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!