Chapter 15

Acids and Bases

Objectives

• Describe the properties of acids and bases

• Recognize acids and bases by definitions

• Compare strong and weak acids/bases

• Describe electrolytes

What Do You Know About Acids and Bases?

Properties of Acids

• Sour taste

– (A VERY bad way to test in the lab!)

• What acids have you tasted?

• Turns blue litmus red

• Reacts with metal to form hydrogen gas

• pH less than 7

• Electrolyte

• Solutions that conduct electrical current

Properties of Bases

• Tastes bitter

– (Also not a good lab practice!)

• Turns red litmus blue

• pH greater than 7

• Electrolyte

Acid/Base Definitions

• Arrhenius Definition

– Acids increase the H+ ion concentration in solution

• Acids have H as the first element

– Bases increase the OH- ion concentration in solution

• Many bases end with OH

– Definition limited to solutions!

Acid/Base Definitions

• Bronsted/Lowry Definition

– Acids are H+ donors

• H+ ion is just a proton

–(often called proton donors)

– Bases are H+ acceptors

–(Often called proton acceptors)

• This is the definition of acids and bases we will use the most.

Strong Acids

• Strong Acids – Acids that completely ionize in solution

– Assume that HA can be an acid

HA H+ + A-

– There will be NO HA left in solution

• Strong Acids – Memorize the first 3

HNO3 HCl H2SO4

HClO4 HBr HI

The Hydronium Ion

• H+ ions can be written one of two ways

– 1st H+

• Easy way to indicate the ion

– 2nd H3O+

• Indicates that the H+ attaches to water molecules

–Hydronium Ion

• Either way is fine

Weak Acids

• Weak Acids – Acids that only partially ionize in solution– Equilibrium is established– Assume HA to be a weak acid

HA + H2O H3O+ + A-

– There will be lots of HA left in solution• Weak Acids – Any acid that is not strong

H2CO3 HF H3PO4

HNO2 HC2H3O2 H2SO3

Inorganic vs Organic Acids

• All acids have H in the formula

• Inorganic acids contain hydrogen and a halogen or hydrogen and a polyatomic ion

• Organic acids have hydrogen and carbon and oxygen (not CO3

-2)

– Carboxyl group

– Carboxylic acids

• All weak

Strong Bases

• Strong Bases – Bases that completely ionize in solution

KOH K+ + OH-

– There will be NO KOH left in solution

• Strong Bases

NaOH KOH

• Group 2 hydroxides are also considered strong bases

Weak Bases

• Weak Bases – Bases that partially ionize in solution

NH3 + H2O NH4+ + OH-

– There will be lots of NH3 left in solution

– Weak bases are ammonia derivates

• They will have nitrogen in them

Electrolytes• Conduct current because charged particles are

free to move about• Acids form ions when they are dissolved in

solution

HCl H+ + Cl-

• Salts form ions when they are dissolved in solution

NaNO3 Na+ + NO3-

• Charged particles complete (or close) the circuit

Closed Circuit

Open Circuit

Another Type of Open Circuit

Closed circuit•Ions in solution close the circuit

• Charges flow to the opposite pole

•Current flows from negative to positive

•Light bulb shines!

Nonelectrolytes

• Solutions that do not conduct an electric current

– No ions are present

• Nothing to conduct the current

• All molecules are nonelectrolytes

– They dissolve into individual molecules

Electrolytes or Nonelectrolytes?

• CH4

– Nonelectrolyte

• KBr

– Electrolyte

• C2H6O

– Nonelectrolyte

• H2SO4

– Electrolyte

Strong and Weak Electrolytes

• Strong electrolytes

– Solutions that conduct current well

• Lots of ions in solution

–Strong acids and bases, salts

• Weak electrolytes

– Solutions that conduct current poorly

• Few ions

–Weak acids and bases

Homework

• p. 625 #42,43,51,55,61,67,70,86,97

Objectives

• Recognize polyprotic acids.

• Compare the strengths of weak acids and bases.

• Describe a neutralization reaction

• Calculate neutralization data

• Determine if a solution is acidic, basic, or neutral

• Explain the autodissociation of water

Polyprotic Acids

• Acids that can donate more than 1 hydrogen

• H2CO3 – Carbonic Acid

– Able to donate 2 H’s

• Diprotic

• H3PO4 – Phosphoric Acid

– Able to donate 3 H’s

• Triprotic

Cont.

• HNO3 – Nitric Acid

– Only 1 H to donate

• Monoprotic

• HC2H3O2 – Acetic Acid

– Only the first H can be donated

• Same with most carboxylic acids

How Weak is Weak?Quantifying Weak Acids and Bases• Weak acids and bases have equilibrium

dissociation values

– The smaller the constant the weaker the acid or base

– The larger the constant the stronger the acid or base

How Weak is Weak?Quantifying Weak Acids and Bases

Acid - HA(aq) H+ + A-

• The Equilibrium Expression

Ka = [H+] [A-] / [HA]

Base - B(aq) + H2O BH+ + OH-

• The Equilibrium Expression

Kb = [BH+] [OH-] / [B]

Sample Ka Values

• Acid Ka

• HClO2 1.2x10-2

• HF 7.2x10-4

• HC2H3O2 1.8x10-5

• HClO 3.5x10-8

• HCN 6.2x10-10

• HIO 2.0x10-11

• What is the strongest / weakest acid?

Sample Kb Values

• Base Kb

• NH3 1.8x10-5

• (C2H5)2NH 1.3x10-3

• (C2H5)3N 4.0x10-4

• CH3NH2 4.4x10-4

• C2H5NH2 5.6x10-4

• What is the strongest / weakest base?

Neutralization Reactions

• The reaction of an acid and a base to yield a salt and water

• HI + KOH HOH + KI

• Net Ionic Equation

• Net Ionic is ALWAYS the same

Example• What volume of 0.25M KOH is required to

react with 50.0 mL of 0.20 M HBr?

Example• What volume of 1.2M NaOH is required to

react with 30.0 mL of 0.70 M H2SO4?

Acidic, Basic, and Neutral Solutions

• Relates to concentrations of H+ and OH-

• Acidic solutions have more H+ than OH-

– H+ from an acid

• Basic solutions have more OH- than H+

– OH- from base

• Neutral solutions have equal amounts of both ions

– Both ions come from water!

Autoionization of Water

• Water molecules dissociate by themselves

H2O(l) H+ + OH-

• In pure water the concentrations are equal• Both 1.00x10-7M• Write the equilibrium expression

• Kw is always 1.00x10-14 at 25ºC

LeChatlier’s Principle (Again)

• The concentration of H+ and OH- can vary in solution.

• When H+ is added, [OH-] decreases

• When OH- is added, [H+] decreases

• That is how solutions become acidic or basic

Example• What is the concentration of H+ and OH- in

a 0.10M solution of HCl? Is the solution acidic, basic, or neutral?

Example• A solution is prepared by dissolving

6.00 grams of KOH in 200.0mL of water. What is the concentration of H+ and OH-? Is the solution acidic, basic, or neutral?

Homework

• p. 626 #80,87,89,92,101,117,119,124,131,134