Summary
Highlights
Entropy is defined as the level of disorder or particle dispersion in a system. Solids have the lowest entropy, followed by liquids, aqueous solutions, and gases. Increasing temperature or volume (for gases) also increases entropy. For chemical reactions, an increase in gas molecules leads to higher entropy.
The entropy of a reaction can be calculated by subtracting the sum of reactants' entropies from the sum of products' entropies. Gibbs Free Energy (ΔG) measures the thermodynamic favorability of a process; a negative ΔG indicates a favored process. Calculations for ΔG, ΔH, and ΔS are typically done at standard conditions (25°C, 1 atm, 1 M solutions).
The universe favors exothermic reactions (negative ΔH) and reactions with increasing entropy (positive ΔS). If both conditions are met, the reaction is favored at all temperatures. If a reaction is endothermic and has decreasing entropy, it's never favored. Other combinations depend on temperature (e.g., positive ΔH and positive ΔS favor high temperatures).
Some thermodynamically favored reactions can be kinetically controlled, meaning they occur very slowly due to high activation energy. Gibbs Free Energy is also related to the equilibrium constant (K) by the equation ΔG = -RT ln K. A negative ΔG results in a large K (lots of product), while a positive ΔG results in a small K (few products).
The solubility of ionic compounds (like KCl) is determined by the balance between enthalpy and entropy changes. Even endothermic dissolution can occur if the entropy increase is significant. Unfavorable reactions can be driven by external energy (like electricity) or by coupling them with more favorable reactions to achieve an overall negative ΔG.
Galvanic cells, or batteries, are thermodynamically favored devices that harness electron flow from redox reactions. Oxidation occurs at the anode, and reduction at the cathode. Electrons flow from anode to cathode via a wire, and a salt bridge balances charge. The cathode's mass increases (CAT gets FAT), and cations move towards the cathode while anions move towards the anode.
Cell potential (voltage) is calculated from standard reduction potentials (cathode potential minus anode potential). ΔG = -nFE, where n is moles of electrons, F is Faraday's constant, and E is cell potential. Positive voltage indicates a favored cell. The Nernst Equation is used for non-standard conditions, showing how changes in concentration affect voltage. A galvanic cell at equilibrium, or a 'dead battery,' has a zero voltage.
Electrolytic cells involve non-favored processes that require an external power source, often used for plating metals. The equation current (amps) = charge (coulombs) / time (seconds) is used for these processes. Stoichiometry with Faraday's constant is used to convert charge to moles and then to grams of substance produced.