1 Hydrogenbonds - - PDF document

▶
1
SMART_READER_LITE
LIVE PREVIEW

1 Hydrogenbonds - - PDF document

Nov 15, 2023 598 likes •715 views

Water:theamazingsubstance AndAqueousGeochemistry Thewatermolecule Thewatermoleculeisasymmetric.Thisresultsinpolarity(apositivelycharged

slide-1
SLIDE 1

1

Water:theamazingsubstance AndAqueousGeochemistry

Thewatermolecule

Thewatermoleculeisasymmetric.Thisresultsinpolarity(apositivelycharged endandanegativelychargedend)and,hence,ahighdielectricconstant.

Thedielectricconstant

  • Thedielectricconstantofasolventisarelative

measureofitspolarity.

slide-2
SLIDE 2

2 Hydrogenbonds

Hydrogenbondsoccurwhenthepositiveendofthemolecule(hydrogenside)is attractedtothenegativeendofadjacentmolecules(oxygenside).Thisattraction isresponsibleformanyoftheimportantpropertiesofwater.

Propertiesofwater

  • OnlysubstanceonEarthtoexistinall

threestatesofmatteratthesametime

  • Highboilingpoint
  • Excellentsolvent
  • Highsurfacetension
  • Highviscosity
  • Highspecificheatcapacity

…therearemanymore

Density…watericefloats

Inliquidwatereachmoleculeishydrogenbondedtoapproximately3.4other watermolecules. Iniceeachmoleculeishydrogenbondedto4othermolecules.

slide-3
SLIDE 3

3

TheT,Pphasediagramforwater

Thecriticalpointisthetemperatureandpressurebeyondwhichthereisno physicaldistinctionbetweentheliquidandthegasphase.AtsupercriticalTandP waterisconsideredasupercriticalfluid.

SolubilityProduct

Mostsimplesalts(simpleionicsaltsofacation andanion)canbeexpressedas asolubilityproduct(theequilibriumconstant). NaCl(s)=Na+(aq)+Cl6(aq)Ksp=[Na+][Cl6] logK=1.5855 CaSO4(s)=Ca+2 +SO4

62

Ksp=[Ca2+][SO462] logK=64.3064 Themagnitudeofthesolubilityproductcangiveyouanideaastohowsoluble amineralis

Henry’sLaw

  • Thesolubilityofagasisproportionaltoitspartial

pressure CO2(g)=CO2(aq) K=CO2(aq)/CO2(g)so… CO2(aq)=K*CO2(g) …KisoftenwrittenasKH

slide-4
SLIDE 4

4

CO2(g)=CO2(aq) logKH =61.4689 O2(g)=O2(aq) logKH =62.8983 Thedataabovewascalculatedat25Cand1atmospheretotalpressure. Whichgasismoresolubleinwaterattheseconditions? Whydidn’tIdothecalculationusinganequilibriumconstantat25Candthe partialpressureofthegasphase?

  • 0.00000001

0.0000001 0.000001 0.00001 0.0001 0.001 0.01 0.1 1 0.2 0.4 0.6 0.8 1

  • CO2(aq)

O2(aq)

Units

  • Sincewearenowdiscussingthesolubilityofaqueousspecies,wemust

introducetheunitsthatareusedinsolubilitycalculations Thethreemostcommonlyusedunitsare… Partspermillion(ppm) =mg(solute)/Kg(solution)…mg/kg Molality (m) =mols(solute)/kg(water)…mol/kg Molarity (M)=mols(solute)/Liter(water)…mol/L Wheneverwearedoingacalculationusinganequilibriumconstantweshould usemolality. Whydowewanttousemassunitsandnotvolumeunits?

slide-5
SLIDE 5

5

ChargeBalance:

AllEarthmaterialstendtobeelectricallyneutral.Wesawthisinmineralformulas andbalancedchemicalreactions. Thesameholdstrueforaqueoussolutions… IntheNa2O6H2Osystem,chargebalancewouldbewrittenas mNa+ +mH+ =mOH6 IntheCaO6H2Osystem,chargebalancewouldbewrittenas 2*mCa2+ +mH+ =mOH6 WhydowemultiplymCa2+ by2? Multiplyingtheconcentration(mol/kg)ofanionbyitschargeresultsinequivalents(eq/kg)

pH

  • pH=6log[H+]

Asamatteroffactp(anything)means… –log(anything) ex.pe…pK…pOH… ThewaterreactioncanbewrittenasH2O=H+ +OH6 Iftheactivityofwateris1thentheequilibriumconstantforthisreactionis essentiallyasolubilityproduct IfwecalculatedeltaGr at25Cand1atmospherethevalueofKis10614 Or…thepK=14…soK=[H][OH]…whenH=OHwehaveneutralityand [H]=(K)0.5 orpH=(6logK)/2… neutralpH=7 Whatifthetemperatureandpressureisnot25Cand1atmosphere?

pHofnaturalwaters

  • ThepHrangeofnaturalwatersnearoron

theEarth’ssurfacerangesfromabout668.

  • Why?
  • Theansweristhecarbonatesystem
slide-6
SLIDE 6

6

WhenCO2(g)dissolvesinwater, fourspeciesareproduced

H2CO3 (carbonicacid) CO2(aq)(dissolvedCO2)

(takentogetherwithH2CO3 wegetH2CO3

*)

HCO3

6 (bicarbonate)

CO3

26 (carbonate)

pH

2 4 6 8 10 12 14

logi

68 67 66 65 64 63 62 6.35 10.33 H2CO3* HCO3

6

CO3

26

H+ OH6

CommonpH rangeinnature

Bjerrum plotforcarbondioxide

CarbonateAlkalinity

Alkalinityisthecapacityofawatertoaccept protons.Itisthesumofallbasesinthewater minustheprotons…inmostwaters,the carbonatesystemdominatesthealkalinity. Alk =mHCO36 +2mCO366 +mOH6 6 mH+ Alkalinityisalsoequaltothechargeimbalanceby subtractingtheconservativeanions(Cl,Br…) fromtheconservativecations (Na,Mg,K…)

slide-7
SLIDE 7

7

mNa+mK+mH+2mCa=mCl+mHCO3+2mCO3+mOH Ifwerearrangewecanget… (mNa+mK+2mCa)6mCl=mHCO3+2mCO3+mOH6mH

pHofraininequilibriumwithCO2(g) CO2+H2O=H2CO3(KCO2) H2CO3=HCO3

6+H+(K1)

HCO36=CO3

26 +H+(K2)

Chargebalanceinwaterforthissystemis… mH+ =mHCO3

6 +2*mCO3 26 +mOH6

  • WaterinequilibriumwithatmosphericCO2(g)

pressure(1063.5)at25CfixespHat~6(5.66)

  • Waterinequilibriumwithcalciteand

atmosphericCO2(g)pressureat25CfixespHat ~8(8.26)

  • HencethenaturalrangeofwaterpH

(additionalsolutesinwateranddifferentTandP causesthesepHvaluestovaryalittlebit)… ThepHofseawateris~7.568.5

slide-8
SLIDE 8

8 Hydrolysis

  • Reactionofamineralwithwater.

Anorthite+4H2O=Ca++ +2Al+++ +2SiO2(aq)+8OH6 H2O=H+ +OH6 Anorthite+8H+ =4H2O+Ca++ +2Al+++ +2SiO2(aq)

∆Gr =615.44kJ/mol

slide-9
SLIDE 9

9

Congruentvs Incongruent Dissolution

  • Congruent6allcomponentsofthemineral

dissolve(Ksp)

Gypsum=Ca++ +SO466 +2H2O

  • Incongruent6thereisaninsolublesecondary

phase

(aluminumissoinsolublethatasecondaryaluminosilicate phaseformsduring hydrolysisofmostsilicateminerals) Anorthite+H2O+2H+=Ca+++Kaolinite 2Orthoclase+H2O+2H+=2K++Kaolinite+4SiO2(aq)

SaturationState

  • Ifamineralisinequilibriumwithwater,itis

saidtobesaturated.

  • Amineralwillprecipitatefromwaterwhen

themineralissupersaturatedanddissolve inwaterwhenundersaturated.

  • Thestateofsaturationisdeterminedfrom

thesaturationindex

SaturationIndex

  • ThesaturationindexisdefinedaslogQ/K

KistheequilibriumconstantforthereactionandQ isthereactionquotient. ThereactionquotientissimilartoK,buttheactual measuredchemistryisusedtocalculateavalue forQ. WhenQ=Kthemineralisinequilibriumand, therefore,saturated

slide-10
SLIDE 10

10 logQ/K

  • IfQ=KthenQ/K=1andlogQ/K=0

saturated

  • IfQ>KthenQ/K>1andlogQ/K=(+)

supersaturated

  • IfQ<KthenQ/K<1andlogQ/K=(6)

undersaturated

Example

TheaverageconcentrationofNa+andCl6 intheoceansis 0.479mol/kgand0.558mol/kgrespectively… Ksp forHalite…Ksp=[Na+][Cl6]wherelogKsp=1.593at25C and1atmosphere(K=10^1.593=39.174) Q=[Na+][Cl6]=[0.479][0.558]=0.267 logQ/K=62.166 Q<KsoHaliteisundersaturated inseawaterat25Cand1 atmosphere

Example#2

TheaverageconcentrationofCa++andSO466 intheoceansis0.011 mol/kgand0.029mol/kgrespectively… Ksp forGypsum…Ksp=[Ca++][SO466]where logKsp =64.443at25Cand1atmosphere(K=10^64.443=3.6E65) Q=[Ca++][SO466]=[0.011][0.029]=3.19E64 logQ/K=0.947 Q>KsoGypsumissupersaturatedinseawaterat25Cand1 atmosphere(DidIdothiswrong?)

slide-11
SLIDE 11

11

Staf f ThuNov082007

10 12 14 16 18 20 22 24 –3 –2 –1 1 2 3

Temperature(C) Saturation,someminerals(logQ/K)

Anhydrite Aragonite Calcite Dolomite Gypsum Halite