Chemistry (university) · Part 08
Acids, bases and electrochemistry
pH, weak acids, buffers, titrations, galvanic cells, cell potentials and electrolysis.
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- Brønsted-Lowry acids and basesA Brønsted-Lowry acid gives away a proton (H⁺). A base accepts one. When an acid loses a proton, it becomes its conjugate base. When a base gains one, it becomes its conjugate acid.
- pH and KwpH = −log[H⁺]. Water ionizes slightly: Kw = [H⁺][OH⁻] = 1.0×10⁻¹⁴ at 25 °C. So pH + pOH = 14. Each pH step is a factor of 10 in [H⁺].
- Weak acids and KaA weak acid only partly ionizes: HA ⇌ H⁺ + A⁻, with Ka = [H⁺][A⁻]/[HA]. A smaller Ka means a weaker acid. Use an ICE table, and often x is small enough to ignore beside the starting concentration.
- Buffer solutionA buffer resists pH changes. It holds a weak acid and its conjugate base together. Added acid is used up by the base, and added base is used up by the acid. Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]).
- Acid-base titrationIn a titration, a solution of known concentration (the titrant) is added slowly to a measured sample until the reaction is just complete. This point is the equivalence point. An indicator or pH meter shows when it is reached.
- Galvanic cellA galvanic (voltaic) cell turns a spontaneous redox reaction into electric current. Oxidation happens at the anode and reduction at the cathode. Electrons flow through the wire from anode to cathode. A salt bridge keeps the charge balanced.
- Standard cell potentialThe standard cell potential is E°cell = E°cathode − E°anode, using standard reduction potentials in volts. A positive E°cell means the reaction is spontaneous. ΔG° = −nFE°, where F = 96,485 C/mol.
- Nernst equationThe Nernst equation gives the cell potential when concentrations are not standard: E = E° − (RT/nF) ln Q. At 25 °C, E = E° − (0.0592/n) log Q. As the cell runs, Q grows, E falls, and at equilibrium E = 0.
- ElectrolysisElectrolysis uses electricity to force a non-spontaneous reaction. Charge Q = current × time, and moles of electrons = Q / 96,485 C/mol. The moles of product follow from the electron count in the half-reaction.