Clear Notion
Login

Concepts

Read through concept notes with worked explanations and diagrams, chapter by chapter.

Concept 02 / 08Equilibrium

Acids and Bases: Arrhenius, Bronsted-Lowry, Lewis Concepts and the pH Scale

Arrhenius, Bronsted-Lowry and Lewis definitions of acids and bases, conjugate acid-base pairs and relative strengths, the ionic product of water and its temperature dependence, the pH scale, and pH calculations for strong acids, strong bases, their mixtures and very dilute solutions.

Figure 1
Arrhenius concept. Acids give H+\mathrm{H^{+}} (really H3O+\mathrm{H_{3}O^{+}}) in water; bases give OH−\mathrm{OH^{-}}. Limited to aqueous solutions and cannot explain the basicity of NH3\mathrm{NH_{3}} without invoking NH4OH\mathrm{NH_{4}OH}. Bronsted-Lowry concept. An acid is a proton donor; a base is a proton acceptor. Every acid-base reaction transfers a proton and forms a conjugate pair on each side: HCl+H2O⇌H3O++Cl−\mathrm{HCl + H_{2}O \rightleftharpoons H_{3}O^{+} + Cl^{-}}; NH3+H2O⇌NH4++OH−\mathrm{NH_{3} + H_{2}O \rightleftharpoons NH_{4}^{+} + OH^{-}}. A conjugate pair differs by one proton (HCl/Cl−\mathrm{HCl/Cl^{-}}, NH4+/NH3\mathrm{NH_{4}^{+}/NH_{3}}). The stronger the acid, the weaker its conjugate base: Cl−\mathrm{Cl^{-}} is a negligible base, while CH3COO−\mathrm{CH_{3}COO^{-}} is a noticeable one. Species such as H2O\mathrm{H_{2}O}, HCO3−\mathrm{HCO_{3}^{-}} and HSO4−\mathrm{HSO_{4}^{-}} can act either way (amphiprotic). Lewis concept. A Lewis acid accepts an electron pair; a Lewis base donates one. Lewis acids: electron-deficient species (BF3\mathrm{BF_{3}}, AlCl3\mathrm{AlCl_{3}}), cations (H+\mathrm{H^{+}}, Ag+\mathrm{Ag^{+}}, Fe3+\mathrm{Fe^{3+}}), molecules with vacant d orbitals or multiple bonds to electronegative atoms (SiF4\mathrm{SiF_{4}}, SO2\mathrm{SO_{2}}, CO2\mathrm{CO_{2}}). Lewis bases: NH3\mathrm{NH_{3}}, H2O\mathrm{H_{2}O}, OH−\mathrm{OH^{-}}, F−\mathrm{F^{-}}. This definition covers reactions with no protons at all, such as BF3+NH3→F3B←NH3\mathrm{BF_{3} + NH_{3} \rightarrow F_{3}B \leftarrow NH_{3}}. Ionic product of water. Water self-ionises: H2O+H2O⇌H3O++OH−\mathrm{H_{2}O + H_{2}O \rightleftharpoons H_{3}O^{+} + OH^{-}}. Kw=[H+][OH−]=1.0×10−14K_{w} = [\mathrm{H^{+}}][\mathrm{OH^{-}}] = 1.0 \times 10^{-14} at 298 K298\,\mathrm{K}. Self-ionisation is endothermic, so KwK_{w} increases with temperature (about 5.5×10−135.5 \times 10^{-13} at 373 K373\,\mathrm{K}). Pure water is always neutral ([H+]=[OH−][\mathrm{H^{+}}] = [\mathrm{OH^{-}}]), but its pH is below 77 when hot (about 6.16.1 at 100∘C100^{\circ}\mathrm{C}). pH scale. pH=−log⁡[H+]\text{pH} = -\log[\mathrm{H^{+}}], pOH=−log⁡[OH−]\text{pOH} = -\log[\mathrm{OH^{-}}], and pH+pOH=pKw=14\text{pH} + \text{pOH} = \text{p}K_{w} = 14 at 298 K298\,\mathrm{K} (see figure). Strong acids and bases ionise completely: 0.01 M0.01\,\mathrm{M} HCl\mathrm{HCl} has pH 22; 0.01 M0.01\,\mathrm{M} Ba(OH)2\mathrm{Ba(OH)_{2}} has [OH−]=0.02[\mathrm{OH^{-}}] = 0.02, pOH 1.701.70, pH 12.3012.30. Mixtures of strong acid and strong base: find the excess moles of H+\mathrm{H^{+}} or OH−\mathrm{OH^{-}} and divide by the total volume. Very dilute acids: when the acid concentration is near 10−7 M10^{-7}\,\mathrm{M}, the H+\mathrm{H^{+}} from water cannot be ignored. Solve [H+]=c+Kw[H+][\mathrm{H^{+}}] = c + \frac{K_{w}}{[\mathrm{H^{+}}]}. Key Takeaways: - Bronsted: proton transfer; Lewis: electron-pair transfer. - Stronger acid means weaker conjugate base. - Kw=10−14K_{w} = 10^{-14} at 25∘C25^{\circ}\mathrm{C}; increases with temperature. - pH+pOH=14\text{pH} + \text{pOH} = 14 at 25∘C25^{\circ}\mathrm{C}. - Pitfall: 10−8 M10^{-8}\,\mathrm{M} HCl\mathrm{HCl} does not have pH 88; an acid can never give a basic solution. Brain-Teaser 1: Find the pH of 10−8 M10^{-8}\,\mathrm{M} HCl. Solution: [H+]=10−8+10−14[H+][\mathrm{H^{+}}] = 10^{-8} + \frac{10^{-14}}{[\mathrm{H^{+}}]}. Let h=[H+]h = [\mathrm{H^{+}}]: h2−10−8h−10−14=0h^{2} - 10^{-8}h - 10^{-14} = 0, so h=10−8+10−16+4×10−142=1.05×10−7 Mh = \frac{10^{-8} + \sqrt{10^{-16} + 4 \times 10^{-14}}}{2} = 1.05 \times 10^{-7}\,\mathrm{M}. pH =6.98= 6.98: slightly acidic, as it must be. Brain-Teaser 2: 50 mL50\,\mathrm{mL} of 0.1 M0.1\,\mathrm{M} HCl is mixed with 50 mL50\,\mathrm{mL} of 0.2 M0.2\,\mathrm{M} NaOH. Find the pH. Solution: H+\mathrm{H^{+}}: 5 mmol5\,\mathrm{mmol}; OH−\mathrm{OH^{-}}: 10 mmol10\,\mathrm{mmol}. Excess OH−=5 mmol\mathrm{OH^{-}} = 5\,\mathrm{mmol} in 100 mL100\,\mathrm{mL}: 0.05 M0.05\,\mathrm{M}. pOH =1.30= 1.30, pH =12.70= 12.70. Brain-Teaser 3: Identify the conjugate acid and conjugate base of HCO3−\mathrm{HCO_{3}^{-}}, and classify BF3\mathrm{BF_{3}}, NH3\mathrm{NH_{3}} and Fe3+\mathrm{Fe^{3+}} as Lewis acids or bases. Solution: Conjugate acid H2CO3\mathrm{H_{2}CO_{3}}; conjugate base CO32−\mathrm{CO_{3}^{2-}}. BF3\mathrm{BF_{3}} (incomplete octet) and Fe3+\mathrm{Fe^{3+}} (cation with vacant orbitals) are Lewis acids; NH3\mathrm{NH_{3}} (lone pair) is a Lewis base.

Practice this concept

Free · no account required · instant solutions

Acids and Bases: Arrhenius, Bronsted-Lowry, Lewis Concepts and the pH Scale — DPP

10 questionsSee the solution the moment you answer

Practice