Thermodynamics
Definition
Thermodynamics is the science of the relationships between heat and other forms of energy.
Thermochemistry
Thermochemistry is the study of the quantity of heat energy absorbed or evolved during physical or chemical changes.
It is also called energetics and is largely based on the first law of thermodynamics.
Enthalpy Change
Definition
The sum of all the possible, potential and kinetic energies of a system is called its heat content or enthalpy denoted by H.
Mathematical Relationship
ΔH = (Heat content of products, HP) – (Heat content of reactants, HR)
Exothermic vs Endothermic Reactions
| Property | Exothermic | Endothermic |
|---|---|---|
| Heat Transfer | System to surroundings | Surroundings to system |
| ΔH Sign | Negative (-) | Positive (+) |
| Temperature | Increases | Decreases |
| Examples | Combustion, Neutralization | Photosynthesis, Melting |
Energy Profile Diagrams
Activation Energy
The minimum amount of energy required by reactant molecules to cross the energy barrier is called activation energy (Ea).
Standard Enthalpy Changes
Standard conditions: Temperature = 25°C (298 K), Pressure = 1 atm
Standard enthalpy of reaction (ΔHr): Enthalpy change when stoichiometric amounts of reactants in standard states react completely.
Standard enthalpy of combustion (ΔHc): Enthalpy change when one mole of substance is completely burnt in excess oxygen.
Standard enthalpy of formation (ΔHf): Enthalpy change when one mole of compound is formed from its elements.
Standard enthalpy of neutralization (ΔHn): Enthalpy change when one mole of water is formed by acid-base reaction.
Hess’s Law of Heat Summation
Statement
The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same.
Applications
1. Calculating enthalpy of formation using enthalpy of combustion data
2. Calculating enthalpy change of reaction using enthalpies of formation
3. Calculating bond energies
Key Formula: ΔHreaction = ΣΔHf(products) – ΣΔHf(reactants)
Entropy and Free Energy
Entropy (S)
Entropy is a measure of the number of ways energy can be distributed within a system at a specific temperature.
It is a measure of randomness or disorder in a system.
Gibbs Free Energy (G)
ΔG = ΔH – TΔS
Where: ΔG = Gibbs free energy change, ΔH = enthalpy change, T = temperature (K), ΔS = entropy change
Spontaneity Criteria
ΔG < 0: Reaction is spontaneous
ΔG > 0: Reaction is non-spontaneous
ΔG = 0: System is at equilibrium