Reactivity 3 - What are the mechanisms of chemical change?
Reactivity 3 explains the pathways of chemical change. The exam tests acid–base equilibria, redox and electrochemistry, and organic reaction mechanisms.
Acids, bases and pH
- Brønsted–Lowry acids donate protons; bases accept them. Conjugate pairs differ by H⁺.
- Strong acids ionise completely; weak acids establish an equilibrium with Ka.
- pH = −log[H⁺] and [H⁺] = 10^(−pH). At 298 K, Kw = 1.0 × 10⁻¹⁴, so pH + pOH = 14.
- For a weak acid, [H⁺] ≈ √(Ka × c) when ionisation is small.
Redox and cells
- Oxidation is loss of electrons; reduction is gain. Oxidation numbers track electron transfer.
- The oxidising agent is reduced; the reducing agent is oxidised.
- In a voltaic cell, oxidation occurs at the anode and reduction at the cathode; E°cell = E°cathode − E°anode.
Organic mechanisms
- Curly arrows show electron-pair movement from a lone pair or bond to an atom or bond.
- Halogenoalkanes undergo nucleophilic substitution, with the rate depending on the halogenoalkane and the nucleophile.
- Alkenes undergo electrophilic addition.
Worked example
Find the pH of 0.100 M CH₃COOH (Ka = 1.8 × 10⁻⁵). [H⁺] = √(1.8 × 10⁻⁵ × 0.100) = √(1.8 × 10⁻⁶) = 1.34 × 10⁻³ mol dm⁻³. pH = −log(1.34 × 10⁻³) = 2.87.
Traps
- Confusing strong with concentrated.
- Drawing arrows from the wrong end of a bond.
- Forgetting that pH is logarithmic, so a change of 1 unit is a tenfold change in [H⁺].