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Acids and Bases

Operational definitions are based on observed properties. Compounds can be Classified as acid or base by observing these sets of properties.

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Properties of Acids Taste sour (acere – Latin for sour) (Lemons, vinegar)

Cause certain organic dyes to change colour (Turns blue litmus paper to red –

BAR)

Acid properties are destroyed by Bases (React with bases to form a salt and

water)

Acid solutions are Electrolytes (substance in solution that conduct an electric

current – Acids can be strong or weak electrolytes)

Acids react (corrode) with active metals (Group I and II as well as Zn and

Aluminum) (Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g))

Acids react with carbonates (CO32-) and hydrogen carbonates (HCO3

1-) to

produce carbon dioxide gas {2HCl(aq) + Na2CO3(s) → 2NaCl(aq) + H2O(l) + CO2(g)}

Certain nonmetal oxides will dissolve to produce acid solutions. (SO3(g) + H2O →

H2SO4(aq) (SO3(g) is the acid anhydride – without water)

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Properties of Bases Bases taste bitter; mustard and soapBases taste bitter; mustard and soap

Bases cause weak organic acids (dyes) to change colour (red Bases cause weak organic acids (dyes) to change colour (red

litmus paper to blue {BB} Basic Bluelitmus paper to blue {BB} Basic Blue

Acids destroy base properties - react with acids to form salts Acids destroy base properties - react with acids to form salts

and water and water

Bases are electrolytes {strong or weak}Bases are electrolytes {strong or weak}

Feel soapy, slipperyFeel soapy, slippery

Bases are formed when the oxide of some metals dissolve in Bases are formed when the oxide of some metals dissolve in

water (CaO(s) + Hwater (CaO(s) + H22O O → Ca(OH)→ Ca(OH)22(aq)(aq) {CaO is the base anhydride} {CaO is the base anhydride}

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Acid/Base definitions• Definition #1: Arrhenius (traditional)

– Acids are compounds with ionizable hydrogen– produce H+ ions (or hydronium ions H3O+) in solution

– Bases are compounds that produce OH- ions in solution (problem: some bases don’t have hydroxide ions!)

The reaction between an acid and a base:H+(aq) + OH-(aq) → H2O (l)

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Arrhenius acid is a substance that produces H+ (H3O+) in water.The HCl molecule is ionized. (ionization)

Arrhenius base is a substance that produces OH- in water. The ions are dissociated. (dissociation)

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Some acids have more than one ionizable hydrogen

• H2SO4 → H+(aq) + HSO41-(aq

• HSO41- → H+(aq) + SO4

2-(aq) H2SO4 is diprotic

• H3PO4(aq) → H+(aq) + H2PO41-(aq)

• H2PO41-(aq) → H+(aq) + HPO4

2-(aq)

• HPO42-(aq) → H+(aq) + PO4

3-(aq)

Phosphoric acid is a triprotic acid.

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Water self-ionization

H2O ↔ H+(aq) + OH-(aq) [H+] = [OH-] = 10-7M at SATP

Keq = [H+][ OH-] [H2O(l)]Kw = [H+][ OH-] = 10-7 x 10-7 (at 25ºC)Kw = 10-14 at SATP

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H2O ↔ H+(aq) + OH-(aq) What happens to this equilibrium if HCl(g) dissolves in the water? HCl(g) + H2O(l) → H3O+(aq) + Cl-(aq)

• Increasing Decreasing• H2O ↔ H3O+(aq) + OH-(aq)• [H+] > [OH-] = acidic

• What happens when sodium hydroxide dissolves? NaOH(s) + H2O → Na+(aq) + OH-(aq)

• Decreasing Increasing• H2O ↔ H3O+(aq) + OH-(aq)• [H+] < [OH-] = basic (alkaline solution)

If [HIf [H++] = 10] = 10-7-7 then [OH then [OH--] = 10] = 10-7-7 solution is neutral (SATP) solution is neutral (SATP)

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pH and logs• [H+] is important in the study of acid-base

chemistry. pH is the widely used scale to show [H+].

• pH = -log[H+] or pH = 1 . log[H+][H+] = 10 – pH (the antilog)A logarithm is the power to which ten must

be raised to get a number.log1000 = log(103) = 3

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pH calculations• For a neutral solution • pH = -log[H+]• pH = -log [10-7]• pH = - [-7]• pH = 7 at SATP

• Example:• [H+] = 5 x 10-3

• pH = -log [5 x 10-3]• pH = -log [0.005]• pH = - (-2.3) = 2.3

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pH and pOH• pOH = - log [OH-] or [OH-] = 10 - pOH

• Kw = [H+] x [OH-] = 1 x 10-14 (at 25ºC)• pKw= pH + pOH• 14= pH + pOH

• Example:• If pH = (2.3) what is the [OH-]?• pH + pOH = 14• pOH = 14 – pH• pOH = 14 – 2.3• pOH = 11.7• pOH = -log [OH-]• [OH-] = inverse log -11.7 or (10 - 11.7)• [OH-] = 2.0 x 10-12

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[H[H33OO++], [OH], [OH--] and pH] and pH• What is the pH of the 0.0010 M NaOH What is the pH of the 0.0010 M NaOH

solution? solution? • [OH[OH--] = 0.0010 (or 1.0 X 10] = 0.0010 (or 1.0 X 10-3-3 M) M)• pOH = - log 0.0010pOH = - log 0.0010• pOH = 3pOH = 3• pH + pOH = 14pH + pOH = 14• pH = 14 – 3 = 11pH = 14 – 3 = 11• OR Kw = [HOR Kw = [H33OO++] [OH] [OH--]]• 1.0 x101.0 x10-14-14 = [H = [H3OO++] x 1.0 X 10] x 1.0 X 10-3-3 • [H[H3OO++] = 1.0 x 10] = 1.0 x 10-11-11 M M• pH = - log (1.0 x 10pH = - log (1.0 x 10-11-11) = 11.00) = 11.00

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Problem 1: The pH of rainwater collected in a certain region of the northeastern New Brunswick on a particular day was 4.82. What is the H+ ion concentration of the rainwater?

Problem 2: The OH- ion concentration of a blood sample is 2.5 x 10-7M. What is the pH of the blood?

Problem 3: A chemist dilutes concentrated hydrochloric acid to make two solutions: (a) 3.0 M and (b) 0.0024 M. Calculate the [H3O+], pH, [OH-], and pOH of the two solutions at 25°C.

[H+] = 1.51 x 10-5

pOH = 6.6 pH = 7.4

Problem 4: What is the [H3O+], [OH-], and pOH of a solution with pH = 3.67? Is this an acid, base, or neutral?

Problem 5: Problem #4 with pH = 8.05?

a) [H3O+] = [3.0], pH = - 0.48, pOH = 14.48, [OH-] = 3.3 x 10-15

b) [H3O+] = [2.4x10-3], pH = 2.62, pOH = 11.38, [OH-] = 4.2 x 10-12

[H3O+] = 2.14 x10-4, pOH = 10.33, [OH-] = 4.68x 10-11 It is an acid.

[H3O+] = 8.92 x10-9, pOH = 5.95, [OH-] = 1.12x 10-6 It is an acid.

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Acid/Base Definitions

• Definition #2: Brønsted – Lowry

– Acids – proton donor A “proton” is a hydrogen ion (the atom lost it’s electron)

– Bases – proton acceptor (accepts a hydrogen ion) No longer needs to contain the OH- ion

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A Brønsted-Lowry acid is a proton donorA Brønsted-Lowry base is a proton acceptor

acid conjugate base

base conjugate acid

BaseAcidAcidBaseNH4

+ + OH-NH3 + H2O

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The Bronsted-Lowry conceptThe Bronsted-Lowry concept

• Acids and bases are identified based on whether they donate or accept H+.

• “Conjugate” acids and bases are found on the products side of the equation. A conjugate base is the same as the starting acid minus H+.

+Cl HH

HO

+H

HH O Cl+

acid base conjugate acid conjugate base

conjugate acid-base pairs

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Practice problemsPractice problemsIdentify the acid, base, conjugate acid, conjugate base, and conjugate acid-base pairs:

acid base conjugate acidconjugate baseCH3OOH(aq) + H2O(l) CH3COO–(aq) + H3O+(aq)

conjugate acid-base pairs

acidbase conjugate acidconjugate baseOH

–(aq) + HCO3–(aq) CO3

2–(aq) + H2O(l)

conjugate acid-base pairs

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Base Conjugate acid \ \NH3(g) + H2O(l) ↔ NH4

+(aq) + OH-(aq) / / Acid Conjugate Base

HCl(aq) + H2O(l) ↔ H3O+(aq) + Cl-(aq)Acid Base Conjugate Conjugate

Acid Base

The water has acted as both an acid and a base, depending on what it is mixed with. Substances that can act as both an acid and a base are amphoteric (also called amphiproteric).

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Strong acid and base :HA(aq) + H2O(l) ↔ H3O+(aq) + A-(aq)B(aq) + H2O(l) ↔ BH+(aq) + OH-(aq)At equilibrium the ionic form is favored

Weak acid and base :HA(aq) + H2O(l) ↔ H3O+(aq) + A-(aq)

B(aq) + H2O(l) ↔ BH+(aq) + OH-(aq)

At equilibrium the molecular form is favored

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CH3COOH(aq) + H2O(l) ↔ H+(aq) + CH3COO-(aq)

Keq= [H+] [CH3COO-] . [CH3COOH] [H2O(l)][H2O] is a constant, so collect the constants(Keq)[H2O(l)] = [H+] [CH3COO-]

[CH3COOH] (Keq)[H2O] is represented Ka(ionization constant for an acid) Ka = [H+] [CH3COO-] = 1.8 x 10-5

[CH3COOH]Ka < 1 weak acidGeneral Formula for the ionization constant of a weak acid.

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