Unit 8: Acids and Bases

Acid properties

• sour

• pH < 7

• can be a strong (completely) or weak (partially ionized/dissociated) electrolyte in water

• ex: coffee, soda

Base properties

• bitter

• feel slippery

• pH > 7

• can be a strong or weak electrolyte in water'

• ex: baking soda, soap

Arrhenius Acids and Bases

• acids make H+ ion(s) in aq solution and can be mono, di, or triprotic

  • HCl, H2CO3, or H3PO4

• bases make OH- ion(s) in aq solution

  • NaOH

Bronsted-Lowry Acids and Bases

• acids are H+ donors and bases are H+ acceptors

• NH3: A Bronsted Base but NOT Arrhenius Base

  • NH3 has a basic pH, but doesn’t have an O

  • In water, it accepts a H+, producing an OH

Conjugate Acid-Base Pairs

• most acid base reactions are reversible and at equilibrium

• the products and reactants are related by the transfer of an H+ ion (proton)

Lewis Acids and Bases

• instead of focusing on protons like Arrhenius and Bronsted-Lowry, Lewis focused on role of electron pairs

• Lewis Acid= electron pair acceptor

• Lewis Base= electron pair donor

• Na+= Lewis Acid (always)

Ex: NH3 + H+ ←> NH4

de- ae-

LB LA

OH- is always a

base (never an acid)

H30+ is always an

acid (never a base)

Theories Pyramid

Arrhenius

• (most selective)

Bronsted-Lowry

• includes Arrhenius too

Lewis

• includes Arrhenius and Bronsted

Na+

• in a corner at bottom of pyramid

• Lewis Acid always (never higher)

Hydrogen Ions in H2O

• water naturally self-ionizes into H+ and OH- ions

  • H2O ←> H+ + OH- Kw=[H+][OH-]

• Water is amphoteric

  • behave as both an acid and a base

• when the [H+] equals the [OH-], the solution is neutral

• H+ is called a proton or hydronium ion, Oh- is a hydroxide ion

Ion Product Constant (Keq) for water

Keq= [H+][OH-]= 1x10^-14

• x= 1×10^-7 ← [H] or [OH]

We determine how acidic/basic a solution is by looking at the

concentration of H+ and OH- in solution

• an acidic solution has more H ions

• a basic solution has more OH ions

• increasing one decreases the others (inverse)

pH and pOH

• pH= -log[H+] (decrease=acidic)(increase=basic)

• pOH= -log[OH-] (decrease=basic)(increase=acidic)

pH and pOH for water

• pH= -log[1×10^-7] = 7

• pOH= -log[1×10^-7] = 7

[H+] =

10^-pH

[OH-] =

10^-pOH

pH + pOH =

14

neutral pH

7

acidic pH

pH < 7

basic pH

pH > 7

“p”

mathematical operator

• variable that denotes an operation to be done

• = -log( )

Measuring pH

• pH meters measure [H+] concentration

• we can also use indicators

  • indicators are chemicals that change colors at different pHs (phenolphthalein and thymol blue)

Acid-Base Neutralization Reactions

• reactions between acids and bases are double replacement reactions

• they generally produce water and an ionic compound (a neutral solution)

NaOH +HCl → H2O + NaCl

Titration

• you can determine the molarity of an unknown acid or base by titrating it with a given amount of a known acid or base

• when the [H+]=[OH-], the solution is neutral

• we call this the equivalence point

• we use an indicator to determine when the equivalence point has been reached

MaVa=MbVb

• this is important if acid and base are not 1:1

• Ma= [H+]

• Mb= [OH-]

M of H: 3MHCl

3MH+

M of H: 3MH2SO4

6MH+

M of OH: 3Mal(OH)3

9MOH-

Titration Curves

• equivalence point is in the middle of the curve

start low go high

• titration curve of strong acid with a strong base

• start w/ acid, go up w/ pH

start high go low

• titration curve of strong base with strong acid

• start w/ base, go down with pH

Concentrated vs Dilute

• measured in molarity (M)

• generally <1 M is dilute, and >1M is concentrated

strong vs weak

strong acids and bases ionize (dissociate) completely in solution

• not reversible (→)

weak acids and bases ionize only partially in solution

• reversible reactions at equilibrium (←>)

• only a few break apart

• ex: CH3COOH + H20 ←> CH3COO + H3O

weak acids and bases

• because these are ←> reactions, we can write a Keq expression for these reactions

• Ka and Kb are acid and base dissociation constants

• the larger the Ka or Kb, the stronger the acid or base and the more ions present

• Ka= [H][A]/[HA]

• Kb= [B][OH]/[BOH]

H3O=

H

Ka/b =

x²/initial concentration of reactants

• x² uses products

memorize strong acids

• HCl

• HBr

• HI

• HNO3

• H2SO4

memorize strong bases

• NaOH

• KOH

• RbOH

• CsOH

• Ca(OH)2

• Sr(OH)2