Thermochemistry Exothermic reactions release heat to the surroundings. Fe 2 O Al  2 Fe + Al 2 O kJ Potassium Permanganate Reaction Demo Or ΔH = kJ Thermite Reaction Demo

Thermochemistry Endothermic reactions absorb heat from the surroundings. Ba(OH) NH 4 NO kJ  Ba(NO 3 ) NH H 2 O Barium Hydroxide Reaction Demo Or ΔH = kJ

[

Enthalpy Energy that is gained or lost by substances during a reaction. Symbolized by the letter H. [

Enthalpy Example: Given equal amounts (mass) of both, which produces more energy: Methane (Natural Gas!) or Octane (Gasoline!)

Enthalpy Combustion of Methane (Natural Gas!) CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O(g) ΔH = kJ (Exothermic!) 22

Enthalpy Taken from reaction ratio! Your furnace turns on and burns 32.0 g of methane, how much heat is produced? 32.0 g CH g CH 4 1 mol CH kJ = kJ CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O(g) ΔH = kJ

Enthalpy Combustion of Octane (Gasoline!) C 8 H 18 (l) + O 2 (g)  CO 2 (g) + H 2 O(g) ΔH = kJ (Exothermic!)

Enthalpy Taken from reaction ratio! Your car turns on and burns 32.0 g of octane, how much heat is produced? 32.0 g C 8 H g C 8 H 18 1 mol C 8 H 18 2 mol C 8 H kJ = -762 kJ 2 C 8 H 18 (l) + 25 O 2 (g)  16 CO 2 (g) + 18 H 2 O(g) ΔH = kJ

Enthalpy Methane produces more energy when burned…as long as mass is constant.

Enthalpy of Formation This is the amount of energy (enthalpy) that is involved in “creating” a compound. [

Enthalpy of Formation The “ΔH” given in a reaction is calculated by using these values in this equation:

Enthalpy of Formation Example: CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O (l) What is ∆H for this reaction? 1 st Step - Balance the Equation 22

CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O (l) Enthalpy of Formation 2 nd Step – Find Enthalpy Values Using Chart -75 kJ/mol kJ/mol -286 kJ/mol Any element that is “pure”, has an enthalpy of 0 kJ/mol

Enthalpy of Formation 3 rd – Plug into ΔH equation and solve: ΔH = [ (-286) + (-393.5)] – [(-74.8) + 0] Coefficient from balanced equation! ΔH = (H products ) – (H reactants ) 2

Enthalpy of Formation Final Answer: kJ/mol No significant digits needed since all the numbers are found on tables!

Hess’s Law A series of reactions can be added together to find their overall enthalpy. N 2 + O 2  2 NO 2 NO + O 2  2 NO 2 + N O 2  2 NO 2 ΔH = +181 kJ ΔH = -131 kJ ΔH = +68 kJ

Hess’s Law Example: C 2 H 2 + H 2  C 2 H 4 ΔH = kJ C 2 H 6  C 2 H 4 + H 2 ΔH = kJ What is the enthalpy for the net reaction: C 2 H H 2  C 2 H 6

C 2 H 2 + H 2  C 2 H 4 C 2 H 6  C 2 H 4 + H 2 + C 2 H H 2  C 2 H 6 ΔH = kJ ΔH = kJ ΔH = kJ If you cannot simply add to find the net reaction, “flip” a reaction to make it work! C 2 H 4 + H 2  C 2 H 6 ΔH = kJ