Chem 1310 5

Information about Chem 1310 5

Published on January 2, 2008

Author: Noormahl

Source: authorstream.com

Content

Chem 1310: Introduction to physical chemistry Part 5: Buffers and solubility:  Chem 1310: Introduction to physical chemistry Part 5: Buffers and solubility Peter H.M. Budzelaar Buffers:  Buffers Water has a very small [H3O+] (10-7). Adding just a little bit of acid or base can change the pH drastically. Add 0.001 M HCl: pH goes from 7 to 3! For many applications this sensitivity is undesirable. One of the best ways to prevent pH swings is buffering: the use of a mixture of a weak acid and its conjugate base (which will be a weak base). Buffers:  Buffers Two important aspects: What will be the resulting pH? What will be the buffer capacity (how much acid/base can be absorbed before the pH starts to change drastically)? The pH of a buffer solution:  The pH of a buffer solution Take a mixture of HOAc and NaOAc (both 0.1 M): HOAc + H2O ⇋ OAc- + H3O+ The pH of a buffer solution (2):  The pH of a buffer solution (2) We usually assume x « buffer concentration, so (Always check afterwards! If not valid, solve the full quadratic equation) The pH of a buffer solution (3):  The pH of a buffer solution (3) General formula (using the same x assumption): where we replace the actual [HA], [A-] by the amounts weighed in (Henderson-Hasselbalch). Buffer capacity:  Buffer capacity How much added acid or base can a buffer absorb? At most until the buffer acid or its conjugate base is consumed. If you have 1L of a buffer containing 0.2 M HOAc and 0.35 M NaOAc, this can absorb up to 0.35 moles of acid (all NaOAc consumed) or 0.2 moles of base (all HOAc consumed). As long as you do not exceed the buffer capacity, you can calculate the new pH using Henderson-Hasselbalch. Buffer capacity (2):  Buffer capacity (2) 0.03 moles of HCl is added to 1L of a buffer of 0.1 M each of HOAc and NaOAc. What is the resulting pH? New [HOAc] = 0.1+0.03 = 0.13, new [NaOAc] = 0.1-0.03=0.07: Titration:  Titration Slowly add acid of known concentration from a burette to a solution of base (or vv). Use an indicator to detect moment of fast pH change (happens at equivalence point). Strong acid, base largest pH change, almost any indicator will work. Weak acid titrated with strong base: Solution will originally not be very acidic, but will go till very basic. Use indicator for pH > 7, e.g. phenolphtalein. Titration:  Titration Weak base titrated with strong acid: Solution will originally not be very basic, but will go till very acidic. Use indicator for pH < 7, e.g. methyl red. Do not titrate a weak acid with a weak base! No clear equivalence point. Solubility in water:  Solubility in water Just another equilibrium (see MSJ p839 for Ksp table): AgCl(s) ⇋ Ag+(aq) + Cl-(aq) KC = Ksp = [Ag+][Cl-] The standard rules for writing equilibrium constants apply: Mg3(PO4)2 (s) ⇋ 3 Mg2+(aq) + 2 PO43-(aq) Ksp = [Mg2+]3[PO43-]2 No AgCl, because that is a pure solid. Calculating the solubility of a compound in pure water:  Calculating the solubility of a compound in pure water Add excess AgCl to water; it starts to dissolve: [Ag+][Cl-] = x2 = Ksp = 1.8·10-10 x » 1.3·10-5 mol/L Calculating the solubility of a compound in pure water (2):  Calculating the solubility of a compound in pure water (2) Add excess PbCl2 to water; it starts to dissolve: [Pb2+][Cl-]2 = 4x3 = Ksp = 1.7·10-5 x » 0.015 mol/L Calculating solubility in the presence of "common ions":  Calculating solubility in the presence of "common ions" Dissolve AgCl in a solution of 0.1 M NaCl: [Ag+][Cl-] = x(0.1+x) = Ksp = 1.8·10-10 Calculating solubility in the presence of "common ions" (2):  Calculating solubility in the presence of "common ions" (2) Assume x « 0.1: x(0.1+x) » 0.1x = Ksp = 1.8·10-10 Þ x = 1.8·10-9 mol/L (verify: x « 0.1!) A lot less soluble than in pure water! Without assumption: solve the quadratic equation. This is often not a good idea! Calculating whether a salt will precipitate:  Calculating whether a salt will precipitate Calculate Qsp = [...][...] (same formula as for Ksp) Qsp < Ksp: more could dissolve Qsp = Ksp: saturated solution Qsp > Ksp: super-saturated: salt will precipitate (c.f. Q and K for other equilibria) Solubility calculations are not always straightforward...:  Solubility calculations are not always straightforward... The real solution equilibrium might be more complicated: PbCl2(s) ⇋ PbCl+ + Cl- ⇋ Pb2+ + 2 Cl- The original Ksp expression is still valid, but we cannot assume all Pb in solution is present as Pb2+. There will also be some PbCl+, so the amount of Pb that goes into solution will be higher than expected. Solubility calculations are not always straightforward...:  Solubility calculations are not always straightforward... Added reagents may complex with the solutes and reduce their concentrations, setting up new equilibria: AgCl(s) ⇋ Ag+(aq) + Cl-(aq) Ag+(aq) + 2 CN-(aq) ⇋ Ag(CN)2- (aq) AgCl(s) + 2 CN-(aq) ⇋ Ag(CN)2- (aq) + Cl-(aq) (Hess's law) Solubility and complexation:  Solubility and complexation We have 1L of a solution 0.1 M in NaCN. Will it dissolve 0.01 moles of AgCl? Assuming complete conversion to Ag(CN)2-: [Cl-] = 0.01, [Ag(CN)2-] = 0.01, [CN-] = 0.008: a) b) Either way: it will easily dissolve! Solubility and acid-base reactions:  Solubility and acid-base reactions CaCO3(s) ⇋ Ca2+(aq) + CO32-(aq) CO32- + H2O ⇋ HCO3- + OH- Part of CO32- removed via reaction with water Þ more will dissolved than you would calculate from Ksp. With added acid: CO32- + H3O+ ® HCO3- + H2O HCO3- + H3O+ ® H2CO3 + H2O H2CO3 ® CO2 + H2O Solubility and acid-base reactions:  Solubility and acid-base reactions Eventually, all CaCO3 dissolves in acid! This happens with many poorly soluble salts of weak acids (S2-, CO32-, F-), except when Ksp is really very small (PbS, HgS, ...).

Related presentations


Other presentations created by Noormahl

CADBURY
24. 02. 2008
0 views

CADBURY

Team Work
06. 07. 2007
0 views

Team Work

Mischel and Bandura
24. 02. 2008
0 views

Mischel and Bandura

Celebrities Through The Years
01. 08. 2007
0 views

Celebrities Through The Years

India - Be a Proud Indian
14. 08. 2010
0 views

India - Be a Proud Indian

Secular Bull Market
18. 04. 2008
0 views

Secular Bull Market

super-gatos
15. 04. 2008
0 views

super-gatos

guest lecture
13. 04. 2008
0 views

guest lecture

Strategists ToolboxV1 2003
10. 04. 2008
0 views

Strategists ToolboxV1 2003

ch17
09. 04. 2008
0 views

ch17

Song list
07. 04. 2008
0 views

Song list

WS2 5 Resource paper
30. 03. 2008
0 views

WS2 5 Resource paper

11 Routing
07. 10. 2007
0 views

11 Routing

ming
12. 10. 2007
0 views

ming

Retail Supplier Link
07. 11. 2007
0 views

Retail Supplier Link

ch18
13. 11. 2007
0 views

ch18

Cause and Effect Outline
16. 11. 2007
0 views

Cause and Effect Outline

DrawDiagrams
19. 11. 2007
0 views

DrawDiagrams

019 Traditional Medicine
17. 12. 2007
0 views

019 Traditional Medicine

enzymes 2
30. 12. 2007
0 views

enzymes 2

Weir
07. 01. 2008
0 views

Weir

celticcoolness
06. 11. 2007
0 views

celticcoolness

fourthlecture
11. 12. 2007
0 views

fourthlecture

Ch02Rev
04. 01. 2008
0 views

Ch02Rev

TDT6
21. 11. 2007
0 views

TDT6

cassini
15. 11. 2007
0 views

cassini

3 Pam Howard ICON Clinic al
18. 03. 2008
0 views

3 Pam Howard ICON Clinic al

The Copernican Revolution
21. 03. 2008
0 views

The Copernican Revolution

10aKansasv Hendricks
01. 01. 2008
0 views

10aKansasv Hendricks

levi Wrestling
21. 06. 2007
0 views

levi Wrestling

Food as MedBend
04. 03. 2008
0 views

Food as MedBend

Icc3
28. 12. 2007
0 views

Icc3

nwwr long
04. 01. 2008
0 views

nwwr long

HighDBPs
07. 11. 2007
0 views

HighDBPs

Class FaceBook Template R1
14. 03. 2008
0 views

Class FaceBook Template R1

wetlands ins
03. 01. 2008
0 views

wetlands ins

Sedimentation
09. 11. 2007
0 views

Sedimentation

546 wk3
30. 11. 2007
0 views

546 wk3

Marolf
28. 11. 2007
0 views

Marolf

GLD EH Assy061003
10. 12. 2007
0 views

GLD EH Assy061003

London R3 Panel Presentation
10. 03. 2008
0 views

London R3 Panel Presentation

qcrypto
07. 01. 2008
0 views

qcrypto

2005CON Building FLL Robots
02. 01. 2008
0 views

2005CON Building FLL Robots

Summers UROP
28. 12. 2007
0 views

Summers UROP

UR Pantelic
29. 11. 2007
0 views

UR Pantelic

KOS NAL Finch
05. 12. 2007
0 views

KOS NAL Finch

Sandorfi
16. 11. 2007
0 views

Sandorfi

speakers kit slide templates
31. 12. 2007
0 views

speakers kit slide templates

JUG Danijel
04. 10. 2007
0 views

JUG Danijel

McKenna
29. 12. 2007
0 views

McKenna

03 GP science apps
15. 11. 2007
0 views

03 GP science apps

2007 SBIR Overview
02. 11. 2007
0 views

2007 SBIR Overview