enthalpy

heat (new definition)

change in thermal energy of a substance

enthalpy

• H

• total heat content in a system (at constant pressure)

• therefore the difference of thermal energy from reactants to products

ΔH

• change in enthalpy

• measures the energy absorbed or released during a chemical reaction

• H_reactants - H_products

Q vs. ΔH

• both represent energy transfer

• Q measures total heat added or removed in any process

• ΔH heat of reaction (energy per mole) at constant pressure, to determine if endothermic or exothermic

• at constant pressure, Q_rxn = ΔH_rxn

additive reaction enthalpies

• uses Hess’s law

• states that the enthalpy change of a physical or chemical process depends only on reactants and products

• enthalpy change is independent of the pathway of the process and number of steps in the process

• enthalpy change will be the same even if more steps are taken to get to the same product

summation of enthalpies

• hess’s law

• for any chemical change in several steps, net ΔH = sum of ΔH of separate steps

• total enthalpy change for a chemical reaction is the same, regardless of whether the reaction occurs in one step or several steps

standard molar enthalpy of formation

• quantity of energy associated with the formation of one mole of a substance from its elements in their standard states (at SATP)

• ΔH°_f

• units: kJ/mol