Chemistry unit guide
Mechanisms of chemical change
Redox, electrochemical cells, electrolysis and organic reaction mechanisms.
Reactivity 3 splits into two halves that feel like different subjects. The redox and electrochemistry half is procedural and rewards a reliable method for oxidation states and half-equations. The organic mechanism half rewards precision in drawing - curly arrows start at a lone pair or a bond and end where the electrons go, and an arrow drawn from the wrong place loses the mark even when the product is right.
Topics in this unit
What gets examined
- Assigning oxidation states and identifying what is oxidised and reduced.
- Balancing half-equations in acidic and basic conditions and combining them.
- Deducing cell potential from standard electrode potentials and predicting whether a reaction is spontaneous.
- Distinguishing voltaic from electrolytic cells, and identifying the anode and cathode in each.
- Predicting products of electrolysis of molten salts and of aqueous solutions.
- Drawing nucleophilic substitution mechanisms, including the SN1 and SN2 distinction at HL.
How to revise it
Use one oxidation-state method every time
Oxygen is -2, hydrogen is +1, the sum equals the overall charge. Applying the same three rules mechanically is faster and more reliable than reasoning about each compound.
Remember oxidation is loss at the anode, in both cell types
The anode is where oxidation happens whether the cell is voltaic or electrolytic. What changes between them is the sign, not the chemistry, and that is precisely what gets examined.
Draw arrows from electrons to where they go
Every curly arrow starts at a lone pair or a bond and ends at an atom or a bond. An arrow starting on an atom rather than its electrons is the most commonly penalised drawing error.
Learn the SN1 and SN2 evidence
Tertiary favours SN1 through carbocation stability, primary favours SN2 through less steric hindrance. Questions ask you to justify the choice, not just name it.
Where marks get lost
- Balancing charge before atoms in a half-equation, or forgetting to balance charge at all.
- Swapping anode and cathode when moving between voltaic and electrolytic cells.
- Forgetting that water can be oxidised or reduced in preference to the ions in aqueous electrolysis.
- Drawing a curly arrow that begins on the atom instead of its lone pair.
- Omitting the intermediate carbocation in an SN1 mechanism.
Practise this unit
Reading about a unit only goes so far. Work through real IB-style questions on it, with mark schemes.