Chemical Thermodynamics
Easy Overview
Why do some reactions happen on their own while others need a push? Thermodynamics answers that. It's basically the energy accounting of chemistry — who loses energy, who gains it, and whether the whole thing is worth it.
First law and enthalpy
Energy can't be created or destroyed — just moved around. Like money in a closed economy. Enthalpy (H) is just the heat content of a system. If a reaction gives off heat (ΔH negative), it's exothermic. Fireworks, burning wood. If it takes in heat (ΔH positive), it's endothermic — like cooking an egg.
Hess's law
The total enthalpy change is the same no matter how many steps you take. Like climbing a mountain — whether you go straight up or take the scenic route, the height difference is the same. Super useful for calculating ΔH of reactions that are hard to measure directly.
Second law and entropy
Entropy is just a fancy word for disorder. The universe loves chaos. Your room naturally gets messier, not cleaner. Same with energy — it spreads out. A reaction that increases disorder (solid → gas, more molecules) has positive ΔS and is more likely to happen.
Gibbs free energy
ΔG = ΔH - TΔS. This is THE equation. If ΔG is negative, the reaction happens spontaneously. If positive, it won't. If zero, it's at equilibrium. Gibbs energy combines enthalpy and entropy into one number — like a final report card for whether the reaction will occur.
Spontaneity and temperature
Some reactions flip from spontaneous to non-spontaneous depending on temperature. If ΔH is negative and ΔS is positive, it's always spontaneous — free energy. If ΔH is positive and ΔS is negative, it's never spontaneous. The other combos depend on temperature — like ice melting only above 0°C.
Key Points
- •ΔH = heat change at constant pressure
- •Exothermic: ΔH < 0; Endothermic: ΔH > 0
- •Hess's law: ΔH depends only on initial and final states
- •Entropy (S) = measure of disorder; ΔS > 0 for spontaneous reactions
- •ΔG = ΔH - TΔS; ΔG < 0 means spontaneous
- •At equilibrium, ΔG = 0
- •Standard state conditions: 1 bar, 298 K
Practice Questions
- Calculate the enthalpy change for a reaction using standard enthalpies of formation.
- State and explain Hess's law of constant heat summation.
- For a reaction, ΔH = +30 kJ and ΔS = +100 J/K. At what temperature does it become spontaneous?
- Explain why ΔG must be negative for a spontaneous process.