Ch 5: Thermochemistry

Nature of Energy (vocabulary) Thermodynamics: study of energy and its transformations Thermochemistry: study of relationships between energy changes involving heat and chem rxns Energy (E): capacity to do work or transfer heat Heat: E transferred between objects due to temp difference Joule (J): SI unit for heat Calorie (cal): informal unit of heat 1 cal = 4.18 J

1 st Law of Thermodynamics Also called the “ Law of Conservation of Energy ” Energy cannot be created or destroyed, it just changes between forms (ex. KE to PE) Energy transfers between system (what we are studying) to surroundings (everything else) Exothermic processes – heat is released Endothermic processes – heat is absorbed

Enthalpy (H) Enthalpy - heat gained or lost between the system and the surroundings Enthalpy is a state function (depends only on the present state of system, not the path it took to reach its present state) Change in enthalpy (ΔH) Positive for endothermic process Negative for exothermic process

Enthalpies of Reaction ΔH is usually expressed in kJ or kJ/mol Consider the combustion of methane CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g) …. ΔH = kJ Is this rxn endo or exo? Exo (ΔH is negative) How many kJ of heat are absorbed when 25.0g of methane burn in air? (next slide)

CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g) …. ΔH = kJ 1. Convert grams CH 4 to moles 2. Find ratio of moles CH 4 to kJ from balanced equation 3. Solve for kJ

CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g) …. ΔH = kJ 1. FM of methane = 16.0 g/mol 25.0g / 16.0 g per mol = 1.56 mol of methane 2. Ratio is 1 mol CH4 to – 802 kJ per mo 1.56 mol * kJ 1 1 mol 3. Answer = kJ

How many kJ of heat are produced when 200.g of sulfur dioxide reacts completely with oxygen to produce sulfur trioxide? 2SO 2 (g) + O 2 (g)  2SO 3 (g) … ΔH = kJ Follow same steps as previous problem Pay attention to ratio of moles sulfur dioxide to kJ (next click gives answer) Answer = 308 kJ

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