Kinetics and Redox/Electrochemistry
What the exam tests
Kinetics and Redox/Electrochemistry covers reaction rates and collision theory, factors affecting rate, oxidation numbers, redox balancing, electrochemical cells, standard electrode potentials and electrolysis.
Core rules
- Rate increases with concentration, surface area and temperature, and with a catalyst.
- Collision theory requires collisions with sufficient energy and correct orientation.
- A catalyst lowers activation energy without being consumed.
- Oxidation is loss of electrons; reduction is gain. Oxidation numbers: free element 0, simple ion equals charge, oxygen usually −2, hydrogen usually +1.
- In a cell, oxidation occurs at the anode and reduction at the cathode. The more negative electrode potential is the anode.
- Electrolysis: the cathode attracts cations; the anode attracts anions. Q = It, and one mole of electrons is 96 500 C.
Worked example
Find the oxidation number of manganese in KMnO₄. K is +1 and each O is −2; four oxygens give −8. The sum must be 0. +1 + x − 8 = 0, so x = +7. Manganese is in the +7 state and is reduced when permanganate acts as an oxidising agent.
Traps
- Confusing anode and cathode in a cell versus an electrolytic cell (the labels stay the same, the polarity differs).
- Forgetting that fluorine and peroxides break the usual oxygen rule.
- Balancing oxygen with water in acidic and basic conditions in the same way.
- Treating rate and equilibrium as the same thing; a catalyst speeds both directions equally.
- Using minutes in Q = It instead of seconds.