Acids and Bases: Qualitative and Quantitative Approaches

SCH4U1 Unit 5

Acids and Bases So Far…

Grade 10• Nomenclature and formulas• Properties of acids and bases• Arrhenius definition (HX = acid; XOH = base)

Grade 11• Brønsted-Lowry definition• Strong vs weak, concentrated vs. dilute • pH scale• Acid-base titrations

Qualitative Properties of Acids and Bases

Qualitative Properties of Acids and Bases

Acids• Soluble is water• Sour taste• Watery feel• Electrolyte• React w/ metals to make H2

• Formed by non-metal oxides + water (SO3 + H2O)

• Neutralize bases• Turns litmus red

Bases• Soluble or insoluble• Bitter taste• Soapy feel• Electrolyte• No reaction w/ metals• Formed by metal oxides

+ water (Na2O + H2O)• Neutralize acids• Turns litmus blue

Chemical Definitions of Acids & Bases

1) Arrhenius Theory (1884)Based on dissociation in water.

2) Brønsted-Lowry Theory (1923)Based on proton (H+) transfer in reactions.

3) Lewis Theory (1923)Based on electron pair transfer.

Electrolytes: compounds that dissociates or ionizes when dissolved in water (e.g. NaCl)

NaCl Na+ (aq) + Cl-

(aq)

Electrolytes conduct electricity when dissolved. Water is a very weak electrolyte.

≈ 0%

H2O no reaction

Acid-Base Key Terms

100%

Strong Acid/Base: Acids or bases that ionize (dissociate) 100%.

Strong Acids: HCl, HBr, HI, HNO3, HClO4, H2SO4* (1 H)

HBr(aq) H+ (aq) + Br- (aq) 100%

Strong Bases: LiOH, NaOH, KOH, RbOH, CsOH (soluble) Mg(OH)2, Ca(OH)2, Sr(OH)2, Ba(OH)2 (insoluble)

CsOH (s) Cs+ (aq) + OH- (aq)

100%

All other acids or bases are weak.

Two Important Ions

• Hydronium (protonated water)

• Hydroxide (deprotonated water)

• Monoprotic: Acids containing only one ionizable hydrogen (e.g. HCl).

• Diprotic: Acids containing two… (e.g. H2CO3)

• Triprotic: Acids containing three… (e.g. H3PO4)

Polyprotic Acids

Amphoteric: A substances capable of acting like acids OR bases in BL acid-base reactions.

HSO4- (aq) + H2O ⇌ SO4

2- (aq) + H3O+ (aq)

BL Acid BL Base CB CA

HSO4- (aq) + H2O ⇌ H2SO4

2- (aq) + OH- (aq)

BL Base BL Acid CA CB

Quantitative Properties of Acids and Bases

Quantitative Properties of Acids and Bases

Water itself never consists of “pure” H2O. Water constantly undergoes autoionization to produce these ions:

H2O (l) H+ (aq) + OH− (aq)

or more accurately,

H2O + H2O (l) H3O+ (aq) + OH− (aq)

acidbase conj. acid conj. base

Ion Product Constant of Water (Kw)

The equilibrium constant equation for this autoionization process can be written as:

Kw = [H3O+][OH−] =

Therefore for “pure” water: [H3O+] = [OH−] =

1.0 x 10-14 (at 25oC)

1.0 x 10-7 mol/L

So what really is an acid & base?!

Water undergoes autoionization in all aqueous solutions. If an acid or a base is dissolved in water, then [H3O+] ≠ [OH−], but the product of their concentration always equals 1.0 x 10-14 or Kw.

In all aqueous solutions:2H2O (l) H3O+

(aq) + OH− (aq)

Kw = [H3O+][OH−] = 1.0 x 10-14

Defining Neutral, Acidic & Basic

For an aqueous equilibrium:

Solution is neutral: [H3O+] = [OH−]

Solution is acidic: [H3O+] > [OH−]

Solution is basic:[H3O+] < [OH−]

Problem Solving with Strong Acids and Bases

• Refer to the sample questions on quantitative aspects of acids and bases.