Enthalpy

EnthalpyChapter 5: Energetics and ThermodynamicsIB Standard Level ChemistryEnthalpyYou know that enthalpy (H) is the heat or energy content in a system.A reactions reactants and products also have enthalpy.H = Hproducts Hreactants(i.e. final initial)

This is usually expressed as an overall enthalpy change, H.

SystemSurroundingsEnthalpy Changes: FormationHf is the energy change on forming a substance from its individual partsNa(s) + Cl2(g) -> NaCl(s)

+->Enthalpy Changes: CombustionHc is the energy change on completely combusting a substance2H2(s) + O2(g) -> 2H2O(s)

+->

+Enthalpy ChangesAlways include state symbols for enthalpy change equations

H can be positive or negativeIf H < 0, the products have less energy than the reactantsIf H > 0, the products have more energy than the reactants

Enthalpies of ReactionWhen H is positive, the reaction is endothermicThe system takes in energy, and the surroundings lose energyA thermometer will detect a decrease in energy

When H is negative, the reaction is exothermicThe system loses energy, and the surroundings gain energyA thermometer will detect an increase in temperature

QuestionUsing the values from the Data Booklet, write a formula for the combustion of hydrogen, along with its enthalpy value. State if it is endothermic or exothermic.