Mec chapter 8

1,327 views

Published on

Published in: Business, Technology
0 Comments
2 Likes
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
1,327
On SlideShare
0
From Embeds
0
Number of Embeds
1
Actions
Shares
0
Downloads
22
Comments
0
Likes
2
Embeds 0
No embeds

No notes for slide

Mec chapter 8

  1. 1. Copyright© The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 8 Acids and Bases and Oxidation-ReductionDennistonToppingCaret7th Edition
  2. 2. 8.1 Acids and Bases• Acids: Taste sour, dissolve some metals, cause plant dye to change color• Bases: Taste bitter, are slippery, are corrosive• Two theories that help us to understand the chemistry of acids and bases 1. Arrhenius Theory 2. Brønsted-Lowry Theory
  3. 3. Arrhenius Theory of Acids and Bases8.1 Acids and Bases • Acid - a substance, when dissolved in water, dissociates to produce hydrogen ions – Hydrogen ion: H+ also called “protons” HCl is an acid: HCl(aq) → H+(aq) + Cl-(aq)
  4. 4. Arrhenius Theory of Acids and Bases8.1 Acids and Bases • Base - a substance, when dissolved in water, dissociates to produce hydroxide ions NaOH is a base NaOH(aq) → Na+(aq) + OH-(aq)
  5. 5. Arrhenius Theory of Acids and Bases8.1 Acids and Bases • Where does NH3 fit? • When it dissolves in water it has basic properties but it does not have OH- ions in it • The next acid-base theory gives us a broader view of acids and bases
  6. 6. Brønsted-Lowry Theory of Acids and Bases8.1 Acids and Bases • Acid - proton donor • Base - proton acceptor – Notice that acid and base are not defined using water – When writing the reactions, both accepting and donation are evident
  7. 7. Brønsted-Lowry Theory of Acids and Bases8.1 Acids and Bases HCl(aq) + H2O(l) → Cl-(aq) + H3O+(aq) acid base What donated the proton? HCl Is it an acid or base? Acid What accepted the proton? H2O Is it an acid or base? Base
  8. 8. .Brønsted-Lowry Theory of Acids and Bases8.1 Acids and Bases NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) base acid Now, let us look at NH3 and see why it is a base. Did NH3 donate or accept a proton? Accept Is it an acid or base? Base What is water in this reaction? Acid
  9. 9. 8.1 Acids and Bases Acid-Base Properties of Water • Water possesses both acid and base properties – Amphiprotic - a substance possessing both acid and base properties – Water is the most commonly used solvent for both acids and bases – Solute-solvent interactions between water and both acids and bases promote solubility and dissociation
  10. 10. 8.1 Acids and Bases Acid and Base Strength • Acid and base strength – degree of dissociation – Not a measure of concentration – Strong acids and bases – reaction with water is virtually 100% (Strong electrolytes)
  11. 11. 8.1 Acids and Bases Strong Acids and Bases • Strong Acids: – HCl, HBr, HI Hydrochloric Acid, etc. – HNO3 Nitric Acid – H2SO4 Sulfuric Acid – HClO4 Perchloric Acid • Strong Bases: – NaOH, KOH, Ba(OH)2 – All metal hydroxides
  12. 12. 8.1 Acids and Bases Weak Acids • Weak acids and bases – only a small percent dissociates (Weak electrolytes) • Weak acid examples: – Acetic acid: CH3COOH(aq) + H2O(l) CH3COO-(aq) + H3O+(aq) – Carbonic Acid: H2CO3(aq) + H2O(l) HCO3-(aq) + H3O+(aq)
  13. 13. 8.1 Acids and Bases Weak Bases • Weak base examples: – Ammonia: NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) – Pyridine: C5H5NH2(aq) + H2O(l) C5H5NH3+(aq) + OH-(aq) – Aniline: C6H5NH2(aq) + H2O(l) C6H5NH3+(aq) + OH-(aq)
  14. 14. 8.1 Acids and Bases Conjugate Acids and Bases • The acid base reaction can be written in the general form: HA + B A– + HB+ acid base • Notice the reversible arrows • The products are also an acid and base called the conjugate acid and base
  15. 15. 8.1 Acids and Bases HA + B A- + HB+ acid base base acid • Conjugate acid - what the base becomes after it accepts a proton • Conjugate base - what the acid becomes after it donates its proton • Conjugate acid-base pair - the acid and base on the opposite sides of the equation
  16. 16. 8.1 Acids and Bases Acid-Base Dissociation HA + B A– + HB+ • The reversible arrow isn’t always written – Some acids or bases essentially dissociate 100% – One way arrow is used • HCl + H2O → Cl- + H3O+ – All of the HCl is converted to Cl- – HCl is called a strong acid – an acid that dissociates 100% • Weak acid - one which does not dissociate 100%
  17. 17. 8.1 Acids and Bases Conjugate Acid-Base Pairs • Which acid is stronger: HF or HCN? HF • Which base is stronger: CN- or H2O? CN -
  18. 18. 8.1 Acids and Bases Acid-Base Practice • Write the chemical reaction for the following acids or bases in water • Identify the conjugate acid-base pairs 1. HF (a weak acid) 2. H2S (a weak acid) 3. HNO3 (a strong acid) 4. CH3NH2 (a weak base) Note: The degree of dissociation also defines weak and strong bases
  19. 19. 8.1 Acids and Bases The Dissociation of Water • Pure water is virtually 100% molecular • Very small number of molecules dissociate – Dissociation of acids and bases is often called ionization H2O(l) + H2O(l) H3O+(aq) + OH-(aq) • Called autoionization • Very weak electrolyte
  20. 20. 8.1 Acids and Bases Hydronium Ion • H3O+ is called the hydronium ion • In pure water at room temperature: – [H3O+] = 1 x 10-7 M – [OH-] = 1 x 10-7 M • What is the equilibrium expression for: H2O(l) + H2O(l) H3O+(aq) + OH-(aq) + K eq = [H 3O ][OH ] - Remember, liquids are not included in equilibrium expressions
  21. 21. 8.1 Acids and Bases Ion Product of Water • This constant is called the ion product for water and has the symbol Kw + K w = [H 3O ][OH ] - • Since [H3O+] = [OH-] = 1.0 x 10-7 M, what is the value for Kw? – 1.0 x 10-14 – It is unitless
  22. 22. 8.2 pH: A Measurement Scale for Acids and Bases• pH scale - a scale that indicates the acidity or basicity of a solution – Ranges from 0 (very acidic) to 14 (very basic)• The pH scale is rather similar to the temperature scale assigning relative values of hot and cold• The pH of a solution is defined as: pH = -log[H3O+]
  23. 23. A Definition of pHScale for Acids and Bases 8.2 pH: A Measurement • Use these observations to develop a concept of pH – if know one concentration, can calculate the other – if add an acid, [H3O+] ↑ and [OH-] ↓ – if add a base, [OH-] ↑ and [H3O+] ↓ – [H3O+] = [OH-] when equal amounts of acid and base are present • In each of these cases 1 x 10-14 = [H3O+][OH-]
  24. 24. Measuring pHScale for Acids and Bases 8.2 pH: A Measurement • pH of a solution can be: – Calculated if the concentration of either is known • [H3O+] • [OH-] – Approximated using indicator / pH paper that develops a color related to the solution pH – Measured using a pH meter whose sensor measures an electrical property of the solution that is proportional to pH
  25. 25. Calculating pHScale for Acids and Bases 8.2 pH: A Measurement • How do we calculate the pH of a solution when either the hydronium or hydroxide ion concentration is known? • How do we calculate the hydronium or hydroxide ion concentration when the pH is known? • Use two facts: pH = -log[H3O+] 1 x 10-14 = [H3O+][OH-]
  26. 26. Calculating pH fromScale for Acids and Bases 8.2 pH: A Measurement Acid Molarity What is the pH of a 1.0 x 10-4 M HCl solution? – HCl is a strong acid and dissociates in water – If 1 mol HCl is placed in 1 L of aqueous solution it produces 1 mol [H3O+] – 1.0 x 10-4 M HCl solution has [H3O+]=1.0x10-4M pH = -log[H3O+] = -log [H3O+] = -log [1.0 x 10-4] = -[-4.00] = 4.00
  27. 27. Calculating [H3O+] From pHScale for Acids and Bases 8.2 pH: A Measurement What is the [H3O+] of a solution with pH = 6.00? pH = -log[H3O+] • 4.00 = -log [H3O+] • Multiply both sides of equation by –1 • -4.00 = log [H3O+] • Take the antilog of both sides • Antilog -4.00 = [H3O+] • Antilog is the exponent of 10 • 1.0 x 10-4 M = [H3O+]
  28. 28. Calculating the pH of a BaseScale for Acids and Bases 8.2 pH: A Measurement What is the pH of a 1.0 x 10-3 M KOH solution? • KOH is a strong base (as are any metal hydroxides) • 1 mol KOH dissolved and dissociated in aqueous solution produces 1 mol OH- • 1.0 x 10-3 M KOH solution has [OH-] = 1.0 x 10-3 M 1 x 10-14 = [H3O+][OH-] • Solve equation for [H3O+] = 1 x 10-14 / [OH-] • [H3O+] = 1 x 10-14 / 1.0 x 10-3 = 1 x 10-11 • pH = -log [1 x 10-11] pH = -log[H3O+] = 11.00
  29. 29. Calculating pH from AcidScale for Acids and Bases 8.2 pH: A Measurement Molarity What is the pH of a 2.5 x 10-4 M HNO3 solution? • We know that as a strong acid HNO3 dissociates to produce 2.5 x 10-4 M [H3O+] pH = -log[H3O+] • pH = -log [2.5 x 10-4] • = 3.60
  30. 30. Calculating [OH-] From pHScale for Acids and Bases 8.2 pH: A Measurement What is the [OH-] of a solution with pH = 4.95? • First find [H3O+] pH = -log[H O+] 3 • 4.95 = -log [H3O+] • [H3O+] = 10-4.95 • [H3O+] = 1.12 x 10-5 1 x 10-14 = [H3O+][OH-] • Now solve for [OH-] • [OH-] = 1 x 10-14 / 1.12 x 10-5 = 1.0 x 10-9
  31. 31. 8.2 pH: A MeasurementScale for Acids and Bases The pH Scale
  32. 32. For a strong acid For a strong baseScale for Acids and Bases HCl molarity pH 8.2 pH: A Measurement NaOH molarity pH 1.0 x 100 0.00 1.0 x 100 14.00 1.0 x 10-1 1.00 1.0 x 10-1 13.00 More basic 1.0 x 10-2 2.00 1.0 x 10-2 12.00 1.0 x 10-3 3.00 1.0 x 10-3 11.00 1.0 x 10-4 4.00 1.0 x 10-4 10.00 1.0 x 10-5 5.00 1.0 x 10-5 9.00 1.0 x 10-6 6.00 1.0 x 10-6 8.00 ci d c Aer o M 1.0 x 10-7 7.00 1.0 x 10-7 7.00 Each 10 fold change in concentration i changes the pH by one unit
  33. 33. The Importance of pH andScale for Acids and Bases 8.2 pH: A Measurement pH Control Any change that takes place in aqueous solution generally has at least some pH dependence – Agriculture - crops grow best in soil with proper pH – Physiology - blood pH shift of 1 pH is fatal – Acid Rain - lowers pH of water in aquatic systems causing problems for native fishes – Municipal services - sewage treatment and water purification require optimal pH – Industry - many processes require strict pH control for cost-effective production
  34. 34. 8.3 Reactions Between Acids and Bases• Neutralization reaction - the reaction of an acid with a base to produce a salt and water HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) Acid Base Salt Water• Break apart into ions: H+ + Cl- + Na+ + OH- →Na+ + Cl- + H2O• Net ionic equation – Show only the changed components – Omit any ions appearing the same on both sides of equation = Spectator ions H+ + OH- → H2O
  35. 35. Net Ionic Neutralization Reaction8.3 Reactions Between • The net ionic neutralization reaction is more Acids and Bases accurately written: H3O+(aq) + OH-(aq) → 2H2O(l) • This equation applies to any strong acid / strong base neutralization reaction • An analytical technique to determine the concentration of an acid or base is titration • Titration involves the addition of measured amount of a standard solution to neutralize the second, unknown solution • Standard solution - solution of known concentration
  36. 36. 8.3 Reactions Between Acid – Base Titration Buret – long glass Standard solution tube calibrated in mL is slowly added Acids and Bases which contains the until the color standard solution changes The equivalence Indicator – a point is when the substance which moles of H3O+ changes color as pH changes and OH- are equal Flask contains a solution of unknown concentration plus indicator
  37. 37. 8.4 Acid-Base Buffers• Buffer solution - solution which resists large changes in pH when either acids or bases are added• These solutions are frequently prepared in laboratories to maintain optimum conditions for chemical reactions• Buffers are also used routinely in commercial products to maintain optimum conditions for product behavior
  38. 38. 8.4 Acid-Base Buffers The Buffer Process • Buffers act to establish an equilibrium between a conjugate acid – base pair • Buffers consist of either – a weak acid and its salt (conjugate base) – a weak base and its salt (conjugate acid) CH3COOH(aq) + H2O(l) CH3COO-(aq) + H3O+(aq) – Acetic acid (CH3COOH) with sodium acetate (CH3COONa) • An equilibrium is established in solution between the acid and the salt anion • A buffer is Le Chatelier’s principle in action
  39. 39. Addition of Base (OH-) to a Buffer Solution8.4 Acid-Base Buffers • Adding a basic substance to a buffer causes changes – The OH- will react with the H3O+ producing water – Acid in the buffer system dissociates to replace the H3O+ consumed by the added base – Net result is to maintain the pH close to the initial level • The loss of H3O+ (the stress) is compensated by the dissociation of the acid to produce more H3O+ CH3COOH(aq) + H2O(l) CH3COO-(aq) + H3O+(aq)
  40. 40. Addition of Acid (H3O+) to a Buffer Solution8.4 Acid-Base Buffers • Adding an acidic substance to a buffer causes changes – The H3O+ from the acid will increase the overall H3O+ – Conjugate base in the buffer system reacts with the H3O+ to form more acid – Net result is to maintain the H3O+ concentration and the pH close to the initial level • The gain of H3O+ (the stress) is compensated by the reaction of the conjugate base to produce more acid CH3COOH(aq) + H2O(l) CH3COO-(aq) + H3O+(aq)
  41. 41. 8.4 Acid-Base Buffers Buffer Capacity • Buffer capacity - a measure of the ability of a solution to resist large changes in pH when a strong acid or strong base is added • Also described as the amount of strong acid or strong base that a buffer can neutralize without significantly changing pH

×