AP Chemistry Unit 3: Presentation D Acids and Bases www.njctl.org - - PDF document

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Unit 3: Presentation D Acids and Bases

AP Chemistry Slide 2 / 33 Acid/Base Reactions

CaCO3(s) + 2H+(aq) --> H2O(l) + CO2(g) + Ca2+(aq) Caves are formed by the acid/base reaction of limestone with acid which causes the limestone rock to dissolve away.

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Acid and Base Definitions

The most widely used definition of acids and bases in the Bronsted- Lowry definition. ACID BASE H+ Donor H+ acceptor NH4+(aq) + H2O(l) --> NH3(g) + H3O+(aq) Acid: H+ Donor Base: H+ Acceptor

Slide 4 / 33 Conjugate Acids and Bases

After an acid donates an H+ ion, the resulting species can always behave as a base and grab the H+ back again. NH4+(aq) + H2O <--> NH3(g) + H3O+(aq) Acid: H+ Donor Base: H+ Acceptor Therefore, NH3 acts as the conjugate base of NH4+ and H3O+ acts as the conjugate acid of H2O.

NH4+(aq) + H2O(l) <--> NH3(g) + H3O+(aq)

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Predicting the formula of a conjugate acid or base involves writing the formula

• f the species

produced after the donation or acceptance of an H+ ion.

CN-(aq) --> HCN(aq) HCO3-(aq) --> CO32-(aq) CH3NH2(aq) --> CH3NH3+(aq) CH3COOH(aq) --> CH3COO-(aq)

base base

• Conj. acid
• Conj. acid

acid acid

• Conj. base
• Conj. base

Conjugate Acids and Bases Slide 6 / 33

Amphoteric Molecules

Amphoteric molecules can behave as either acids or bases depending on their environment. Water is a classic example H2O donates H+ accepts H+ OH- H3O+ Whether water acts as an acid or base will depend on the relative "strength" of other acids or bases it is reacted with.

• Conj. acid
• Conj. base

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Common amphoteric molecules are the conjugate bases of diprotic or polyprotic acids

H2C2O4(aq) HC2O4-(aq) C2O42-(aq) Acid Amphoteric Base H3PO4(aq) H2PO4-(aq) HPO42-(aq) PO43-(aq) Acid Amphoteric Amphoteric Base

Amphoteric Molecules Slide 8 / 33

1 Which of the following is NOT an acid/base reaction? A HCN(aq) + H2O(l) --> CN-(aq) + H3O+(aq) B NH3(aq) + HC2H3O2(aq) --> NH4+(aq) + C2H3O2-(aq) C H+(aq) + H2O(l) --> H3O+(aq) D Pb2+(aq) + 2I-(aq) --> PbI2(s) E All are acid/base reactions

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1 Which of the following is NOT an acid/base reaction? A HCN(aq) + H2O(l) --> CN-(aq) + H3O+(aq) B NH3(aq) + HC2H3O2(aq) --> NH4+(aq) + C2H3O2-(aq) C H+(aq) + H2O(l) --> H3O+(aq) D Pb2+(aq) + 2I-(aq) --> PbI2(s) E All are acid/base reactions

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Answer D

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2 In which of the following reactions does the underlined substance behave as an acid? A I only B II only C III only D II and III only E I, II, and III

• I. CH3NH2(g) + H2O(l) --> CH3NH3+(aq) + OH-(aq)
• II. HCOOH(aq)

+ NH2-(aq) --> HCOO-(aq) + NH3(aq)

• III. HClO(aq) + OH-(aq) --> ClO-(aq) + H2O(l)

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2 In which of the following reactions does the underlined substance behave as an acid? A I only B II only C III only D II and III only E I, II, and III

• I. CH3NH2(g) + H2O(l) --> CH3NH3+(aq) + OH-(aq)
• II. HCOOH(aq)

+ NH2-(aq) --> HCOO-(aq) + NH3(aq)

• III. HClO(aq) + OH-(aq) --> ClO-(aq) + H2O(l)

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Answer D

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3 What would be the expected products of the following reaction... A H3PO4(aq) + H2O(l) B H2PO42-(aq) + H2O(l) C HPO4-(aq) + H3O+(aq) D HPO42-(aq) + H2O(l) E None of these

H2PO4-(aq) + OH-(aq) --> ____________

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3 What would be the expected products of the following reaction... A H3PO4(aq) + H2O(l) B H2PO42-(aq) + H2O(l) C HPO4-(aq) + H3O+(aq) D HPO42-(aq) + H2O(l) E None of these

H2PO4-(aq) + OH-(aq) --> ____________

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Answer D

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4 Which of the following is the conjugate acid of the chlorite ion (ClO2-)? A HClO2 B HClO2+ C HClO D HClO3 E HClO2-

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4 Which of the following is the conjugate acid of the chlorite ion (ClO2-)? A HClO2 B HClO2+ C HClO D HClO3 E HClO2-

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Answer A

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5 Which of the following has the acid paired with it's appropriate conjugate base? A I only B II only C III only D I and III only E I and II only

• I. HPO42-(aq)/PO42-(aq)
• II. HBrO2(aq)/BrO-(aq)
• III. H2CrO4(aq)/HCrO4-(aq)

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5 Which of the following has the acid paired with it's appropriate conjugate base? A I only B II only C III only D I and III only E I and II only

• I. HPO42-(aq)/PO42-(aq)
• II. HBrO2(aq)/BrO-(aq)
• III. H2CrO4(aq)/HCrO4-(aq)

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Answer C

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6 Which of the following would be amphoteric? A CO32-(aq) B HC2H3O2(aq) C HC2O4-(aq) D F-(aq) E H3O+(aq)

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6 Which of the following would be amphoteric? A CO32-(aq) B HC2H3O2(aq) C HC2O4-(aq) D F-(aq) E H3O+(aq)

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Answer C

Slide 14 (Answer) / 33 Strong Acids

Strong acids dissociate 100% in aqueous solutions thereby donating ALL of their H+ ions. Therefore, much like a soluble ionic compound, any strong acid must be written as being broken up into it's ions. Strong Acid Written as... HCl(aq) H+(aq) + Cl-(aq) HNO3(aq) H+(aq) + NO3-(aq) H2SO4(aq) 2H+(aq) + SO42-(aq) HClO4(aq) H+(aq) + ClO4-(aq) HBr(aq) H+(aq) + Br-(aq) HI(aq) H+(aq) + I-(aq)

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Conjugate bases of strong acids are not able to accept H+ ions back and therefore do not participate in acid/base reactions. H+(aq) + NO3-(aq) + NH3(aq) --> NH4+(aq) H+(aq) + NH3(aq) --> NH4+(aq) H+(aq) + Br-(aq) + ClO-(aq) --> HClO(aq) H+(aq) + ClO-(aq) --> HClO(aq)

Strong Acids Slide 16 / 33 Writing Acid/Base Reactions

First write the correct formulas for all species involved, breaking compounds into ions if soluble ionics or strong acids. Example: Aqueous solutions of methlyamine (CH3NH2) and hydroiodic acid are mixed. CH3NH2(aq) + H+(aq) + I-(aq) --> Predict products based on acid/base principles. CH3NH2(aq) + H+(aq) + I-(aq) --> CH3NH3+(aq) CH3NH2(aq) + H+(aq) --> CH3NH3+(aq)

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7 Which of the following species would be an extremely weak base? A Br- B F- C NO2- D ClO3- E NH3

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7 Which of the following species would be an extremely weak base? A Br- B F- C NO2- D ClO3- E NH3

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Answer A

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8 Why are strong acids written as separate ions? A Strong acids are dangerous B Strong acids have strong conjugate bases C Strong acids do not have conjugate bases D Strong acids donate all of their H+ ions E None of these

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8 Why are strong acids written as separate ions? A Strong acids are dangerous B Strong acids have strong conjugate bases C Strong acids do not have conjugate bases D Strong acids donate all of their H+ ions E None of these

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Answer D

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9 Which of the following is NOT a strong acid? A HNO3 B HF C HBr D HClO4 E H2SO4

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9 Which of the following is NOT a strong acid? A HNO3 B HF C HBr D HClO4 E H2SO4

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Answer B

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10 Which of the following best represents the balanced equation for the acid/base reaction of aqueous potassium hydroxide and chromic acid (H2CrO4)? A OH- + H2CrO4 → H2O + CrO42- B 2K+ + H2CrO4 → 2H+ + K2CrO4

C 2OH- + H2CrO4 → 2H2O + CrO42-

D 2K+ + H2CrO4 → 2KH + CrO42- E None of these

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10 Which of the following best represents the balanced equation for the acid/base reaction of aqueous potassium hydroxide and chromic acid (H2CrO4)? A OH- + H2CrO4 → H2O + CrO42- B 2K+ + H2CrO4 → 2H+ + K2CrO4

C 2OH- + H2CrO4 → 2H2O + CrO42-

D 2K+ + H2CrO4 → 2KH + CrO42- E None of these

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Answer C

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11 Which of the following best represents the balanced equation for the acid/base reaction if solid strontium hydroxide is added to a solution of acetic acid (HC2H3O2)? A Sr(OH)2 + 2HC2H3O2 → 2C2H3O2- + Sr2+ + 2H2O B 2OH- + 2HC2H3O2 → 2C2H3O2- + 2H2O

C Sr(OH)2 + 2C2H3O2- → Sr(C2H3O2)2 + 2OH-

D Sr(OH)2 + HC2H3O2 → C2H3O2- + Sr2+ + H2O E None of these

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11 Which of the following best represents the balanced equation for the acid/base reaction if solid strontium hydroxide is added to a solution of acetic acid (HC2H3O2)? A Sr(OH)2 + 2HC2H3O2 → 2C2H3O2- + Sr2+ + 2H2O B 2OH- + 2HC2H3O2 → 2C2H3O2- + 2H2O

C Sr(OH)2 + 2C2H3O2- → Sr(C2H3O2)2 + 2OH-

D Sr(OH)2 + HC2H3O2 → C2H3O2- + Sr2+ + H2O E None of these

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Answer A

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12 Which of the following best represents the balanced equation for the acid/base reaction when solutions of sodium hypobromite (NaBrO) and sulfurous acid (H2SO3) are mixed? A 2Na+ + SO32- → Na2SO3 B 2OBr- + H2SO3 → 2HBr + SO42-

C 2OBr- + H2SO3 → 2OH- + Br2SO3

D 2OBr- + H2SO3 → 2HOBr + SO32- E None of these

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12 Which of the following best represents the balanced equation for the acid/base reaction when solutions of sodium hypobromite (NaBrO) and sulfurous acid (H2SO3) are mixed? A 2Na+ + SO32- → Na2SO3 B 2OBr- + H2SO3 → 2HBr + SO42-

C 2OBr- + H2SO3 → 2OH- + Br2SO3

D 2OBr- + H2SO3 → 2HOBr + SO32- E None of these

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Answer D

Slide 23 (Answer) / 33 Acid/Base Stoichiometry

Normal stoichiometrical calculations are routinely carried out with acid/ base reactions however there are some specific applications that will be addressed here. Application Purpose Titration Determine (M) of acid or base by reacting it with a known quantity of acid or base Molar Mass Determination Determine molar mass (g/mol) of acid or base by reacting it with a known quantity of acid or base.

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Titrations

In a titration, a known(standardized) solution of acid or base is carefully added to the unknown concentration of acid or base until the reaction is complete. standardized acid or base solution of known M (titrant) unknown concentration of acid or base solution (analyte) By measuring the moles of titrant added, the moles of the analyte can be determined and hence the concentration (M).

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Example: What is the concentration of a 10 mL acetic acid solution if 34.1 mL of 0.2 M KOH was needed to titrate it? Write Reaction OH-(aq) + HC2H3O2(aq) --> C2H3O2-(aq) + H2O(l) Determine moles of titrant reacted and moles of analyte present. 0.0341 L x 0.2 n OH- x 1 mol HC2H3O2 = 0.00682 mol 1 L 1 mol OH- Find Molarity 0.00682 mol/0.010 L = 0.682 M

Titrations Slide 26 / 33 Using Titrations to Find Molar Mass

The identity of an acid can be discerned if the molar mass of the acid is known. The molar mass is determined by titrating a known mass of an acid dissolved in water with a standardized base. Molar Mass = grams/mol

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Using Titrations to Find Molar Mass

Example: If 3.1 grams of an acid are dissolved up with water to make a 25 mL solution it was determined that 45.9 mL of a 2.5 M NaOH standardized solution is needed to titrate the acid. What is the molar mass of the acid? HA(aq) + OH-(aq) --> H2O(aq) + A-(aq)

unknown acid

0.0459 L x 2.5 n OH- x 1 n HA = 0.1148 mol HA 1 L 1 n OH- Molar Mass = g/mol ..... 3.1 g/0.1148 mol = 27 g/mol (HCN)

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13 How many mL of 0.310 M KOH would be needed to completely titrate 100. mL of 0.200 M HCl?

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13 How many mL of 0.310 M KOH would be needed to completely titrate 100. mL of 0.200 M HCl?

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Answer

64.5 mL

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14 How many mL of 0.0340 M Ca(OH)2 would be needed to titrate 120. mL of 0.220 M nitrous acid (HNO2)?

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14 How many mL of 0.0340 M Ca(OH)2 would be needed to titrate 120. mL of 0.220 M nitrous acid (HNO2)?

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Answer

388 mL

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15 What is the molarity of a 50.0 mL solution of acetic acid (CH3COOH) if it took 5.0 mL of 1 M NaOH to fully titrate it?

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15 What is the molarity of a 50.0 mL solution of acetic acid (CH3COOH) if it took 5.0 mL of 1 M NaOH to fully titrate it?

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Answer

0.10 M

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16 What is the molar mass of a monoprotic acid if after 0.430 gram sample of it is dissolved in water, 45.0 mL of 0.300 M KOH is needed to titrate it completely?

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16 What is the molar mass of a monoprotic acid if after 0.430 gram sample of it is dissolved in water, 45.0 mL of 0.300 M KOH is needed to titrate it completely?

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Answer

31.9 g/mol

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17 Which of the following would be the identity of an acid if after 0.398 grams of it are dissolved in water, 6.89 mL of 1.1 M NaOH was needed to titrate it? A HBrO B HCN C HClO D HI E HC3H6O3

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17 Which of the following would be the identity of an acid if after 0.398 grams of it are dissolved in water, 6.89 mL of 1.1 M NaOH was needed to titrate it? A HBrO B HCN C HClO D HI E HC3H6O3

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Answer C

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