Enthalpy and Entropy

Also called

enthalpy · enthalpy change ΔH · exothermic · endothermic · calorimeter · coffee cup calorimeter · entropy · disorder · spontaneous process · free expansion into vacuum · mixing of gases · Gibbs energy · ΔG = ΔH − TΔS · spontaneity · एन्थैल्पी · एन्ट्रॉपी · गिब्स ऊर्जा

Topics

• Enthalpy change ΔH is the heat taken in or given out at constant pressure; a calorimeter measures it.
• Exothermic: ΔH < 0. Dissolving NaOH gives out heat, so the water and the thermometer warm up.
• Endothermic: ΔH > 0. Dissolving NH₄Cl takes in heat from the water, so the thermometer falls.
• Entropy S measures disorder: the number of ways the particles and their energy can be arranged.
• An ideal gas spreads into a vacuum with no heat change (ΔH = 0); the rise in entropy alone drives it.
• Two gases mix and never unmix by themselves: in an isolated system a spontaneous change has ΔS > 0.
• Gibbs energy decides: ΔG = ΔH − TΔS. A change is spontaneous when ΔG < 0 and at equilibrium when ΔG = 0.
• Ice melting has ΔH = +6.0 kJ mol⁻¹ and ΔS = +22 J K⁻¹ mol⁻¹, so ΔG < 0 only above 273 K.

About this model

A GLB file of about 458 KB, shown in Lably's 3D viewer and, on a phone, placed in your room with AR. Credit: Model, science and animation: Lably (built procedurally with three.js; no downloaded meshes). | CC BY 4.0. Original geometry and animation made for Lably.

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