AP Chemistry



AP Chemistry 6: Thermodynamics Name __________________________

A. Enthalpy (H): Bond Energy (5.3 to 5.5, 8.8)

1. chemical reactions typically involve breaking bonds between reactant atoms and forming new bonds

2. breaking bonds takes energy ∴ chemical system gains bond energy; surroundings lose energy (heat, etc.)

3. forming bonds releases energy ∴ chemical system loses energy, surroundings gain energy

4. change in energy called “change in enthalpy”—ΔH

a. when energy required to break bonds > energy released to form new bonds, +ΔH (endothermic)

1. products at a higher energy state than reactants (weaker bonds)

2. surroundings lose energy (cool down)

b. when energy required to break bonds < energy released to form new bonds, –ΔH (exothermic)

1. products at a lower energy state than reactants (stronger bonds)

2. surroundings gain energy (heat up)

5. thermochemical equation

a. chemical equation with ΔH

1. listed to the right of equation

2. included as reactant (endothermic) or product (exothermic)

b. ΔH can be used in dimensional analysis process

6. ΔH from calorimetry

a. reactants are put in an insulated container filled with water, where heat is exchanged between reactants and water, but no heat is lost

b. by conservation of energy: ΔHreaction = –Qwater

1. Q = mcΔT for simple coffee cup calorimeter—aqueous reactions

a. m = mass of water

b. c = specific heat of water (4.18 J/g•K)

c. ΔT = change in temperature (Tf – Ti)

temperature can stay in oC, since

1 oC = 1 K (don't add 273 to ΔToC!)

2. Q = (C + mc)ΔT for “bomb" calorimeter

a. C = “bomb constant” accounts for all non-water components that change temperature

b. all other letters are the same as the simple calorimeter

7. ΔH using bond energy (B.E.) data

|Bond Energies in (kJ/mol) |

|Single |Multiple |

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B. Entropy (S): Disorder (19.2)

1. atoms/molecules have inherent disorder depending on

a. number of atoms—more internal motion = disorder

b. spacing of molecules—farther apart = disorder

c. speed of molecules—faster = disorder

2. predict increase in disorder for physical changes (+ΔS)

a. spread out: evaporation, diffusion and effusion

(solution: spread out solute and solvent (+ΔS), but bond solute-solvent (-ΔS) ∴ ?, but usually +ΔS)

b. motion: melting and boiling

3. predict increase in disorder for chemical changes (+ΔS): moles gaseous products > moles gaseous reactants

C. Thermodynamic Data (5.6 to 5.7, 19.4)

|Species |ΔHfo (kJ/mol) |So (kJ/mol•K) |

|Al |0.0 |+0.0283 |

|Al3+ |-531.0 |-0.3217 |

|Al2O3 |-1675.7 |+0.0509 |

1. standard heat of formation (ΔHfo) data

a. ΔHo for the formation of one mole of compound from its elements at standard temperature (25oC)

Al: Al(s) → Al(s) ∴ no reaction

Al3+: Al(s) → Al3+ + 3 e-

Al2O3: 2 Al(s) + 3/2 O2(g) → Al2O3(s)

b. ΔHfo for elements in natural state = 0.0

c. more negative = more stable (harder to decompose)

2. standard entropy (So) data

a. amount of disorder compared to H+ (simplest form of matter), which is zero by definition

b. listed in J/mol•K on AP exam, so you will have to convert to kJ/mol•K for most calculations

3. calculations using the thermodynamic data chart

a. altering ΔHfo

1. opposite sign for the reverse reaction

C + 2 Cl2 → CCl4 = –139.4 kJ

∴ CCl4 → C + 2 Cl2 = +139.4 kJ

2. multiply by number of moles (coefficient)

1 mole CCl4= –139.4 kJ

∴ 2 mole CCl4 = –278.8 kJ

b. calculate ΔH for a reaction using ΔHfo

1. Hess’s Law: ΔH for a multi-step reaction equals the sum of ΔH for each step

| CH4(g) → C + 2 H2 |-(-74.8) |

| C + O2 → CO2(g) |-393.5 |

|+ 2 H2 + O2 → 2 H2O(g) |2(-241.8) |

|CH4(g) + 2 O2 → CO2(g) + 2 H2O(g) -802.3 |

2. ΔH ≈ ΔHo = ΔHfoproducts – ΔHforeactants

c. calculate ΔS for a reaction using So

ΔS ≈ ΔSo = Soproducts – Soreactants

D. Gibbs Free Energy (G): Overall Energy State (19.5 to 19.6)

1. combination of enthalpy and entropy: G = H + TS

2. for a chemical or physical change: ΔGo = ΔHo – ToΔSo

a. To = 298 K

b. where T ≠ 298 K: ΔG ≈ ΔHo – TΔSo

3. determining if a process is spontaneous (ΔG < 0)

a. lower potential energy (-ΔH)—chemical reactions

b. greater disorder (+ΔS)—physical changes

c. depends on temperature

1. threshold temperature (Tthreshold)

2. occurs when ΔG = 0 ∴ Tthreshold = ΔHo/ΔSo

d. summary chart

|ΔH |ΔS |Spontaneous Process (ΔG ................
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