Grade 12 Chemistry SCH4U0 Mr. Krstovic Thermochemistry Grade 12 Chemistry SCH4U0 Mr. Krstovic

What did we learn so far? Heat is energy transferred from one place to another Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. Calorimetry is the science of measuring the heat of chemical reactions or physical changes.

What did we learn so far? Q = mcT A calorimeter is a device used for calorimetry. Q = mcT Specific heat capacity (c) has units of J/gC Difference in heat capacity of oil and water (recall the demo in class)

Remember these three processes?

Exothermic and Endothermic Processes If heat is transferred from surrounding to system. Process is endothermic. Surroundings feel COOLER! If heat is transferred from system to surroundings. Process is exothermic. Surroundings feel HOTER!

Enthalpy ΔHsystem = - Qsurrounding Chemists give a special symbol, ΔH(delta H) to the heat change in a reaction This heat change is called ENTHAPLY ΔH (enthalpy) = energy absorbed or released to the surroundings when a system changes from reactants to products. ΔHsystem = - Qsurrounding

Potential Energy Diagrams

Molar Enthalpy ΔHx: the enthalpy change for a physical, chemical or nuclear change involving 1 mol of a substance [Units: kJ/mol] Types of Molar Enthalpies: ΔHvap ΔHsol ΔHrx ΔHfr ΔHcomb (See Table1 p.306)

Types of Molar Enthalpies Example: ΔHcomb CH4(g) + 2 O2(g) ® CO2(g) + 2 H2O(l) ΔHcomb = - 802 kJ/mol

Enthalpies of Reaction What is DH for the combustion of 11.0 g of CH4 in excess oxygen? 11.0 g CH4 = -550 kJ

Example Question In a calorimetry experiment, 7.46 g of KCl is dissolved in 100.0 mL of water at an initial T of 24.1 C. The final T of the solution is 20C. What is the molar enthalpy of solution of KCl?

Methods of Communicating Enthalpy Changes Method 1: Energy value as part of the reaction H2O (l) + 285.8 kJ  H2 + 1/2O2 Mg (s) + 1/2O2 (g)  MgO(s) + 601.6 kJ

Method 2 Thermochemical Equations with ΔH values next to the reaction H2O (l)  H2 + 1/2O2 ΔH = +285.8 kJ Mg (s) + 1/2O2 (g)  MgO(s) ΔH = - 601.6 kJ

Method 3 Standard Molar Enthalpy of Reaction measured at SATP conditions (100 kPa; 25 C) ΔHorx - this symbol distinguishes standard molar enthalpies from molar enthalpies ΔHocomb = -726 kJ/mol CH3OH

Method 4 Potential Energy Diagrams