Heat vs. Enthalpy, Hess's Law, Reaction Types, and Chemical Properties
Classified in Chemistry
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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).