Thermodynamics
Every physical/chemical change is accompanied by change in energy Thermodynamics: branch of chemistry that studies energy changes specifically: changes in heat energy
Tells us if a reaction will occur 2 considerations: Thermodynamics Tells us if a reaction will occur 2 considerations: enthalpy (heat energy) entropy (chaos/randomness)
Enthalpy, H enthalpy: heat content of system at constant pressure use symbol: H
changes in Enthalpy are measurable cannot measure enthalpy content of system directly can measure changes in enthalpy! symbol = H H = Hfinal – Hinitial = Hproducts - Hreactants
net gain in energy Endothermic Process: energy absorbed Hfinal > Hinitial so Hfinal – Hinitial result: H is positive
net loss in energy Exothermic Process: energy released Hfinal Hinitial so Hfinal – Hinitial result: H is negative [ see footnote to table I]
energy of universe is conserved Which arrow represents an endothermic change? ? exothermic change B environment A energy can move between system and the environment system A B
Change in Energy choose how measure energy change depends on how set up experiment monitor the system monitor the environment (this is easier)
Energy lost = Energy gained how do you know energy has moved? can measure energy gained/lost by environment equals energy lost/gained by system change in temperature!
reaction is carried out in water in styrofoam cup - cup is the universe! temperature of water is monitored - water is the environment!
Q = mCT Q = energy change m = mass of water c = specific heat of water T = temperature change = Tf – Ti
different types of H’s H of dissolving: heat of solution H of phase change: heat of fusion/heat of vaporization H of reaction: heat of reaction categorized by rxn type
Table I: Heats of Reaction rxns #1-6: combustion rxns H: heat of combustion rxns #7-18: formation reactions (synthesis) H: heat of formation rxns #19-24: dissolving equations H: heat of solution
energy depends on amount example: it takes more energy to heat up water in bathtub than to make a cup of tea
CH4(g) + 2O2(g) CO2(g) + 2H2O (l) H = -890.4 kJ 1 mole of methane + 2 moles of oxygen → 1 mole of carbon dioxide gas & 2 moles of liquid water reaction is exothermic (negative sign for ΔH) 890.4 kJ energy released per mole of CH4(g) burned
Energy depends on amount CH4(g) + 2O2(g) CO2(g) + 2H2O (l) H = -890.4 kJ burn 2 moles of CH4(g) with 4 moles of O2(g), get 2 times as much energy out remember stoichiometry! (2)(890.4 kJ) = 1780.8 kJ is released
Phase Change: Energy depends on direction endothermic exothermic gas melting/fusion boiling/ vaporization sublimation condensation freezing deposition PE liquid solid
Reactions: Energy depends on direction too! N2(g) + 3H2(g) 2NH3(g) H = -91.8 kJ If look at reverse reaction, then need to reverse sign of H 2NH3(g) N2(g) + 3H2(g) H = 91.8 kJ
Thermochemical Equations balanced chemical equations show physical state of all reactants & products energy change can be given in 2 ways 1. energy term written as reactant or product OR 2. H is given right after equation
Exothermic Rxn: energy = product 4Fe(s) + 3O2(g) 2Fe2O3(s) H = -1625 kJ OR 4Fe(s) + 3O2(g) 2Fe2O3(s) + 1625 kJ exothermic if (-)
Endothermic Rxn: energy = reactant NH4NO3(s) NH4+1(aq) + NO3-1(aq) H = 27 kJ OR NH4NO3(s) + 27 kJ NH4+1(aq) + NO3-1(aq) endothermic if (+)
Changes of State H2O(s) H2O(l) Hf = 334 J/g at 0oC H2O(l) H2O(s) Hf = -334 J/g at 0oC energy is absorbed when water melts & energy is released when water freezes! H2O(l) H2O(g) Hv = 2260 J/g at 100oC H2O(g) H2O(l) Hv = -2260 J/g at 100oC energy is absorbed when water evaporates & energy is released when water condenses!