Notes

Outline

Thermochemistry Part 1-Energy Exchanges and Calorimetry

Forms of Energy Kinetic Energy-Energy of motion(Thermal, Electrical. Magnetic,etc) KE = ½(mass)(velocity)2 = ½ mv2 Examples-moving object; molecular motion; electron motion Example Problem Potential Energy-Energy of Position PE = (mass)(acceleration due to gravity)(distance from reference point)= mgh Examples- coiled spring; energy locked within molecules

Relationship Between Kinetic and Potential Energy Total Energy = KE + PE KE--à PE Reactions that absorb kinetic energy to produce new product molecules PE -à KE Chemical Reaction that produces energy from product formation

Units of Energy Calorie(cal)- amount of energy required to raise 1 gram of water by 1 degree Celsius Joule(j) 1 calorie = 4.18 Joules Kcalorie (Kcal) 1000 calories = 1 Kcal = 4180 j Kilojoule (Kj) 1 Kj = 1000 j

T-6.4 Relationship Between A Thermodynamic System and Its Environment

Thermal Exchanges Between System and Environment Endothermic- Energy enters the System from the Environment(ie: absorbed by System) Energy taken from environment lowering its temperature Temperature lowering indicative of a reaction system that is endothermic Exothermic- Energy is given off by or exits the System to the environment Energy is added to the environment increasing its temperature Temperature increasing indicative of a reaction system that is exothermic.

Heats of reaction(Enthalpy Change per mole) Heats of Reaction ( DH )- The energy exchanged by a reaction system and its environment Types of Heats of Reaction                      ( D H = Hf – Hi) Heat of solution Heat of sublimation Heat of Combustion Heat of Compound Formation

Heats of Reaction And Energy Exchange If D H > 0 (ie:positive) Then reaction is endothermic Product Energy State (enthalpy) > reactant energy state If  D H <0 (ie:negative) Then reaction is exothermic Product energy state < Reactant energy state

T-46 Enthalpy Diagrams Endo vs Exo

T-6.8 Enthalpy Diagram

Thermochemical Equations and D H Thermochemical Equation- balanced equation with an energy term included A   +   B   --à   C   +   D   + energy Exothermic reaction with D H < 0 A   +   B   +   energy --à C   +   D Endothermic reaction with D H > 0 If the equation is multiplied by a common factor then the D H is also multiplied by the same factor If equation is reversed then D H changes sign

T-47 Reversing A Chemical Reaction

Mol-Energy Relationships In A Thermochemical Equation 2H2  +  O2  -à  2H2O   DH = -483.7 Kj 2 mols H2 = -483.7 Kj 1 mol O2 = -483.7 Kj 2 mols H2O = -483.7 Kj

Determining Energy Exchange From A Mass of Component Involved In The Reaction Thermo chemical Stoichiometry Problems                           1                       2 Grams of given -à mols of given-à Kj of requested Grams of given  x  1 mol given / molecular mass in grams = mols of given Mols of given  X  D H / 1 mol given = Kj energy requested Example

Heat Flow Equation Heat Flow = q = (mass of system) (specific heat of system) (final temp – Initial Temp) Q = mcD t = mc(tf – ti) Example