Energy Changes and Electrochemistry
This domain tests energy transfers in reactions and the chemistry of electrolysis. You must interpret energy level diagrams, calculate bond-energy changes and predict electrode products.
Energy changes
- Exothermic reactions release energy to the surroundings; temperature rises. Products are lower in energy than reactants.
- Endothermic reactions absorb energy; temperature falls. Products are higher in energy.
- Activation energy is the energy needed to start the reaction.
- Breaking bonds absorbs energy; making bonds releases energy. If energy released exceeds energy absorbed, the reaction is exothermic.
Electrolysis
Electrolysis uses electrical energy to decompose an ionic compound. The anode is the positive electrode and oxidation occurs there. The cathode is the negative electrode and reduction occurs there. Positive ions move to the cathode; negative ions move to the anode. Molten compounds give the elements; aqueous solutions may give hydrogen or oxygen instead of the metal, depending on the reactivity series and concentration.
Worked example
Molten lead(II) bromide is electrolysed using inert electrodes. Predict the products and write the electrode equations. Lead(II) bromide contains Pb²⁺ and Br⁻ ions. At the cathode (negative), Pb²⁺ ions gain electrons: Pb²⁺ + 2e⁻ → Pb, so lead metal forms. At the anode (positive), Br⁻ ions lose electrons: 2Br⁻ → Br₂ + 2e⁻, so bromine vapour forms. The molten electrolyte conducts because its ions are free to move.
Traps that cost marks
- Saying positive ions go to the positive electrode; they go to the cathode, which is negative.
- Reversing anode and cathode signs.
- Forgetting that a cell and an electrolytic cell have opposite electrode conventions.
- Adding bond energies with the wrong sign; breaking bonds is endothermic.