Thermochemistry Chapter 10

Energy Chemical potential energy (PE) Measuring energy Energy stored in bonds Unstable compounds have lots of PE More PE more reactive Measuring energy Experimentally using q=mcΔT and q=mΔH Theoretically using bond energy, heat of formation, energy diagrams

Heat vs. Temperature Heat ≠ Temperature Temperature: average KE of molecule Heat: amount energy exchanged due to temperature difference Heat depends on AMOUNT of substance Heat either given off or absorbed (lose or gain heat due to temp difference) Heat is directional (moves from area of hot to cold) ENERGY LOST = ENERGY GAINED

Calorimetry q = mcΔT q = heat in Joules (J) m = mass in grams (g) c = specific heat capacity J/g˚C ΔT= change in temp in ˚C or K At constant pressure, heat = change in enthalpy (ΔH) Enthalpy: total energy of system (KE + PE) Heat = Energy = Enthalpy (FYI: Used interchangeably)

Energy Measurement and conversions A joule is the SI unit of heat and energy A calorie (cal) is the quantity of heat that raises the temperature of 1 gram of pure water 1 degree Celsius. A food Calorie (Cal) is a kilocalorie (kcal). 1000 calories = 1 Calorie 1 Calorie = 1000 calories = 1 kcal = 4186 Joules Specific heat capacity of water: 4.18 J/g◦C or 1.00 cal/g◦C

Enthalpy Change in energy = change in enthalpy (at constant pressure) Enthalpy: measure of energy content of substance q = ΔH Endothermic ΔH > 0 Exothermic ΔH < 0 Reactants Products Energy Reaction Coordinate Activated Complex

Energy Diagram Change in energy is change in enthalpy Reaction Coordinate Activation Energy H < 0 Change in energy is change in enthalpy Reaction is exothermic Activation energy ALL reactions require activation energy!

Endothermic Reaction Energy Reaction Coordinate Activation Energy H > 0 Reactants ENDOTHERMIC REACTION Likelyhood of endothermic rxn taking place not likely (why?) Non-spontaneous Substances like to be in a lower energy state Exothermic rxns usually spontaneous

Summary/ Review Thermochemistry How does a reaction happen? Energy is added Activation energy! Used for BREAKING bonds between reactants Activated complex or intermediate formed New bonds formed that RELEASE energy Change in energy is enthalpy (ΔH)

Energy of Phase Changes A few definitions Melting point: temperature at which all energy absorbed causes change of state from solid to liquid Freezing point: same temperature as melting point causes change from liquid to solid Pure substances freezing and melting points are at the same temp, not true always for mixtures Boiling point: temperature at which all energy absorbed causes change of state from liquid to gas or gas to liquid (condensation point)

Heating Curve 1. Solid 2.Melting point 3.Liquid 4.Boiling point 5. Gas time Heating Curve 1 2 3 4 5

Phase Changes Changing temperature, calculating heat exchanged use: Q = mCΔt Remember C is dependent on phase (state of matter) Melting/ freezing/ boiling/etc, calculating heat exchanged use: Q = mΔH Units J/mol or J/gram M = mass or moles (depending on ΔH)

Other Vocabulary: Heat of Reaction: Hrxn heat produced in a chemical reaction Heat of Combustion: Hcomb heat produced by a combustion reaction Heat of Neutralization: neutral heat produced in a neutralization reaction (when an acid and base are mixed to get water, pH = 7) Heat of solution: Hsol heat produced by when something dissolves Heat of Fusion: Hfus heat produced when something melts Heat of Vaporization: Hvap heat produced when something evaporates Heat of Sublimation: Hsub heat produced when something sublimes Heat of formation: Hf change in enthalpy that accompanies the formation of 1 mole of compound from it’s elements