⚗️ Visual Chemistry
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Acids and Bases

25min Part 1 / Ch2 / Lesson 4
Prerequisites: 1-2-2

Objectives

  • Define acids and bases using the Arrhenius definition
  • Distinguish between strong and weak acids/bases
  • Understand pH and calculate it

Defining Acids and Bases

Arrhenius definition:

  • Acid: produces H+\mathrm{H^+} ions in water
  • Base: produces OH−\mathrm{OH^-} ions in water
AcidIonization
Hydrochloric acidHCl→H++Cl−\mathrm{HCl \rightarrow H^+ + Cl^-}
Sulfuric acidH2SO4→2H++SO42−\mathrm{H_2SO_4 \rightarrow 2H^+ + SO_4^{2-}}
Acetic acidCH3COOH⇌H++CH3COO−\mathrm{CH_3COOH \rightleftharpoons H^+ + CH_3COO^-}
BaseIonization
Sodium hydroxideNaOH→Na++OH−\mathrm{NaOH \rightarrow Na^+ + OH^-}
Calcium hydroxideCa(OH)2→Ca2++2OH−\mathrm{Ca(OH)_2 \rightarrow Ca^{2+} + 2OH^-}
AmmoniaNH3+H2O⇌NH4++OH−\mathrm{NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-}

Strong vs. Weak

TypeDegree of ionizationIn waterExamples
Strong acid≈ 1 (complete)Nearly all molecules release H+\mathrm{H^+}HCl, H2SO4\mathrm{H_2SO_4}, HNO3\mathrm{HNO_3}
Weak acid≪ 1Only some ionizeCH3COOH\mathrm{CH_3COOH}, H2CO3\mathrm{H_2CO_3}
Strong base≈ 1 (complete)Nearly all release OH−\mathrm{OH^-}NaOH, KOH, Ca(OH)2\mathrm{Ca(OH)_2}
Weak base≪ 1Only some ionizeNH3\mathrm{NH_3}

Classification by degree of ionization

Degree of ionization α=molecules ionizedmolecules dissolved\alpha = \dfrac{\text{molecules ionized}}{\text{molecules dissolved}}

Strong acids/bases: α≈1\alpha \approx 1 (complete ionization) Weak acids/bases: α≪1\alpha \ll 1 (partial ionization)

Hydrogen Ion Concentration and pH

pH=−log⁡10[H+]\mathrm{pH} = -\log_{10}[\mathrm{H^+}]

  • pH < 7: acidic
  • pH = 7: neutral
  • pH > 7: basic (alkaline)
01234567891011121314AcidicNeutralBasic
The pH scale

Examples:

  • [H+]=0.001=10−3 mol/L[\mathrm{H^+}] = 0.001 = 10^{-3}\ \mathrm{mol/L} → pH=3\mathrm{pH} = 3
  • [H+]=10−5 mol/L[\mathrm{H^+}] = 10^{-5}\ \mathrm{mol/L} → pH=5\mathrm{pH} = 5
Common Misconception

“Lower pH is safer” → pH near 0 is strongly acidic, near 14 is strongly basic — both are dangerous. pH around 7 is neutral.

Check Your Understanding

Q1 What is the pH of 0.01 mol/L HCl (strong acid)?

Q2 Which is a weak acid?

Q3 A solution with pH 11 is:

Q4 If [H⁺] = 10⁻⁴ mol/L, what is the pH?

Exercises

Q1. Find the pH of a 0.050 mol/L0.050\ \mathrm{mol/L} NaOH solution. Use log⁡102=0.30\log_{10} 2 = 0.30.

Solution

NaOH is a strong base (complete ionization): [OH−]=0.050 mol/L[\mathrm{OH^-}] = 0.050\ \mathrm{mol/L}

Using the water ion product: [H+][OH−]=10−14[\mathrm{H^+}][\mathrm{OH^-}] = 10^{-14}

[H+]=10−140.050=10−145.0×10−2=2.0×10−13[\mathrm{H^+}] = \frac{10^{-14}}{0.050} = \frac{10^{-14}}{5.0 \times 10^{-2}} = 2.0 \times 10^{-13}

pH=−log⁡10(2.0×10−13)=13−log⁡102=13−0.30=12.7\mathrm{pH} = -\log_{10}(2.0 \times 10^{-13}) = 13 - \log_{10} 2 = 13 - 0.30 = 12.7