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Acids and Bases: Qualitative and Quantitative Approaches
SCH4U1 Unit 5
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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
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Qualitative Properties of Acids and Bases
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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
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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.
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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%
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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.
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Two Important Ions
• Hydronium (protonated water)
• Hydroxide (deprotonated water)
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• 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
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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
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Quantitative Properties of Acids and Bases
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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
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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
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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
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Defining Neutral, Acidic & Basic
For an aqueous equilibrium:
Solution is neutral: [H3O+] = [OH−]
Solution is acidic: [H3O+] > [OH−]
Solution is basic:[H3O+] < [OH−]
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Problem Solving with Strong Acids and Bases
• Refer to the sample questions on quantitative aspects of acids and bases.