8.4 Thermochemical Equations Pages

Thermochemical Equations A thermochemical equation is a balanced chemical equation that includes the physical states of all reactants and products, and energy changethermochemical equation In a thermochemical equation, the enthalpy of change for the reaction can be written as either a reactant or a product

Thermochemical Equations How to write thermochemical equations: Endothermic (positive ΔH) 2NaHCO 3 (s)+ 129kJ Na 2 CO 3 (s) + H 2 O(g) + CO 2 (g) Exothermic (negative ΔH) CaO(s) + H 2 O(l) Ca(OH) 2 (s) kJ

Write the thermochemical equation for the addition of oxygen gas to solid iron (III) if the ΔH rxn = kJ Fe(s) + O 2 (g)→ Fe 2 O 3 (s) kJ Fe(s) + O 2 (g)→ Fe 2 O 3 (s) kJ Write the thermochemical equation for the decomposition of sulfur trioxide gas into sulfur dioxide gas and oxygen gas if the ΔH rxn = 198 kJ SO 3 (g) kJ → SO 2 (g) + O 2 (g) SO 3 (g) kJ → SO 2 (g) + O 2 (g) 4 32 B. Thermochemical Equation Practice 22 Exothermic Endothermic

Three Rules of Thermochemistry The magnitude of  H is directly proportional to the amount of reactant or product The  H for a reaction is equal in magnitude, but opposite in sign to  H for the reverse reaction The value of  H for a reaction is the same whether it occurs in one step, or in a series of steps (Hess’s Law)

Hess’s Law: Significance Hess’s law makes it possible to figure out the  H of a reaction that normally would be difficult to measure in a calorimeter. C(s) O 2 (g)  CO(g)  H = ? Germain Hess - Published law in 1840 in St. Petersburg C(s) + O 2 (g)  CO 2 (g)  H = kj 2CO(s) + O 2 (g)  2CO 2 (g)  H = kj Can rearrange these two equations to solve  H for first equation

HH

Homework #73 Read pages Problems 21, 23, 34, 35 Due Friday

Hess’s Law Practice #1 Known: N 2 (g) + O 2 (g)  2NO(g)  H = -180 kJ 2NO(g) + O 2 (g)  2NO 2 (g)  H = -112 kJ N 2 (s) + 2O 2 (g)  2NO 2 (g)  H = ? Build your equation:

Hess’s Law Practice #2 Known: 2H 2 (g) + O 2 (g)  2H 2 O(l)  H = -572 kJ H 2 (g) + O 2 (g)  H 2 O 2 (l)  H = -188 kJ Build your equation: 2H 2 O 2 (l)  2H 2 O(l) + O 2 (g)  H = ?

Hess’s Law Practice #3 Known: S(s) + O 2 (g)  SO 2 (g)  H = -297 kJ 2SO 3 (g)  2SO 2 (g) + O 2 (g)  H = 198 kJ 2S(s) + 3O 2 (g)  2SO 3 (g)  H = ? Build your equation:

Hess’s Law Practice #4 Known: 2B (s) + 3/2O 2 (g)  B 2 O 3 (s)  H = kJ B 2 H 6 (g) + 3O 2 (g)  B 2 O 3 (s) + 3H 2 O (g)  H = kJ H 2 (g) + ½ O 2 (g)  H 2 O (l)  H = -286 kJ H 2 O (l)  H 2 O (g)  H = 44 kJ 2B(s) + 3H 2 (g)  B 2 H 6 (g)  H = ? Build your equation: