Enthalpy, Thermochemical Equations, and Combustion

Enthalpy and the First Law of Thermodynamics

• ΔU = q + w
• At constant pressure
• q = ΔH and W = -PΔV
• or ΔU = ΔH -PΔV (enthalpy change for a process at constant pressure)
• Enthalpy (H) is used to quantify heat flow into or out of a system in a process that occurs at constant pressure
• ΔH = H (products) - H (reactants)
• ΔH = heat given off or absorbed by the system at constant pressure

Thermochemical Equations

• The stoichiometric coefficients always refer to the number of moles of a substance

• If you reverse a reaction, the sign of ΔH changes

• If you mulitply both sides of an equation by some constant factor (m), then ΔH must be multiplied by that same factor (m)

• The physical states of all reactants and products must be specified in thermochemical equations