Thermochemical Equations

1. Thermochemical Equations Sam Wickert, Emily Szabo, MikeyMilici

2. What are thermochemical equations? • Themochemical Equations are chemical equations that include the enthalpy change of a reaction. CaO(s) + H2O(l) → Ca(OH)2(s) + 65.2 kJ (Heat is givenoff, 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).

3. Heat of Reaction • 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) + H2O(l) → Ca(OH)2(s) ∆H = -65.2 kJ (Heat is given off, an exothermic Reaction) • Standard conditions are that the reaction is carried out at 101.3 kPa and that the reactants and products are in their usual physical states at 25˚C.

4. Endothermic Thermochemical Equations • Along with the previous examples of Exothermic thermochemical equations, other thermochemical equations may absorb heat from the surroundings. 2NaHCO3(s) + 129 kJ → Na2CO3(s) + H2O(g) + CO2(g) • Remember that ∆H is positive for endothermic reactions, therefore the above reaction can be written as: 2NaHCO3(s) → Na2CO3(s) + H2O(g) + CO2(g) ∆H = 129 kJ

5. Heat of Combustion • 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.