Heat vs. Enthalpy, Hess's Law, Reaction Types, and Chemical Properties

Heat vs. Enthalpy

Enthalpy is a property of a system, sometimes termed "heat content." It represents the system's internal energy (U) plus the product of pressure and volume (pV).

Heat is energy in transit, transferred between systems. Enthalpy (H) is a state function defining the system's state.

Hess's Law and Enthalpy

Hess's law states that the total enthalpy change for a multi-step reaction equals the sum of enthalpy changes for each individual step. This principle, grounded in the conservation of energy, allows for calculating ΔH even when direct measurement is impossible. It enables the determination of standard enthalpies of formation, useful for designing complex syntheses.

Enthalpy and Reaction Types

Enthalpy determines whether a reaction is exothermic (heat-releasing) or endothermic (heat-absorbing). At constant pressure: an endothermic reaction absorbs heat (q > 0, ΔH > 0); an exothermic reaction releases heat (q < 0, ΔH < 0).

Types of Chemical Reactions

Acid-Base Reactions

Acids and bases react to produce water and a salt.

Precipitation Reactions

Cations and anions in aqueous solution form an insoluble solid precipitate.

Redox Reactions

Electrons transfer between elements. Reduction is gaining an electron; oxidation is losing an electron.

Physical vs. Chemical Properties

Physical properties are measurable without changing the substance's identity (e.g., color, odor, melting point, boiling point, density). Chemical properties require a chemical change to be observed (e.g., acidity, basicity, combustibility).