Loading
Unit 9
7–9% of examWhy do some reactions happen on their own while others need a push? This unit answers that with entropy and Gibbs free energy, connects free energy to the equilibrium constant, and then applies it all to batteries and electrolysis, where redox reactions produce or use electricity.
Longer videos that cover the whole unit. Good for a first pass or a final review.
Entropy (S) increases when matter or energy becomes more spread out. You can predict the sign of ΔS: it's usually positive when a solid melts, a liquid boils, a gas expands, the temperature rises, or a reaction makes more moles of gas than it uses up.
Key terms
Data tables list each substance's standard molar entropy, S°, in J/(mol·K). To find ΔS° for a reaction, add up S° for the products and subtract the total for the reactants, multiplying each value by its coefficient in the balanced equation.
Key terms
A process is thermodynamically favored when ΔG° is negative, and ΔG° = ΔH° − TΔS° (watch your units: ΔH° is usually in kJ/mol and ΔS° in J/(mol·K), so convert one before you combine them). A negative ΔH° with a positive ΔS° is favored at every temperature, the opposite pair never is, and the mixed cases depend on temperature.
Key terms
A reaction can be thermodynamically favored and still not happen at a noticeable rate, usually because its activation energy is high. A reaction like that is under kinetic control, and the fact that nothing seems to happen doesn't mean it's at equilibrium.
Key terms
ΔG° and K are linked by ΔG° = −RT ln K. A negative ΔG° means K > 1, so products are favored at equilibrium; a positive ΔG° means K < 1; and a ΔG° close to zero means K is close to 1.
Key terms
Dissolving a solid involves breaking apart the solid, rearranging the solvent, and forming new attractions between the dissolved particles and the solvent. Each step has its own enthalpy and entropy change, and because they partly cancel out, predicting the overall ΔG° of dissolving is tricky.
Key terms
An unfavorable process can be driven by an outside energy source, like electricity in electrolysis or light in photosynthesis, or by pairing it with a favorable reaction that shares an intermediate. Add the reactions and their ΔG° values: if the total is negative, the overall process is favored, which is how living cells use ATP.
Key terms
A galvanic cell runs a favored redox reaction to produce electricity, while an electrolytic cell uses outside electrical energy to force an unfavored one. In both, oxidation happens at the anode and reduction at the cathode; electrons travel through the wire while ions move through the solutions and salt bridge to keep the charges balanced.
Key terms
Find a cell's standard potential from standard reduction potentials: E°cell = E°(cathode) − E°(anode). A positive E°cell means the reaction is favored, and ΔG° = −nFE° converts the voltage into a free energy change.
Key terms
E° assumes standard conditions, where Q = 1. As a cell runs toward equilibrium its voltage drops, reaching zero when Q = K (a dead battery), and the Nernst equation, E = E° − (RT/nF) ln Q, shows that raising Q lowers the voltage. On the exam you use it to reason about which way the voltage changes, not to grind through calculations.
Key terms
Current is charge per unit time (I = q/t), and one mole of electrons carries about 96,485 coulombs (the Faraday constant). From the current and the time, you can work out the moles of electrons that flowed and then how much metal plates out or gas forms at an electrode.
Key terms