Acids and Bases Chapter 19. Describing Acids and Bases

Acids and Bases

Chapter 19

Describing Acids and Bases

Mini-Project

Work with a partner. Organize the following formulas into two groups with four formulas in each group:HNO3, NaOH, H2SO4, H2CO3, Ca(OH) 2, KOH, H8PO4, Mg(OH) 2One way to organize them into groups is:Group One Group TwoHNO3 NaOHH2SO4 Ca(OH) 2

H2CO3 KOHH8PO4 Mg(OH) 2

Group One formulas represent acids.Group Two formulas represent bases.

ACIDS

• Electrolytes in solution

• Taste sour (lemon, vinegar)

• React with metal (corrosion)

• React with carbonates (makes bubbles of CO2

• Turns blue litmus RED

• In Water forms Hydrogen ION

HCl H+ + Cl-Water

BASES

• Electrolyte in solution

• Taste Bitter (soap, tonic water)

• Feel Slippery (soap)

• Turns Red Litmus Blue

• React with Acids to make water

NaOH Na+ + OH-Water

Why are Some Solutions Acid & Others Base?

• Acid solutions contain more H+ ions than OH- ions.

• Base solutions contain more OH- ions than H+ ions.

• Water is the standard for Acid/Base and is defined as NEUTRAL

• Water has equal amounts of H+ and OH- ions

Arrhenius Model of Acids/Bases

• Substance is an acid if it contains hydrogen and dissociation causes hydrogen ions to form in solution

• Substance is a base if it contains a hydroxide and dissociates to produce hydroxide ions in solution

Bronsted-Lowry Model

• Acid is a proton (hydrogen ion) donor

• Base is a proton (hydrogen ion) receptor

• This is a broader definition than Arrhenius model because there are substances that cause donation or reception without having hydrogen in them.

Example• When an acid dissolves in water, it donates an

H+ ion to a water molecule forming H3O+.• The water molecule acts as a base and accepts

the H+ ion

HX + H2O ⇄ H3O+ + X-

– Conjugate acid = species produced when a base accepts a hydrogen ion from an acid

– Conjugate base = species produces when an acid donates a hydrogen ion to a base

– Conjugate base pair = 2 substances related to each other by donating and accepting a single hydrogen ion

Historical views on acidsHistorical views on acids

• The more recent Bronsted-Lowry concept is that acids are H+ (proton) donors and bases are proton acceptors

Ionization

+Cl HH

HO

+H

HH O Cl+

• O (e.g. H2SO4) was originally thought to cause acidic properties. Later, H was implicated, but it was still not clear why CH4 was neutral.

• Arrhenius made the revolutionary suggestion that some solutions contain ions & that acids produce H3O+ (hydronium) ions in solution.

The Bronsted-Lowry conceptThe Bronsted-Lowry concept• In this idea, the ionization of an acid by water

is just one example of an acid-base reaction.

• 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

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

acid base conjugate acidconjugate baseHC2H3O2(aq) + H2O(l) C2H3O2

–(aq) + H3O+(aq)

conjugate acid-base pairs

acidbase conjugate acidconjugate baseOH

–(aq) + HCO3–(aq) CO3

2–(aq) + H2O(l)

conjugate acid-base pairs

acid base conjugate acidconjugate baseHF(aq) + SO3

2–(aq) F–(aq) + HSO3–(aq)

conjugate acid-base pairs

acidbase conjugate acidconjugate baseCO3

2–(aq) + HC2H3O2(aq) C2H3O2–(aq) + HCO3

–(aq)

conjugate acid-base pairs

acid base conjugate acidconjugate baseH3PO4(aq) + OCl

–(aq) H2PO4–(aq) + HOCl(aq)

conjugate acid-base pairs

Answers: question 18Answers: question 18(a)

(b)

(c)

More

• Amphoteric = substances like water that can act like either an acid or a base

Monoprotic and Polyprotic Acids

• Monoprotic – acids based on formula that can donate only one hydrogen ions– CH3COOH + H2O ⇄ H3O+ + CH3COO-

• Polyprotic – acids that can donate multiple hydrogen ions– H3PO4 + H2O ⇄ H3O+ + H2PO4+– H2PO4

+ + H2O ⇄ H3O+ + HPO4+2– HPO4

+2 + H2O ⇄ H3O+ + PO4+3• Anhydride = oxides that can become acids or bases by adding elements contained in water

Acid Rain

• Acid rain comes from rain collecting gasses from the air to create acids:– Carbon Dioxide = carbonic acid– Sulfur oxides = sulfuric acid– Nitrogen oxides = nitric acid

• Damages statues, buildings, kills forests, kills fish

Acids and Bases in Solution

Chapter 19.2

Acid/Base Strength

• In strong acids, almost all molecules ionize.

• In weak acids, fewer molecules ionize.

Conjugate Pairs Strength

• If an acid is a strong acid, its conjugate pair base is a weak base

• Why?– If HX is strong acid, it ionizes completely.– The conjugate base must be a weak base

because it has a greater attraction to the H+ than HX

– The reaction equilibrium lies far to the right of the equation.

Conjugate Pair Strength

• For a weak acid, the equation equilibrium lies to the right (reactant side)– Conjugate base (Y-) has a stronger attraction

for the H+ ion than the base H2O

– HY + H2O H3O+ + Y-

Acid Ionization Constants

• An “ionization constant” is the tendency of an item to make ions in solution.– Higher the constant, the higher the amount of

ions.– Acid ionization constant is value of the

equilibrium constant expression for a weak acid

– Value Ka indicates whether reactants or products are favored at equilibrium

– Weak acids have low Ka values

Acid Ionization Constants

Acid Ionization Constants

Substance Formula Ka

Acetic Acid HC2H3O2 1.7 x 10-5

Boric Acid H3BO3 5.9 x 10-10

Carbonic Acid H2CO3 4.3 x 10-7

HCO3- 4.8 x 10-11

Hydrogen Sulfide H2S 8.9 x 10-8

HS- 1.2 x 10-13

Hypochlorous Acid HClO 3.5 x 10-8

Nitrous Acid HNO2 4.5 x 10-4

Oxalic Acid H2C2O4 5.6 x 10-2

HC2O4- 5.1 x 10-5

Phosphoric Acid H3PO4 6.9 x 10-3

HSO3 6.3 x 10-8

Base Ionization Constant

• Same Basic Principle as Acid – Measures OH- concentrations

pH Scale

Chapter 19.3

Ionization Constant for Water

• The ionization constant for water is:– 1.0 x 10-14

– [Ka][Kb]– = [1.0x10-7] [1.0 x 10-7]

• Experiments show that the product of [H+] and [OH-] always equals 1.0 x 10-14 at 298°C

• pH scale is a way of showing this relationship of ionization constants

The pH Scale

• pH stands for ‘per Hydrion’• Low pH is Acid• High pH is base• Water is neutral (7.0)

pHpH• There are many ways to consider acids and

bases. One of these is pH.• [H+] is critical in many chemical reactions.• A quick method of denoting [H+] is via pH.• By definition pH = – log [H+], [H+] = 10-pH

• The pH scale, similar to the Richter scale, describes a wide range of values

• An earthquake of “6” is 10 as violent as a “5”• Thus, the pH scale condenses possible

values of [H+] to a 14 point scale (fig. 2, p370)• Also, it is easier to say pH = 7 vs. [H+] = 1 x 10–7

pH

• pH = -log [H+]– [H+] = 10-pH

• pOH = -log [OH-]

• pH + pOH = 14

Calculations with pHCalculations with pH

Ans: 4.2

3.98 x 10–8 M

Q: What is the pH if [H+]= 6.3 x 10–5? pH = – log [H+]

‘(-)’, ‘log’, ‘6.3’, ’10x’, ‘(-)’, ‘5’, ‘)”, ‘)”, ‘ENTER’)

Q: What is the [H+] if pH = 7.4?[H+] = 10–pH mol/L(’10x’, ‘(-)’, ‘7.4’, “)” ‘ENTER‘)

Calculating pH from Strong Acid Solutions

• Strong acids are 100% ionized

• For monoprotic acids, concentration of the acid IS the concentration of the H+ ion

• Use Acid concentration as substitute for H+ ion concentration.– Use Base concentration as substitute for OH-

concentration

Calculating pH from Strong Acid Solutions

• Example: What is the pH of a 0.1M solution of HCl?– 0.1 M HCl = 1 x 10-1 M– Calculate pH = 1

• Example: What is pH of solution that is 7.5 x 10-4 M Ca(OH)2?

– (7..5 x 10-4) x 2 = 1.5 x 10-3M• There are 2 OH- ions per molecule

– Calculate pOH = -log[OH-]• = 2.8• pH = 14-2.8 = 11.2

Calculating Molarity from pH

• Example: what is the molarity of an acid solution with a pH of 2.37?– [H+] = 10-pH

– [H+] = 10-2.37 = 4.27 x 10-3 M

Neutralization

Chapter 15.4

Acid-Base Reactions

• Neutralization reaction is a reaction between an acid and a base– Makes Water + Salt– Solution becomes Neutral (not acid or base)

HCl + NaOH H2O + Na+ + Cl-

Acid-Base Reactions

• Mg(OH)2 + 2 HCl → MgCl2 + 2H2O

• Note:– Cation from base (Mg) is combined with anion

from acid (Cl)

– The salt is MgCl2– The H+ and OH- always combine to form

water

Acid-Base Titration

• Acid/Base Titration is the stoichiometry of acid/base reactions.– Titration is a method for determining the

concentration of a solution by using another solution of known concentration

– Uses an INDICATOR to show when the acid/base reaction is complete (neutral)

• Indicator is a chemical that changes color as determined by acid or base conditions

• There are many indicators with different pH points.

Acid/Base Titration Curve

pH Indicators

Name Acid ColorpH Range of

Color ChangeBase Color

Methyl violet Yellow 0.0 - 1.6 Blue

Thymol blue Red 1.2 - 2.8 Yellow

Methyl orange Red 3.2 - 4.4 Yellow

Bromocresol green Yellow 3.8 - 5.4 Blue

Methyl red Red 4.8 - 6.0 Yellow

Litmus Red 5.0 - 8.0 Blue

Bromothymol blue Yellow 6.0 - 7.6 Blue

Thymol blue Yellow 8.0 - 9.6 Blue

Phenolphthalein Colorless 8.2 - 10.0 Pink

Thymolphthalein Colorless 9.4 - 10.6 Blue

Alizarin yellow R Yellow 10.1 - 12.0 Red

Calculating Molarity from Titration

1. Write the balanced equation2. Calculate the number of moles used in

the ‘known’ solution3. Use the mole ratio from the balanced

equation to calculate moles of reactant in the ‘unknown’ solution

4. Calculate the molarity of the ‘unknown’ solution based on moles used and liters used.

Salt Hydrolysis• When you put salts in water, the resulting

solution can be either acid, base, or neutral– Salts will dissolve to form ions– The anions will accept hydrogens from water– The cations will accept hydroxides from water– Which way it goes depends upon the strength

of the conjugate acids/bases– If conjugate acid is strong, it will be acid– If conjugate base is strong, it will be base– If both are strong, it will be neutral

What are Buffers?

• Buffers are solutions that resist changes in pH when limited amounts of acid or base are added.– Buffer is a weak acid and its conjugate base

or a weak base and it’s conjugate acid– Buffer can accept or donate Hydrogen ions

and shift its equilibrium point left or right– Buffers have limits, but the work for a while– Heavily used in human body, especially blood

Acid Names

1. Binary or hydrohalic acids – HF, HCl, HBr, HI, etc. “hydro____ic acid” are usually strong acids

• If name ends in ‘-ide’– Acid name will be “hydro ____ic acid”

• HF and H2S are weak hydrohalic acid. Although the H-F bond is very polar, the bond is so strong (due to the small F atom) that the acid does not completely ionize.

Acid Naming

2. Oxyacids – contain a polyatomic ion a. Most common form (MCF) “ic” ending –

strong acids (contain 2 oxygen per hydrogen)

b. If chemical name ends in “-ate”a. Acid name will be “___IC Acid”– HNO3 – nitric from nitrate

– H3PO4 - phosphoric from phosphate

– H2SO4 - sulfuric from sulfate

– HClO3 - chloric from chlorate

b. Acids with 1 less oxygen than the MCF “ous” ending- weaker acids

a. Chemical name ends in “-ite”b. Acid name is “___OUS Acid”

HNO2 – nitrous from nitriteH3PO3 - phosphorous from phosphiteH2SO3 - sulfurous from sulfiteHClO2 - chlorous from chlorite c. Acids with 2 less oxygen than the MCF

“hypo___ous” – very weak acidsHNO - hyponitrous H3PO2 - hypophosphorusHClO - hypochorous 

d. Acids with 1 more oxygen than the MCF “per______ic” – very strong acids

HClO4 – perchloric acid

HNO4 - pernitric acid

e. Organic acids – have carboxyl group -COOH - usually weak acids

• Acid names are based on the base organic name or common name

HC2H3O2 - acetic acid

C7H5COOH - benzoic acid