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Acids and Bases

Acids and Bases. Chapter 20. Properties of…. Acids : Sour taste Change the color of indicators Electrolyte React with base to form water and salt React with certain metals to produce H 2 gas. Properties of…. Bases : Bitter taste, slippery feel Change the color of indicators

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Acids and Bases

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  1. Acids and Bases • Chapter 20

  2. Properties of…. Acids: • Sour taste • Change the color of indicators • Electrolyte • React with base to form water and salt • React with certain metals to produce H2 gas.

  3. Properties of…. Bases: • Bitter taste, slippery feel • Change the color of indicators • Electrolyte • React with acid to form water and salt

  4. Names and Formulas of Acids • Acids have the general formula HX, where X is a monatomic or polyatomic anion. • When the compound HCl(g) dissolves in water, to form HCl(aq), it is named as an acid, hydrochloric acid. • How are acids named?

  5. Three rules for naming acids:

  6. Name the following acids: • HNO3 (hydrogen nitrate) is called… • Nitric acid • H2SO3 (hydrogen sulfite) is called… • Sulfurous acid • HF (hydrogen fluoride) is called… • Hydrofluoric acid

  7. Naming bases • A base produces hydroxide ions when dissolved in water. • NaOH is called sodium hydroxide. • What would you call Ca(OH)2?

  8. The Self-Ionization of Water • When water molecules collide, a hydrogen ion is occasionally transferred from one molecule to another in the following reaction, the self-ionization of water: H2O + H2O  H3O+ + OH- H3O+ is named the hydronium ion.

  9. The Self-Ionization of Water • This reaction can also be written as… H2O  H+ + OH- (Basically, the two equations are the same, but the first is more realistic, because H+ always becomes H3O+ in water.)

  10. The Ion-Product Constant for Water (Kw) The product of [H3O+] and [OH] is always constant at 25oC. When the concentrations are multiplied, the product is 1.0x10-14. This is called the… Ion-product constant for water, Kw. [H3O+] x [OH] = 1.0x10-14 = Kw at 25oC

  11. Concentrations of [H3O+] and [OH] • In pure water, H2O + H2O  H3O+ + OH- the [H3O+] will always be equal to the [OH-].1.0 x 10-7

  12. In Water • In pure water, since [H3O+] = [OH-], • water is neither acidic or basic. • Water is neutral. • [H3O+] = [OH-] = 1.0x10-7M

  13. In Acids and Bases… • In acidicsolutions, the [H3O+]is greater than 1.0 x 10-7M [H3O+] > 1.0 x 10-7M • In basicsolutions, the [H3O+]is less than 1.0 x 10-7M [H3O+] < 1.0 x 10-7M

  14. Ion Concentrations

  15. Determine the [H3O+]… • What is the [H3O+] in a solution if the [OH-] = 6.30 x 10-5M? Is the solution acidic, alkaliine (basic) or neutral? • Kw = 1.0x10-14 =[H3O+] x [OH-] • 1.0x10-14 / [OH-] = [H3O+] • 1.0x10-14 / 6.30 x 10-5M = [H3O+] • Answer: [H3O+] = 1.59 x 10-10M, • acidic because [H3O+] < 1 x 10-7M

  16. Determine the [OH-]… • What is the [OH-] in a solution if the [H3O+] = 3.50 x 10-11M? • Kw = 1.0x10-14 =[H3O+] x [OH-] • 1.0x10-14 / [H3O+] = [OH-] • 1.0x10-14 / 3.50 x 10-11M = [OH-] • Answer: [OH-] = 2.86 x 10-4M

  17. Objectives • To understand pH and pOH • To learn to find pH and pOH for various solutions • To use a calculator to find pH • To learn methods for measuring pH of a solution • To learn to calculate the pH of strong acids

  18. The pH Scale • pH is the negative log of the hydrogen ion concentration. • pH = –log [H+] • The “p scale” is used to express small numbers (“p” means –log).

  19. Determine the pH… • What is the pH of a solution that is 1.0 x 10-3M[H+] ? • pH = –log [H+] = –log(1.0 x 10-3M) • Answer:pH= 3.00 • (really, no calculator required for this one. When the coefficient is 1.0, you can figure this without a calculator.)

  20. Determine the pH… • What is the pH of a solution that is 5.6 x 10-8M[H+] ? • pH = –log [H+] = –log(5.6 x 10-8M) • Answer: pH = 7.25 • (there is no unit, because this is a log)

  21. Significant figures for logarithms

  22. Determine the [H+]… • What is the [H+] if the pH of a solution is 9.00? • pH = –log [H+] • 9.00 = –log [H+] • – 9.00 = log [H+] (Move the negative sign) • 10–9 .00 = [H+] (Take the inverse log of both sides) • Answer: [H+] = 1.0 x 10-9M • (Again, no calculator is needed if the pH is a whole number.)

  23. Using a Calculator to Determine the [H+] from pH

  24. Determine the [H+]… • What is the [H+] if the pH of a solution is 2.75? • pH = –log [H+] • 2.75 = –log [H+] • – 2.75 = log [H+] (Move the negative sign) • 10–2.75 = [H+] (Take the inverse log of both sides) • Answer: [H+] = 1.8 x 10-3M

  25. The pH Scale • Because the pH scale is a log scale based on 10, the pH changes by 1 for every power of 10 change in the [H+].

  26. The pH Scale • Acidic solutions have a pH that is less than 7. • Neutral solutions have a pH of 7. • Alkaline (basic) solutions have a pH that is greater than 7.

  27. The pH Scale In acidicsolutions, [H3O+] > 1.0 x 10-7M, so the pH < 7 In neutral solutions, [H3O+] = 1.0 x 10-7M, so the pH = 7 In basicsolutions, [H3O+] < 1.0 x 10-7M, So the pH > 7 (The negative exponent makes this confusing.)

  28. The pOH Scale • pOH is just as valid as pH, but it is not as commonly used, except in Chemistry. • pOH = log [OH]

  29. Determining pH from pOH You can take the “p” (or –log) of each part of the Kw expression, and you get… Kw = 1.0x10-14 =[H3O+] x [OH-] pKw = 14.00=pH + pOH so, pH + pOH = 14.00 You can easily convert pOH to pH using pKw.

  30. Determine the pH… • What is the pH of a solution if the pOH is 11.60? • pH + pOH = 14.00 • 14.00 – 11.60 = pH • Answer: pH = 2.40

  31. Determine the pH… • What is the pH if the [OH-]= 5.74 x 10-4M? • pOH = -log [OH-] = -log (5.74 x 10-4M) • pOH = 3.241 • pH = 14 - 3.241 • Answer: pH = 10.759 • (Remember, you need 3 sig figs, which are places to the right of the decimal for logs.)

  32. Acid Base Indicators • Indicators – weak acids or bases that exhibit different colors in acidic and basic solutions • In an acid solution the indicator will be in the HIn form. • In a basic solution the indicator will be in the In form.

  33. Acid Base Indicators • One common indicator is litmus paper. • Red litmus paper turns blue in base. • Blue litmus paper turns red in acid.

  34. Acid Base Indicators • Another common indicator is bromothymol blue. • Bromothymol blue is blue in base and yellow in acid.

  35. Acid Base Indicators • Another common indicator is phenolphthalein. • Phenolphthalein is bright pink in base, and clear in acid.

  36. Objectives • To learn about two models of acids and bases • To understand the relationship of conjugate acid-base pairs • To understand the concept of acid strength • To understand the relationship between acid strength and the strength of the conjugate base • To learn about the ionization of water

  37. The Arrhenius Model • Svante Arrhenius defined acids and bases: • Arrhenius Acid – produces hydrogen ions in aqueous solution • Example: HCl  H+ + Cl- • Arrhenius Base – produces hydroxide ions in aqueous solution • Example: KOH  K+ + OH-

  38. Monoprotic acids have one ionizable hydrogen (proton). (Example: HCl) • Diprotic acids have 2 ionizable hydrogens. (Example: H2SO4) • Triprotic acids have 3 ionizable hydrogens. (Example: H3PO4) • (Diprotic and triprotic acids are called polyprotic acids.)

  39. Is HC2H3O2 (acetic acid, found in vinegar) a monoprotic, diprotic, or triprotic acid?

  40. The only hydrogen that is ionizable is the one attached to the oxygen (so it is monoprotic). This H is ionizable because it is in a very polar bond, so this hydrogen is slightly positive and attracted to water.

  41. The Arrhenius model does not explain why ammonia (NH3) is a base. • A new model was created by Brønsted and Lowry. This model explains why NH3 and other molecules are bases, even though they contain no hydroxide.

  42. The Brønsted-Lowry Model • Brønsted Acid – Hydrogen ion donor • Brønsted Base – Hydrogen ion acceptor

  43. Brønsted Equations • In the reaction, • NH3 + H2O  NH4++ OH- • The NH3 is acceptingH+ to form NH4+, so the NH3 is abase. • The H2O is donating H+ to form OH-, so the water is anacid.

  44. NH3 + H2O  NH4++ OH- • Base Acid • Now, in the reverse reaction, the NH4+ acts as an acid and the OH- acts as a base. We call these the conjugate acid and conjugate base.

  45. Conjugate Acid-Base Pairs • NH3 + H2O  NH4++ OH- • Base Acid Conjugate Conjugate acid base • The base always forms the conjugate acid and the acid always forms the conjugate base. • NH3 and NH4+ are a conjugate acid-base pair. They differ from each other by one H+ ion.

  46. Identify the base, acid, the conjugate acid and the conjugate base: HNO3 + H2O  NO3- + H30+ • Acid Base CB CA • Here, water is acting as a base.

  47. Identify the conjugate acid-base pairs: • HCl + H2O  H30+ + Cl-

  48. Water: Is it an acid or a base? • Water is amphiprotic. It can behave as either an acid or a base, by donating or accepting H+. • (Some books use the term amphoteric to describe the same property.)

  49. B. Acid Strength • Strong acid – completely ionized or completely dissociated

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