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Wize High School Grade 12 Chemistry Textbook > Acids and Bases

Acid Equations (Ka, pKa)

Autoionization of Water (Kw)Previous Section
Base Equations (Kb, pKb)Next Section
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Acids and their Equations

Strong Acids

Strong acids dissociate completely
Example:
HCl(aq) → H+(aq) + Cl-(aq)

Wize Tip
The following are strong acids that you should memorize!
  • HCl (hydrochloric acid)
  • H2SO4 (sulfuric acid)
  • HNO3 (nitric acid)
  • HClO4 (perchloric acid)
  • HBr (hydrobromic acid)
  • HI (hydroiodic acid)


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Weak Acids

Weak acids dissociate incompletely, so they have a Ka value (acid dissociation constant)

Ka is defined as the equilibrium constant for the reaction below:

HA+H2O⇌A−+H3O+HA+H_2O \rightleftharpoons A^-+H_3O^+

Ka=[H3O+][A−][HA]K_a=\cfrac{[H_3O^+][A^-]}{[HA]}

pKa=−log⁡Ka and Ka=10−pKapK_a=-\log K_a\ and\ K_a=10^{-pK_a}
Acid strength follows the following trends:

  • A weaker acid would have a higher/lower Ka value?
    lower
  • A weaker acid would have a higher/lower pKa value?
    higher
Wize Tip
Ka and pKa are like opposite trends.
The higher Ka goes, the lower pKa will go,
and the lower Ka goes, the higher pKa will go!

  • A weaker acid will have a higher/lower pH value:
    higher
  • A stronger acid will have a higher/lower Ka value?
    higher
  • A stronger acid will have a higher/lower pKa value?
    lower
  • A stronger acid will have a higher/lower pH value?
    lower


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In general:
  • If the Ka > 1, pKa < 0 the acid is strong/weak:
    strong
    and reactants/products:
    products
    are favored
  • If the Ka < 1, pKa > 0 the acid is strong/weak:
    weak
    and reactants/products:
    reactants
    are favored
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Example: pH Problem

C6H5COOH ⇌ H+ + C6H5COO-

Ka of C6H5COOH = 6.5 x 10-7 and the concentration of C6H5COOH is 0.01 M, what is the pH of the solution?

Write Out An ICE Table

I 0.01M 0 0
C -x +x +x
E 0.01-x x x

Write a K expression and Solve for x

Ka = x²/(0.01 - x) Ka = x²/0.01
We can ignore the "-x" part since y/K > 400

x² = Ka·0.01
x² = (6.5 x 10-7)(0.01) x² = 6.5 x 10-9 x = 8.06 x 10-5
x=[H+]

Find pH

pH = -log[H+] pH = -log(8.06 x 10-5) pH = 4.09



Practice: Strong Acids

Which one of the following statements about strong acids is true?
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Acids With Different Numbers of Protons

Monoprotic Acids
  • These are acids that only have 1 acidic proton (H+) that they can lose in a reaction
  • *think: mono means 1 Examples: HCl, HBr, HI, HNO3, HClO4, etc

Diprotic Acids
  • These are acids that have 2 acidic protons
  • *think: di means 2 Example: H2SO4

Triprotic Acids
  • These are acids that have 3 acidic protons
  • *think: tri means 3 Example: H3PO4

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Polyprotic Acids
  • This term is just referring to any acid that has more than 1 proton
  • *think: poly means many Example: H2SO4, H3BO3, H2C2O4


H3PO4 (aq) + H2O (l) ⇋ H2PO4− (aq) + H3O+ (aq)H_3PO_4\ \left(aq\right)\ +\ H_2O\ \left(l\right)\ \leftrightharpoons\ H_2PO_4^-\ \left(aq\right)\ +\ H_3O^+\ \left(aq\right)

H2PO4− (aq) + H2O (l) ⇋ HPO42− (aq) + H3O+ (aq)H_2PO_4^-\ \left(aq\right)\ +\ H_2O\ \left(l\right)\ \leftrightharpoons\ HPO_4^{2-}\ \left(aq\right)\ +\ H_3O^+\ \left(aq\right)

HPO42− (aq) + H2O (l) ⇋ PO43− (aq) + H3O+ (aq)HPO_4^{2-}\ \left(aq\right)\ +\ H_2O\ \left(l\right)\ \leftrightharpoons\ PO_4^{3-}\ \left(aq\right)\ +\ H_3O^+\ \left(aq\right)



Wize Concept
The main concept to take away from this table is just that each ionization reaction has its own Ka value and Ka1>Ka2>Ka3.