Electrochemistry, Redox and Transition-Metal Chemistry
This domain tests redox balancing, electrochemical cells, standard electrode potentials and the characteristic chemistry of transition metals. STPM expects correct sign conventions and electron bookkeeping.
Redox and oxidation numbers
Oxidation is loss of electrons and an increase in oxidation number; reduction is gain of electrons and a decrease in oxidation number. Rules: elements have oxidation number 0; oxygen is usually −2; hydrogen is +1 with nonmetals; the sum of oxidation numbers equals the charge on the ion or molecule. Balance half-equations by adding electrons, then combine so electrons cancel.
Electrochemical cells
A cell converts chemical energy to electrical energy. E°cell = E°cathode − E°anode. A positive E°cell indicates a spontaneous reaction. Standard conditions are 1 mol dm−3, 1 atm and 298 K. The Nernst equation adjusts E for non-standard concentrations.
Transition metals
Transition metals show variable oxidation states, form complex ions with ligands, are often coloured due to d–d transitions, and act as catalysts. Colour arises when d orbitals split and electrons absorb visible light.
Worked example
For Zn|Zn2+||Cu2+|Cu, E°(Cu2+/Cu) = +0.34 V and E°(Zn2+/Zn) = −0.76 V. E°cell = 0.34 − (−0.76) = 1.10 V. The positive value shows the cell reaction is spontaneous under standard conditions.
Traps that cost marks
Do not reverse the sign of the cathode potential. E°cell is cathode minus anode. Standard electrode potentials apply only under standard conditions. In complex ions, work out the oxidation number of the metal carefully. d0 and d10 ions are usually colourless.