Chemistry Week 7

Thermodynamics

• Branch of physical science that deals with how heat and other forms of energy

Forms of Energy

• Thermal energy

• Radiant energy

• Electric energy

• Nuclear energy

• Chemical energy

Law of Conservation of Energy

• Energy can’t be created nor destroyed

Potential Energy

• Energy stored in an object

Kinetic Energy

• Energy due to movement motion

• Formula: KE= ½mv², where m is mass and v is velocity

Joule

• A unit of energy

• Equal to 0.2390 calories (cal)

• Formula: kgm2/s

First Law of Thermodynamics

• Energy of the universe if constant

• Formula: Uuniv = ΔUsys + ΔUsurr = 0

• Formula rearrangement: ΔUsys = − ΔUsurr

• Uuniv = Internal energy of the universe

  Usys = Internal energy of the system

  Usurr = Internal energy of the surroundings

Open System

• Exchanges matter and energy

• Formula: ΔUsys = q + w

• q= heat

• w= work

Closed System

• Exchanges energy but not matter

• Formula: ΔUsys = q + w

• q= heat

• w= work

Isolated System

• Doesn’t truly exist, just theoretical

• Doesn’t exchange energy or matter

• Formula: ΔUsys = q + w

• q= heat

• w= work

Exothermic Reaction

• Heat flows out of the reaction and into the surroundings

• q<0

• Ex: Combustion of gasoline (octane)

  2 C8H18 + 25 O2 turns into 16 CO2 + 18 H2O + heat

ΔHrxn = −5430 kJ/mol

• ΔH < 0

Enthalpy

• Total heat of a system

• Formula: H = U + PV with P being pressure and V being volume

Endothermic Reaction

• Heat flows into reaction

• q>0

• Ex: Dissolution of NH4Cl in water (Ice packs)

  heat + NH4Cl turns into NH4+ + Cl-

ΔHrxn = + 14.78 kJ/mol

• ΔH > 0

Enthalpy

• Bond breaking requires energy input

• Bond forming releases energy

• Extensive property meaning it depends on the amount of substance present

• Same as heat

Types of Enthalpy

• ΔHcomb = combustion

• ΔHfus = melting/freezing

• ΔHvap = boiling/condensing

• ΔHsoln = dissolution

Calorimetry

• The amount of heat releases or absorbed during a chemical reaction

Heat Capacity

• (C)

• The amount of energy needed to raise the temperature of an object by 1 degree celsius

• Fomrula: ΔT = q/C

Specific Heat Capacity (Cs)

• The amount of energy needed to raise the temperature of 1g of an object by 1 degree celsius

• Fomrula: q = mCsΔT

• When two objects come into contact with each other heat flows from hot to cold

• Formula: qcold = -qhot or [mCsΔT]cold + [mCsΔT]hot = 0

Hess Law

• Delta H of Net Reaction = Sum of Delta H for the individual reaction

• Makes reactions reversible as long as the sign of delta H is changed when the reaction reverses

• If reaction is multiplied by a factor Delta H is then multiplied by the same factor

Heat

• The transfer of thermal energy between objects

• Same as enthalpy

Work

• Something changing chemically

• Ex: Changing form, gas to liquid

• Don't worry about this for purposes of chem 101