S AM W ICKERT, E MILY S ZABO, M IKEY M ILICI THERMOCHEMICAL EQUATIONS

W HAT ARE THERMOCHEMICAL EQUATIONS ? Themochemical Equations are chemical equations that include the enthalpy change of a reaction. CaO(s) + H 2 O(l) → Ca(OH) 2 (s) kJ (Heat is given off, an exothermic Reaction) Enthalpy is the heat content of a system at constant pressure. The heat released or absorbed by a reaction at constant pressure is the same as the change in enthalpy (∆H).

H EAT OF R EACTION The heat of reaction is the enthalpy change for the chemical equation exactly as it is written. These are usually reported as ∆H, which is equal to the heat flow at constant pressure. CaO(s) + H 2 O(l) → Ca(OH) 2 (s) ∆H = kJ (Heat is given off, an exothermic Reaction) Standard conditions are that the reaction is carried out at kPa and that the reactants and products are in their usual physical states at 25˚C.

E NDOTHERMIC T HERMOCHEMICAL E QUATIONS Along with the previous examples of Exothermic thermochemical equations, other thermochemical equations may absorb heat from the surroundings. 2NaHCO 3 (s) kJ → Na 2 CO 3 (s) + H 2 O(g) + CO 2 (g) Remember that ∆H is positive for endothermic reactions, therefore the above reaction can be written as: 2NaHCO 3 (s) → Na 2 CO 3 (s) + H 2 O(g) + CO 2 (g) ∆H = 129 kJ

H EAT OF C OMBUSTION The heat of combustion is the heat of reaction for the complete burning of one mole of a substance. Example: burning 1 mol of methane releases 890 kJ of heat. See full list of most common on page 570. Substance∆H (kJ/mol) Hydrogen-286 Carbon-394 Methane-890 Acetylene-1300