCO3, would be uncomfortable for people to swim in (irritating to

eyes), would be unsuitable for most fish, and would be slightly corrosive to the

metal in boat engines.

7. GeminiLakeformedoverdepositsrich inbothcalciumandsodiumcarbonatesandasaresultthe lakewatersare in equilibriumwithbothminerals.Would you expect thepHof the lake

watertobeclosertothatofBlueLakeorSilkLake?

Silk Lake8. Explainyourreasoning.

This is an open system with infinite inputs of CO2(g) from the atmosphere, CaCO3

(s) and Na2CO3 (s) from the underlying geologic formations. The pH of the water

will be controlled by equilibrium reactions between the solubility of CO2(g) from the

atmosphere and the carbonates in the water arising from calcite and natron

dissolution. Since Na2CO3 is much more soluble than CaCO3, its properties will

dominate. The input of the common CO32- ion from Na2CO3 will actually limit CaCO3

solubility so that there is much less total dissolved Ca2+ (in all species) than in Blue

Lake.

Youhavebeenhiredasaconsultantataformerindustrialsitetoadvisethenewlandownersaboutthe

bestwaytostimulatethedegradationoftwocontaminantsfoundonthesite.Soilanalysesindicatethat

theconcentrationof thecontaminants isdecreasingslowly.Youare trying todeterminewhether the

mechanismofdegradation ispredominantlybiotic(duetomicrobialactivity)orabiotic (strictlydueto

chemicalorphysicalprocesses).You collectseveral samplesof soil from thesite, incubate themata

series temperatures for1weekwhile carefullymaintainingaconstantwater contentof thesoil,and

thenanalyzetheconcentrationofcontaminantineachsoilsample.Youobtainthefollowingresults.

Concentrationofcontaminantinsoil(gcontaminant/kgsoil)

contaminant 4C 10C 20C 30C 40C 50C

A 500 320 160 80 40 20

B 275 200 100 20 125 225


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9. Basedonyourresults,doyou think thedegradationmechanismofcontaminantA isbioticorabiotic? __abiotic__

10.Basedonyour results,doyou think thedegradationmechanismofcontaminantB isbioticorabiotic? __biotic____

11.ExplainyourreasoningforbothcontaminantsThe rate of purely chemical reactions tends to double with every 10 increase in

temperature as is observed for contaminant A. The same pattern is not observed

for contaminant B. Rather, the rate is optimal at a particular temperature, which is

more typical of a microbially mediated reaction.

Afarmerisconcernedaboutthenutrientstatusofthesoilinhistomatofields.Hesendsyousamplesof

thesoilwhichyouanalyzefortotalavailablenitrogen(N),phosphorous(P),andpotassium(K).BasedonthenutritionalrequirementsoftomatoesandtheavailableNPKinthesoil,howmuchNPKwillheneed

toaddprovideadequatenutritionforthecrop?

N

Kgperhectare

P

Kgperhectare

K

Kgperhectare

Nutritionalneedsoftomatoes 130 50 260

Soilavailablenutrients 25 100 28

12.nitrogen __105_kg ha-1__13.phosphorous _____0_____14.potassium __232 kg ha-1__

Yourecommendthatthefarmeradda15:5:35NPKfertilizerbecauseitisreadilyavailable. Recallthat

thefertilizerlabelisreferringto0.15kgNperkgfertilizer,0.05kgP2O5perkgfertilizer,and0.35kgK2O

perkgfertilizer.

N O P K

Atomicweights(gmole1) 14.01 16.00 30.97 39.10

Usingtherecommendedfertilizer,howmanykgofeachelementarein1kgfertilizer?


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15. Nitrogen ___0.15_____ e.g., calculation for P16. Phosphorous___0.02__ (0.05 kg P2O5 /kg fert.)(61.94 kg P/141.94 kg P2O5) = 0.0217.Potassium__0.29_____18.Assumingthattheappliedfertilizerwillbecompletelyavailabletothecrop,whatisthe

minimumamountoffertilizerthatthefarmershouldapplyperhectaretoadequatelysupplythe

mostdeficientnutrienttothetomatoes?

He should apply 800 kg fertilizer ha-1

Calculation: (232 kg K required ha-1) / (0.29 kg K kg-1 fertilizer) = 800 kg fertilizer

required to supply K (the most deficient nutrient)

Onamolarbasis(molesperhectaresoil)howmuchofeachelementdothetomatoesrequire?

19.Nitrogen __9285.7 mol ha-1__20.Phosphorous __1614.5 mol ha-1_____21.Potassium ___6649.6 mol ha-1__

TomeasurethepHofthesoilbroughtinbythefarmer,youblend50gsoilwith50mLdeionizedwater

andletthemixtureequilibrateat25C.OnehourlateryouinsertapHelectrodeanddeterminethatthe

pHofthesoilsolutionis8.5.

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22.BasedonthepHofyoursoilsolutionandthechartbelowthatdisplayselementavailabilityasafunctionofsoilsolutionpH,whichother(besidesNPK)nutrientsmightyouexpectthetomatoes

tobecomedeficientinastheygrow?

Iron, manganese, boron, copper, and zinc


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Plants canonlytakeupnitrogenintheformofammonium(NH4+)ornitrate(NO3

).Mostsoilmineral

surfacesarenegativelycharged.

23.Whichformofnitrogenwouldbemostlikelysorbtosoilmineralsurfacesbyelectrostaticattraction?

ammonium

24.Whichformofnitrogenwouldbethemostlikelytoleachthroughthesoilprofileandthreatengroundwateraquifers?

nitrate

25.Phosphorousisgenerallypresentinsoilintheformofphosphate(PO43),hydrogenphosphate(HPO4

2),ordihydrogenphosphate(H2PO4). Notethatthethreespecieshaveanegativecharge.

Recallingthatmostsoilmineralsurfacesarenegativelycharged,intermsofbondinghowwould

youexplaintheobservationthatphosphorousisnotmobileinsoils.Inotherwordswhatkindof

bondingisinvolvedinthestrongadsorptionofphosphatetosoilmineralsurfaces?

Covelant bonding

26.Basedonwhatyouknowaboutphosphorous,wouldyoupredictthatarsenicwouldbehighlymobileorimmobileinsoils?

Assuming oxidizing conditions in the soil so that As would be in the form of

arsentate (same oxidation state as phosphate) I predict that arsenic would be

highly immobile in soils.

27.Explainyourreasoning.Arsenic is in the same group as phosphorous on the periodic table and thus can be

expected to display similar chemical properties and also bond covelantly to soil

mineral surfaces.

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Environmental Chemistry C Solutions