ThermochemistryThermochemistry

OverviewOverview

• Entropy & Second Law ThermodynamicsEntropy & Second Law Thermodynamics

• Predicting SpontaneityPredicting Spontaneity

• Free EnergyFree Energy

Entropy & Second Law Entropy & Second Law ThermodynamicsThermodynamics

• Second Law of Thermodynamics:Second Law of Thermodynamics: Reactions proceed Reactions proceed in the direction that increases the in the direction that increases the entropyentropy of the system of the system plus surroundings.plus surroundings.

• A A spontaneous processspontaneous process is one that proceeds on its own is one that proceeds on its own without any continuous external influence.without any continuous external influence.

• A A nonspontaneous processnonspontaneous process takes place only in the takes place only in the presence of a continuous external influence.presence of a continuous external influence.

Entropy & Second Law Entropy & Second Law ThermodynamicsThermodynamics

• The measure of molecular disorder in a system The measure of molecular disorder in a system is called the system’s is called the system’s entropyentropy; this is denoted ; this is denoted SS..

• Entropy has units of J/K (Joules per Kelvin).Entropy has units of J/K (Joules per Kelvin).

SS = = SSfinalfinal – – SSinitialinitial

– PositivePositive value of value of SS indicates indicates increased disorderincreased disorder..

– NegativeNegative value of value of SS indicates indicates decreased disorderdecreased disorder..

Entropy & Second Law Entropy & Second Law ThermodynamicsThermodynamics

Predicting SpontaneityPredicting Spontaneity

• To decide whether a process is spontaneous, both To decide whether a process is spontaneous, both enthalpy and entropy changes must be considered:enthalpy and entropy changes must be considered:

• Spontaneous process:Spontaneous process: Decrease in enthalpy (–Decrease in enthalpy (–

HH).).

Increase in entropy (+Increase in entropy (+SS).).

• Nonspontaneous process:Nonspontaneous process: Increase in enthalpy (+Increase in enthalpy (+HH).).

Decrease in entropy (–Decrease in entropy (–SS).).

Predicting SpontaneityPredicting Spontaneity

• Predict whether Predict whether SS° is likely to be positive or ° is likely to be positive or negative for each of the following reactions. negative for each of the following reactions. Using tabulated values, calculate Using tabulated values, calculate SS° for each:° for each:

– a. 2 CO(a. 2 CO(gg) + O) + O22((gg) ) 2 CO 2 CO22((gg))

b. 2 NaHCOb. 2 NaHCO33((ss) ) Na Na22COCO33((ss) + H) + H22O(O(ll) + CO) + CO22((gg))

c. Cc. C22HH44((gg) + Br) + Br22((gg) ) CH CH22BrCHBrCH22Br(Br(ll))

d. 2 Cd. 2 C22HH66((gg) + 7 O) + 7 O22((gg) ) 4 CO 4 CO22((gg) + 6 H) + 6 H22O(O(gg))

Free EnergyFree Energy

• Gibbs Free Energy Change (Gibbs Free Energy Change (GG):): Weighs the Weighs the relative contributions of relative contributions of enthalpyenthalpy and and entropyentropy to the to the overall spontaneity of a process.overall spontaneity of a process.

GG = = HH – – TTSS

GG < 0 < 0 Process is spontaneousProcess is spontaneous

GG = 0 = 0 Process is at equilibriumProcess is at equilibrium

GG > 0 > 0 Process is nonspontaneousProcess is nonspontaneous

Free EnergyFree Energy

• Situations leading to Situations leading to GG < 0: < 0: HH is negative and is negative and TTSS is positive is positive HH is very negative and is very negative and TTSS is slightly negative is slightly negative HH is slightly positive and is slightly positive and TTSS is very positive is very positive

• Situations leading to Situations leading to GG = 0: = 0: HH and and TTSS are equally negative are equally negative HH and and TTSS are equally positive are equally positive

• Situations leading to Situations leading to GG > 0: > 0: HH is positive and is positive and TTSS is negative is negative HH is slightly negative and is slightly negative and TTSS is very negative is very negative HH is very positive and is very positive and TTSS is slightly positive is slightly positive

Free EnergyFree Energy

• Which of the following reactions are spontaneous Which of the following reactions are spontaneous under standard conditions at 25°C?under standard conditions at 25°C?

– a. AgNOa. AgNO33((aqaq) + NaCl() + NaCl(aqaq) ) AgCl( AgCl(ss) + NaNO) + NaNO33((aqaq) )

GG° = –55.7 kJ° = –55.7 kJ

– b. 2 C(b. 2 C(ss) + 2 H) + 2 H22((gg) ) C C22HH44((gg))

GG° = 68.1 kJ° = 68.1 kJ

– c. Nc. N22((gg) + 3 H) + 3 H22((gg) ) 2 NH 2 NH33((gg) )

HH° = –92 kJ; ° = –92 kJ; SS° = –199 J/K° = –199 J/K

Free EnergyFree Energy

• Equilibrium (Equilibrium (GG° = 0):° = 0): Estimate the Estimate the temperature at which the following reaction will temperature at which the following reaction will be at equilibrium. Is the reaction spontaneous at be at equilibrium. Is the reaction spontaneous at room temperature?room temperature?

– NN22((gg) + 3 H) + 3 H22((gg) ) 2 NH 2 NH33((gg) )

HH° = –92.0 kJ° = –92.0 kJ SS° = –199 J/K° = –199 J/K

– Equilibrium is the point where Equilibrium is the point where GG° = ° = HH° – ° – TTSS° = 0° = 0