thermodynamics. every physical or chemical change is accompanied by energy change thermodynamics:...
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ThermodynamicsThermodynamics
EveryEvery physical or chemical physical or chemical change change is accompanied by energy changeis accompanied by energy change
• Thermodynamics: branch of chemistry that studies energy changes–specifically: changes in heat energy
ThermodynamicsThermodynamics
• Tells us if a reaction will occur
• 2 considerations:–enthalpy (heat energy)
–entropy (chaos/randomness)
Enthalpy, HEnthalpy, H
• enthalpy: heat content of system at constant pressure – symbol = H
Changes in Enthalpy are Changes in Enthalpy are measurablemeasurable
cannot measure enthalpy content of system directly
can measure changeschanges in enthalpy! symbol = H
H = Hfinal – Hinitial = Hproducts - Hreactants
Net gain in energy
• Endothermic ProcessEndothermic Process: energy absorbed
• Hfinal > Hinitial
so Hfinal – Hinitial results in positive value
H is positiveH is positive
Net loss in energy
• Exothermic ProcessExothermic Process: energy released
• Hfinal Hinitial
so Hfinal – Hinitial results in negative value
H is negativeH is negative[ see footnote to table I]
Energy of universe is conserved
UniverseUniverse
energy can move energy can move between system and between system and the environmentthe environment
EnvironmentEnvironment
System A
B
Which arrow represents an endothermic change?
? exothermic change AB
Change in Energy
choose how measure energy change –depends on how set up experiment
• monitor the system• monitor the environment*
* usually easier
Energy lost = Energy gained
• how do you know energy has moved?
• can measure energy gained or lost by environment – equals energy lost or gained by system
change in temperature!change in temperature!
source
reaction is carried out in water in styrofoam cup
temperature of water is monitored
water is the environment!water is the environment!cup is the universe!cup is the universe!
Q = mCQ = mCTT
• Q = energy change
• m = mass of water
• c = specific heat of water
• T = temperature change = Tf – Ti
Different kinds of Different kinds of HH’’ss
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 ReactionTable I: Heats of Reaction
• rxns #1-6: combustion rxns rxns #1-6: combustion rxns H: heat of combustion
• rxns #7-18: rxns #7-18: formation reactionsformation reactions
–substance is formed from its elements
H: heat of formation
• rxns #19-24: dissolving equations rxns #19-24: dissolving equations
H: heat of solution
Energy depends on amountEnergy depends on amount
• remember – 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
• burn 2 moles of CH4(g) with 4 moles of O2(g), get 2 times as much energy out
• Stoichiometry!
• (2)(890.4 kJ) = 1780.8 kJ is released
Phase Change: Energy depends on Phase Change: Energy depends on directiondirection
PEPE
Solid
Liquid
Gas
Up is endothermic
Down is exothermic
melting/fusion
boiling/ vaporization
sublimation
condensation
freezing
deposition
Reactions: Energy depends on Reactions: Energy depends on direction too!direction too!
• N2(g) + 3H2(g) 2NH3(g) H = -91.8 kJ
• 2NH3(g) N2(g) + 3H2(g) H = 91.8 kJ
If look at reverse reaction, then need to reverse sign of H
Thermochemical EquationsThermochemical Equations
• balanced chemical equations• show physical state of all reactants & products
• energy change can be given in 2 ways– energy term written as reactant or product
OR
– H is given right after equation
ExothermicExothermic Rxn: Rxn: energy = productenergy = product
4Fe(s) + 3O2(g) 2Fe2O3(s) H = -1625 kJ
OROR
4Fe(s) + 3O2(g) 2Fe2O3(s) + 1625 kJ
Exothermic
EndothermicEndothermic Rxn: Rxn: energy = reactantenergy = reactant
NH4NO3(s) NH4+(aq) + NO3
-(aq) H = 27 kJ
OROR
NH4NO3(s) + 27 kJ NH4+(aq) + NO3
-(aq)
Changes of StateChanges of State
H2O(s) H2O(l) Hfusion = 333.6 J/g at 0oC
H2O(l) H2O(s) H = -333.6 J/g at 0oCenergy is absorbed when water melts &energy is released when water freezes!
H2O(l) H2O(g) Hvapor = 2260 J/g at 100oC
H2O(g) H2O(l) H = -2260 J/g at 100oCenergy is absorbed when water evaporates &energy is released when water condenses!