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”—DH
a. when energy required to break bonds > energy released to form new bonds, +DH (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, –DH (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 DH
1. listed to the right of equation
2. included as reactant (endothermic) or product (exothermic)
b. DH can be used in dimensional analysis process
6. DH 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: DHreaction = –Qwater
1. Q = mcDT for simple coffee cup calorimeter—aqueous reactions
a. m = mass of water
b. c = specific heat of water (4.18 J/g•K)
c. DT = change in temperature (Tf – Ti)
temperature can stay in oC, since
1 oC = 1 K (don't add 273 to DToC!)
2. Q = (C + mc)DT 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. DH using bond energy (B.E.) data
Bond Energies in (kJ/mol)Single / Multiple
H / C / N / O / S / F / Cl / Br / I / C=C / 614
H / 436 / 413 / 391 / 463 / 339 / 567 / 431 / 366 / 299 / C=N / 615
C / 348 / 293 / 358 / 259 / 485 / 328 / 276 / 240 / C=O / 799
N / 163 / 201 / 272 / 200 / 243 / N=N / 418
O / 146 / 190 / 203 / 243 / N=O / 607
S / 266 / 327 / 253 / 218 / O=O / 495
F / 155 / 253 / 237 / O=S / 523
Cl / 242 / 218 / 208 / CºC / 839
Br / 193 / 175 / CºN / 891
I / 151 / CºO / 1072
NºN / 941
S=S / 418
a. energy needed to break a bond (i.e. C–H) in a diatomic, gaseous molecule, which contains the bond type
1. is approximately the same for any molecule
2. affected by molecular bonding \ only works for gaseous species
3. positive value (+ B.E.) for breaking bonds
b. forming bonds (– B.E.)
c. DH = B.E.reactants – B.E.products
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 (+DS)
a. spread out: evaporation, diffusion and effusion
(solution: spread out solute and solvent (+DS), but bond solute-solvent (-DS) \ ?, but usually +DS)
b. motion: melting and boiling
3. predict increase in disorder for chemical changes (+DS): moles gaseous products > moles gaseous reactants
C. Thermodynamic Data (5.6 to 5.7, 19.4)
Species / DHfo (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 (DHfo) data
a. DHo 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. DHfo 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 DHfo
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 DH for a reaction using DHfo
1. Hess’s Law: DH for a multi-step reaction equals the sum of DH 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. DH » DHo = DHfoproducts – DHforeactants
c. calculate DS for a reaction using So
DS » DSo = 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: DGo = DHo – ToDSo
a. To = 298 K
b. where T ¹ 298 K: DG » DHo – TDSo
3. determining if a process is spontaneous (DG < 0)
a. lower potential energy (-DH)—chemical reactions
b. greater disorder (+DS)—physical changes
c. depends on temperature
1. threshold temperature (Tthreshold)
2. occurs when DG = 0 \ Tthreshold = DHo/DSo
d. summary chart
DH / DS / Spontaneous Process (DG <0)+ / + / for temperatures above Tthreshold
+ / – / at no temperatures
– / + / at all temperatures
– / – / for temperatures below Tthreshold
Experiments
1. Heat of Reaction Lab—Use calorimetry to determine DH for a series of reactions, compare the results with thermodynamic data, and combine the results to verify Hess' law.
Heat about 75 mL of water to about 70oC. Place a Styrofoam cup in a 250-mL beaker. Add 50.0 mL cold tap water to the cup. Record the temperature TC. Measure out 50.0 mL of the hot water and place in a second Styrofoam cup. Record the temperature TH. Pour the hot water into the cold water, cover the cup, insert the thermometer in the hole, and mix gently. Record the temperature every 20 seconds for 3 minutes. Discard the water.
a. (1) Record the temperatures.
TC / THtime (s) / 20 / 40 / 60 / 80 / 100 / 120 / 140 / 160 / 180
To C
(2) Graph the temperature vs. time data. Draw a best fit straight line (use a ruler).
Temperature (oC)20 / 60 / 100 / 140 / 180
Time (s)
(3) Use the y-intercept to determine Tmix.
Tmix (y-intercept)b. Calculate the following.
(1) Average of the hot and cold temperatures.
Tav = (TH – TC)/2(2) Heat lost from the water.
QL = mc(Tav – Tmix)(3) C.
C = QL/(Tmix – TC)Place a Styrofoam cup in a 250 mL beaker. Add 50.0 mL of 3.00 M NaOH. Record the temperature To. Pour 50.0 mL of 3.00 M HCl into the NaOH, cover, insert the thermometer, and mix gently. Record the temperature every 20 seconds for 3 minutes. Discard the mixture.
c. (1) Record the temperatures.
Totime (s) / 20 / 40 / 60 / 80 / 100 / 120 / 140 / 160 / 180
ToC
(2) Graph the temperature vs. time data. Draw a best fit straight line (use a ruler).
Temperature (oC)20 / 60 / 100 / 140 / 180
Time (s)
(3) Use the y-intercept to determine Tmix.
Tmix (y-intercept)d. Calculate DHreaction per mole of reactant based on the calorimetry data.
DT (K)Qwater (kJ)
DHreaction/mole
e. Calculate DHreaction per mole of reactant based on DHfo.
OH-(aq) + H+(aq) ® H2O(l)DH
D%
Place a Styrofoam cup in a 250 mL beaker. Add 50.0 mL of 3.00 M NH4Cl. Record the temperature To. Pour 50.0 mL of 3.00 M NaOH into the NH4Cl, cover, insert the thermometer, and mix gently. Record the temperature every 20 seconds for 3 minutes. Discard the mixture.
f. (1) Record the temperatures.
Totime (s) / 20 / 40 / 60 / 80 / 100 / 120 / 140 / 160 / 180
ToC
(2) Graph the temperature vs. time data. Draw a best fit straight line (use a ruler).
Temperature (oC)20 / 60 / 100 / 140 / 180
Time (s)
(3) Use the y-intercept to determine Tmix.
Tmix (y-intercept)g. Calculate DHreaction per mole of reactant based on the calorimetry data.
DT (K)Qwater (kJ)
DHreaction/mole
h. Calculate DHreaction per mole of reactant based on DHfo.
NH4+(aq) + OH-(aq) ® NH3(aq) + H2O(l)DH
D%
Place a Styrofoam cup in a 250 mL beaker. Add 50.0 mL of 3.00 M NH3. Record the temperature To. Pour 50.0 mL of 3.00 M HCl into the NH3, cover, insert the thermometer, and mix gently. Record the temperature every 20 seconds for 3 minutes. Discard the mixture.
i. (1) Record the temperatures.
Totime (s) / 20 / 40 / 60 / 80 / 100 / 120 / 140 / 160 / 180
ToC
(2) Graph the temperature vs. time data. Draw a best fit straight line (use a ruler).
20 / 60 / 100 / 140 / 180
Time (s)
(3) Use the y-intercept to determine Tmix.
Tmix (y-intercept)j. Calculate DHreaction per mole of reactant based on the calorimetry data.
DT (K)Qwater (kJ)
DHreaction/mole
k. Calculate DHreaction per mole of reactant based on DHfo.
NH3(aq) + H+(aq) ® NH4+(aq)DH
D%
l. Show that the chemical equations and DH from
part (d) – part (g) = part (j).
Practice Problems
A. Enthalpy
1. 4 Fe(s) + 3 O2(g) ® 2 Fe2O3(s) DH = -1640 kJ
a. How much heat is released to produce 10.0 g Fe2O3?
b. How many grams of iron are needed to generate
1.00 x 104 kJ of heat?
2. CaSO4(s) + CO2(g) ® CaCO3(s) + SO3(g) DH = 224 kJ
a. How much heat is absorbed when 10.0 g CaSO4 react.
b. How much CaCO3 is produced when 500. kJ is absorbed?
3. When 1.51 g of NH4Cl are dissolved in 100. g of water the temperature drops 1.00oC. Determine
a. Qwater for the solution process.
b. the number of moles of NH4Cl dissolved.
c. DH (in kJ) for the dissolving of one mole of NH4Cl.
4. 12.8 g of MgSO4 is dissolved in 250. g of H2O in a coffee cup calorimeter. The temperature of the solution increases from 23.8oC to 33.1oC. Determine
a. Qwater for the solution process.
b. the number of moles of MgSO4 dissolved.
c. DH (in kJ) for the dissolving of one mole of MgSO4.
5. H2(g) + F2(g) ® 2 HF(g)
Estimate DH for the reaction using the bond energy values.
6. C2H2(g) + 2 H2(g) ® C2H6(g)
Estimate DH for the reaction using the bond energy values.
7. A bomb calorimeter with a constant of 921 J/oC contains 1,000 g of water. The combustion of 1.00 g of ethene (C2H4) increases the temperature 9.3oC. Determine
a. Qwater for the combustion process.
b. the number of moles of ethene reacted.
c. DH (in kJ) for the combustion of one mole of C2H4.
d. Write the equation for the combustion of ethene (C2H4).
e. Calculate DH using bond energies.
B. Entropy
8. Predict whether DS > 0, DS < 0 or DS » 0.
> 0 / » 0 / < 0Melting ice at 0oC
CH4(g) + 2 O2(g) ® CO2(g) + 2 H2O(l)
CH4(g) + 2 O2(g) ® CO2(g) + 2 H2O(g)
Distilling alcohol-water mixture
C. Thermodynamic Data
9. 2 Na2O2(s) + 4 HCl(g) ® 4 NaCl(s) + 2 H2O(l) + O2(g)
Determine DH from the thermochemical reactions below.
2 Na2O2(s) + 2 H2O(l) ® 4 NaOH(s) + O2(g) DH1 = -126 kJ
NaOH(s) + HCl(g) ® NaCl(s) + H2O(l) DH2 = -179 kJ
10. C2H2(g) + 5 N2O(g) ® 2 CO2(g) + H2O(g) + 5 N2(g)
Determine DH from the thermochemical equations below.
2 C2H2(g) + 5 O2(g) ® 4 CO2(g) + 2 H2O(g) DH1 = -2512 kJ
N2(g) + ½ O2(g) ® N2O(g) DH2 = 104 kJ
11. NO(g) + O(g) ® NO2(g)
Determine DH for the above reaction using the following thermochemical equations.
NO(g) + O3(g) ® NO2(g) + O2(g) DH1 = -198.9 kJ
O3(g) ® 3/2 O2(g) DH2 = -142.3 kJ
O2(g) ® 2 O(g) DH3 = 495.0 kJ
12. a. Write the equation for the combustion of methanol, CH3OH(l). (other reactants and products are gaseous).
b. Calculate DH using DHfo values.
1.00 g of methanol is burned in a bomb calorimeter that contains 1200 g of water. The temperature increases 3.4 K.
c. Calculate the heat generated by the combustion reaction.
d. Calculate the calorimeter constant of the bomb.
13. Ca(s) + SO3(g) + 2 H2O(l) ® CaSO3•2 H2O(s)
DH = -795 kJ and DS = -0.2535 kJ/K for the reaction.
a. Calculate DHfo for CaSO3•2 H2O.
b. Calculate So for CaSO3•2 H2O.
D. Gibbs Free Energy
14. Consider the reaction at 25oC:
Cu(s) + 4 H+(aq) + 2 NO3-(aq) ® Cu2+(aq) + 2 NO2(g) + 2 H2O(l).
a. Calculate DHo using DHfo values.
b. Calculate DSo using So values.
c. Calculate DGo.
15. NH4NO3(s) ® NH4+(aq) + NO3-(aq)
Determine the following for the above reaction.
a. Is the reaction exothermic or endothermic?
b. Is there an increase or decrease in entropy?
c. Is the reaction spontaneous at 25oC?
16. 2 SO2(g) + O2(g) ® 2 SO3(g)
a. Calculate DHo.
b. Calculate DSo.
c. Calculate DG at 400 K.
d. Determine the temperature range where the reaction is spontaneous.
17. C2H5OH(l) + 3 O2(g) ® 2 CO2(g) + 3 H2O(l)
a. Calculate DHo.
b. Calculate DSo.
c. Calculate DG at 20oC.
d. At which temperature (if any) will the reaction be spontaneous?
18. When H2SO4(l) is dissolved in water, the temperature of the mixture increases. Predict the sign of DH, DS and DG for this process (justify your answer).
+/– / JustificationDH
DS
DG
19. C2H5OH(l) ® C2H5OH(g)