Thermochemistry and Thermodynamics

enthalpy (H)

the amount of heat energy contained within a system

endothermic reactions

heat transfer from the surroundings to the system

ΔH>0

exothermic reactions

heat transfer from system to surroundings

ΔH<0

what are the different phase changes?

• endothermic

  1. sublimation: solid → gas

  2. vaporization: gas → liquid

  3. melting: solid → liquid

• exothermic

  1. deposition: gas → solid

  2. condensation: gas → liquid

  3. freezing: liquid → solid

specific heat capacity (non phase change)

amount of energy reqired to raise the temperature of 1 gram of substance by 1 degree celsius

units: J/gC

• NOTE: specific heat of a molecule changes between its phase

• q=mCΔT → only applies when there is no phase change occuring

specific heat capacity (yes phase change)

q=mCΔH_fus/vap

• ΔH_fusion= heat to go from solid → liquid

• ΔH_vaporization= heat to go from liquid → gas

why does temperature stay constant during phase changes?

all the heat energy goes into altering the substance’s phase and not to raising the temperature

bomb calorimetry

amount of heat produced by a chemical reaction

• q_rxn=-q_cal

• q_cal=CΔT

• C= heat capacity of calorimeter (kJ/C)

standard enthalpy change

the enthaply change that occurs in a chemical reaction under standard conditions

1. bond breakage/formation: ΔH^o=ΣΔH^o _{bonds broken}-ΣΔH^o _{bonds formed}

2. enthalpies of formation: ΔH^o_rxn=Σ(n x ΔH^o_{f products})- Σ(m x ΔH^o_{f reactants})

3. Hess’s law: use enthalpy of intermediates get final value

entropy (S)

amount of disorder and randomness of a system (larger molecules have HIGHER entropy)

• higher s = lower energy = more stable

3rd law of thermodynamics

entropy of a pure susbatnce at absolute 0 temperature is zero → no motion or kinetic energy

how to calculate entropy change

1. ΔS_rxn > 0: entropically favorible → products favored/more disordered

2. ΔS_rxn < 0: entropically unfavorible → reactants favored/more disordered

which state of matter has the highest entropy?

gases (more disordered)

how does dissolution affect entropy?

more dissolution INCREASES entropy

how does temperature increase affect entropy?

entropy INCREASES with temperature INCREASE

how does an increase in molecules affect entropy?

INCREASE in entropy

how do spontaneous reactions affect entropy?

increase entropy

gibbs free energy (ΔG)

criteria for spontaneity

ΔG= ΔH-TΔS

units: (kJ/mol)

gibbs free energy in relation to spontaneity

ΔG > 0 = not spontaneous

ΔG < 0 = spontaneous

ΔG = 0 : equilibrium

how is K_eq related to ΔG