Gibbs Free Energy
Calculate the heat of formation for the following reaction: 8 Al(s) + 3 Fe3O4(s) → 4 Al2O3(s) + 9 Fe(s)ΔH for Fe3O4(s) = -1120.9 kJ/molΔH for Al2O3(s) = -1669.8 kJ/mol (A) -3,316.5 kJ (B) -2,563.6 kJ (C) -1,657.2 kJ (D) -53.9 kJ
(A) -3316.5 kJ For detailed solution, see problem #1 here: https://www.thoughtco.com/heat-of-formation-example-problem-609556
Hess's Law is true for variables such as enthalpy and entropy because these are _________ Variables. (A) Process (B) State (C) Fixed (D) Variable
(B) State Hess's Law is true for variables such as enthalpy and entropy because these are State Variables.
What is another way delta G is represented?
-T delta S (universe)
What is STP?
0ºC/273 K and 1 atm
What is the specific heat of H2O (l)?
1 cal/g(K)
What are the conditions to measure Gibbs free energy?
1) constant temp and pressure: in isolated system, the moles stay the same because no exchange in mass-> constant pressure and temp -> no change in volume-> no PV work 2) reversible reaction 3) no mechanical work done (only PV work: delta H = delta U (internal energy))
What are the two misconceptions in comparing thermodynamics and kinetics?
1) exothermic reactions proceed rapidly 2) the more stable the products the faster the reaction
What are standard conditions?
100kPa/1atm and 298K and 1 M
1 cal = _____ J
4.184 J
How does Gibbs Free Energy satisfy the second law of thermodynamics?
A biological reaction increases the entropy of the universe only if the Gibbs free energy is negative.
Which of the following lists hydrogen halides in terms of increasing standard heats of formation? A. HF < HBr < HCl < HI B. HI < HBr < HCl < HF C. HBr < HF < HCl < HI D. HF < HI < HCl < HBr
A. HF < HBr < HCl < HI The reaction that forms the most stable HX will be the most exothermic and have the most negative standard heat of formation. Fluorine is the smallest, most electronegative halogen and forms the only hydrogen halide of the group that is not a strong acid. HF is the most stable hydrogen halide and will have the most negative heat of formation. This eliminates choices without HF having the lowest heat of formation. Indeed, the heat of formation of HI is actually endothermic. This eliminates choice HF < HI < HCl < HBr.
In the equation CaCO3(s) -><- CaO(s) + CO2 (g), what happens if Ar(g) is added?
Ar is an inert gas and will increase the overall pressure of the equation but does not affect the partial pressure and volume of CO2 and there would be no shift.
What is the relationship between Keq and the spontaneity of the reaction?
As Keq increases, the more spontaneous the reaction is.
Why can a reaction be nonspontaneous at low temperatures only when delta H and delta S are positive?
At higher temperatures, a compound is less likely to release heat and increase disorder.
Why can a reaction be nonspontaneous at high temperatures only when delta H and delta S are negative?
At lower temperatures, a compound is less likely to release heat and increase disorder.
What is the exception in temperature in relation to delta G in Le Chatlier's Principle for a reaction?
At very high temperatures, entropy takes over and has a negative Gibbs free energy and would only favor the right side
According to Le Chatelier's principle, what would happen to the rest of the reactants and products if more A was added in the reaction: A + B -><- C + D?
B would decrease, C and D increase
What property indicates that enthalpy is independent of the pathway of the reaction?
Enthalpy is state function
Explain when entropy increases and decreases in a system.
Entropy increases in the system when energy is distributed into a system at a given temperature. Entropy decreases when energy is distributed out of a system at a given temperature.
According to Le Chatelier's principle, what would happen to the equilibrium if pressure was added to the reactants in the reaction: A + 2B -><- C + D?
Greater pressure in situation with more molecules shift to right to alleviate the high pressure; if added to products, shifted to left
When is Hess's Law used?
It applies to any state function.
What does it mean for enthalpy to be an external property?
It depends on the amount of the system; directly correlated
In the equation CaCO3(s) -><- CaO(s) + CO2 (g), what happens if a catalyst is added?
It would not make a difference and there would be no shift because it would speed the forward and the backward reaction, making no difference.
According to Le Chatelier's principle, what would happen to the equilibrium if heat was added to the reactants in the reaction: A + B -><- C + D?
It would shift to the right with A and B decreasing and C and D increasing in concentration
What are the units of entropy?
J/mol*K
Can q = mcdeltaT be used for phase change reactions? What is used instead? But where does the heat that is added when the phase doesn't change go?
No because phase change reactions (when solid to liquid happens) do not undergo changes in temperature so delta T = 0. Enthalpy is used instead; heat of fusion for solid to liquid; heat of vaporization for liquid to gas The thermal energy added to the compound in a phase overcomes the intermolecular forces between the molecules. Once the phase is over, the temperature can increase again.
Does enthalpy indicate spontaneity? Does entropy indicate spontaneity?
No only exothermic or endothermic; yes
When delta G = 0, what happens to Q?
Q becomes K, the equilibrium constant
In the equation CaCO3(s) -><- CaO(s) + CO2 (g), what happens if the volume is increased?
Shift to right: increase volume = decrease pressure = decrease concentration of the equation = decrease [CO2] since others are solids->shift to right/favor the products
What does Gibbs free energy tell us?
Tells us whether a chemical reaction can occur spontaneously
What is the standard enthalpy of formation?
The enthalpy change when one mole of a substance is formed from its elements
The change in entropy of the universe in a reversible reaction is 0. Why?
The entropy of the reversible system = Q subR/ T1 The entropy of the environment is -Q subR/T1 delta S (universe) = delta S (reversible system) + delta S (environment)
In the reaction of N(g) + 3H2(g)-><- 2NH3 (g) + Heat, what would happen if the temperature was increased?
The equilibrium would shift to the left.
What is the relationship between the alkane reactant and the number of combustion products?
The larger the alkane reactant, the more numerous the combustion products
What is the second law of thermodynamics?
The principle stating that every energy transfer or transformation increases the entropy of the universe.
What are the standard state conditions?
The standard state is 1 M concentration, 1 atm pressure and a temperature of 273 Kelvins.
Why is Gibbs free energy considered a state function?
The variable is independent of the pathway. It is only affected by reactants and products.
What is the work done in a bomb calorimeter? What is the system and what is the surroundings?
There is no work done because W = P(delta V) and constant V -> W = 0. system: the sample plus the oxygen and steel vessel surroundings: water due to isolation from the rest of the universe
In the equation CaCO3(s) -><- CaO(s) + CO2 (g), what happens if more CaCO3(s) is added?
There would be no shift because the equilibrium is not dependent on CaCO3(s) since it is a solid.
True or false? Hess's Law states that the energy change of a process is independent of the path that was taken to get there
True. Hess's Law states that the energy change of a process is independent of the path that was taken to get there
What happens in a constant-volume calorimetry/bomb calorimetry?
a sample of matter is placed in steel decomposition vessel that has mostly pure oxygen gas which is then placed in an insulated vessel with known amount of water. The decomposition vessel is ignited by an electric ignition mechanism, which leads to combustion and the heat that evolves is the heat of the combustion reaction
Methanol reacts with acetic acid to form methyl acetate and water. C-C: 348 kJ/mol C-H: 415 kJ/mol C=O: 805 kJ/mol O-H: 463 kJ/mol C-O: 360 kJ/mol Based on the values in the table above, what is the heat of reaction in kJ/mol? a) 0 b) 464 c) 824 d) 1288
a) 0 Same bonds are breaking and forming/double displacement
Do the following situations result in an increase or decrease in entropy? a) Liquid water --> Ice b) dry ice sublimates into carbon dioxide c) solid salt --> aqueous salt d) 1 mole Nitrogen gas + 3 moles Hydrogen gas --> 2 moles Ammonium gas e) an ice pack is placed on a wound
a) decrease b) increase c) increase d) decrease (fewer moles of gas) e) increase (transfer of heat)
Explosions are necessarily characterized by: a) delta G < 0 b) delta H > 0 c) delta S < 0 d) T < 0
a) delta G < 0
Suppose deltaGº(rxn) =-2000 kJ/mol for a chemical reaction. At 300 K, what is the change in Gibbs free energy in kJ/mol? a) deltaG = -2000 + (300K)(8.314)(ln Q) b) deltaG = -2000 - (300K)(8.314)(ln Q) c) deltaG = -2000 + (300K)(8.314)(log Q) d) deltaG = -2000 - (300K)(8.314)(log Q)
a) deltaG = -2000 + (300K)(8.314)(ln Q)
A chemical reaction has a negative enthalpy and a negative entropy. Which of the following terms necessarily describes this reaction? a) exothermic b) endothermic c) exergonic d) endergonic
a) exothermic
For a reaction to be spontaneous only at low temperatures and nonspontaneous at high temperatures, delta H and delta S must be: a) negative; negative b) positive; positive c) negative; positive d) positive; negative
a) negative; negative
Suppose that a reaction has delta H = -77kJ and delta S = -0.48 kJ. At what temperature will it change from spontaneous and to non spontaneous? a) 47K b) 160 K c) 243 K d) 321 K
b) 160 K
For a certain chemical process, delta Gº = -4.955 kJ/mol. What is the equilibrium constant Keq for this reaction? R = 8.314 J/mol*K a) Keq = 1.0 b) Keq = 7.4 c) Keq = 8.9 d) Keq = 10
b) Keq = 7.4 -4955 = -(8.314)(298)ln Keq-> -4800=-(8)(300)ln Keq-> 2 = ln Keq -> e^2 = Keq = 2.7^2 -> between 4 and 9
Which of the following devices would be the most appropriate to use to measure the heat capacity of a liquid? a) thermometer b) calorimeter c) barometer d) volumetric flask
b) calorimeter
For a reaction to be spontaneous only at high temperatures and nonspontaneous at low temperatures, delta H and delta S must be: a) negative; negative b) positive; positive c) negative; positive d) positive; negative
b) positive; positive
Calculate the enthalpy change for the following reaction: C(s) + 2 H2(g) -> CH4(g) delta H = ? Bond dissociation energies of H-H and C-H bonds are 436 kJ/mol and 415 kJ/mol, respectively. The delta Hf of C (g) is 715 kJ/mol.
bond formation = exothermic delta H = -(delta H(products)-delta H(reactants)) =-((4(415 kJ/mol)+ 2(436 kJ/mol))-715 kJ/mol= -1660+1590=-70 kJ
Which of the following statements is true of the process that is spontaneous in the forward direction? a) delta G > 0 and Keq > Q b) delta G > 0 and Keq < Q c) delta G < 0 and Keq > Q d) delta G < 0 and Keq < Q
c) delta G < 0 and Keq > Q Q < Keq = Keq > Q
At standard temperature and pressure, a chemical process is at equilibrium. What is the free energy of reaction (delta G) for this process? a) delta G > 0 b) delta G < 0 c) delta G = 0 d) There is not enough information to determine the free energy of the reaction.
c) delta G = 0
Which of the following phase changes is associated with the largest decrease in entropy? a) fusion b) solidification c) deposition d) sublimation
c) deposition
What is happening in a constant-pressure calorimeter?
coffee-cup calorimeter You have a dissolving solution inside an insulated container. The incident pressure/atmospheric pressure is the constant. There should be sufficient thermal insulation to prevent heat gain or loss from the environment.
Pure sodium metal spontaneously combusts upon contact with room temperature water. What is true about the equilibrium constant of this combustion reaction at 25ºC? a) Keq < 0 b) 0 < Keq < 1 c) Keq = 1 d) Keq > 1
d) Keq > 1 says spontaneous increase Keq = increase in spontaneity
A reaction has a positive entropy and enthalpy. What can be inferred about the progress of this reaction from this information? a) the reaction is spontaneous b) the reaction is nonspontaneous c) the reaction is at equilibrium d) There is not enough information to determine whether the reaction is spontaneous or not.
d) There is not enough information to determine whether the reaction is spontaneous or not.
Which of the following processes has the most exothermic standard heat of combustion? a) combustion of ethane b) combustion of propane c) combustion of n-butane d) combustion of n-pentane
d) combustion of n-pentane
In the reaction of N(g) + 3H2(g)-><- 2NH3 (g) + Heat, what would happen if the pressure was decreased?
decreasing pressure = increasing volume = shift in direction where there are more molecules present; shift to left
How is Gibbs free energy related to concentration of the products and reactants?
delta G = delta G (standard state conditions) + 2.303RT log Q
How do you determine the free energy change for a reaction that is in progress? At what values of Q/Keq is the reaction spontaneous/nonspontaneous?
delta G(rxn) = delta Gº(rxn) + RTln Q = RT ln Q/Keq Q/Keq less than 1 = spontaneity in forward reaction Q/Keq greater than 1 = spontaneity in reverse reaction
What is the enthalpy of reaction for the reaction 2H2O (g) -> 2 H2 (g) + O2 (g), given the following bond enthalpies: H-H = 436 kJ/mol, O=O = 498 kJ/mol, O-O = 146 kJ/mol, and O-H = 463 kJ/mol?
delta H = bonds broken - bonds formed (4(463))-(2(436)+498) =1840-1378 = 470kJ
What is the equational definition of enthalpy?
delta H = delta U + P (delta V) U = the total internal energy of system P(delta V) = work down to displace environment in creating the current in the system
How is the heat of the reaction determined?
delta H of reaction = delta H of products - delta H of reactants
What is the equation for the second law of thermodynamics?
delta S (system) + delta S (surroundings) = delta S (universe) > 0
When can delta S (universe) be greater than 0?
delta S (system) - delta H (system)/T; if delta S (system) is greater than delta H (system)/T
How can the change in entropy be determined?
delta S = Q(rev)/T Qrev: heat that is gained or lost in a reversible process
How is the second law of thermodynamics determined mathematically?
delta S(universe) = delta S (system) + delta S (surroundings) > 0
How is the heat exchange of a bomb calorimeter determined mathematically?
delta U (system) + delta U (surroundings) = delta U (calorimeter) = Q (calorimeter) - W (calorimeter) = 0-> delta U (system) = - delta U (surroundings) -> q (system) = -q (surroundings)-> m(steel)c(steel)deltaT + m(oxygen)c(oxygen)deltaT = -m(water)c(water)deltaT
What is the first law of thermodynamics mathematically?
delta U = Q - W U: internal energy Q: heat W: work
What is a state function?
describes the system in an equilibrium state; describe how the system got to its current equilibrium
When the system is endothermic, what is the value of Q? When the system is exothermic, what is the value of Q?
endothermic: delta Q > 0 exothermic: delta Q > 0
In the reaction of N(g) + 3H2(g) + Heat -><- 2NH3 (g), what would happen if the temperature was increased?
equilibrium would shift to the right
In the equation CaCO3(s) -><- CaO(s) + CO2 (g), what happens if CO2(g) is added?
favor the reactants
What is the most stable form of carbon?
graphite (C(s))
What is the difference between heat and temperature?
heat: the amount of energy transferred due to a change in temperature temperature: average kinetic energy of molecules
What is the relationship between spontaneity and Gibbs free energy?
high spontaneity = delta G < 0; nonspontaneity = delta G > 0
What are the units of Gibbs free energy?
joules/mole
What is the zeroth law of thermodynamics?
objects are in thermal equilibrium only when their temperatures are equal
What are the 8 state functions?
pressure, density, Gibbs free energy, enthalpy, entropy, temperature, volume, internal energy
What is the relationship between the system and surroundings in terms of heat?
q (system) = - q (surroundings)
How is the amount of heat released or absorbed determined mathematically?
q = (m)(c)(delta T)
How is the enthalpy of fusion or vaporization determined mathematically?
q = mL L: latent heat; the enthalpy of an isothermal process, units: cal/g
One cup containing 100 grams of water at 300 K is mixed into another cup containing 200g of water at 450K. What is the equilibrium temperature of the system?
q(system) = -q(surroundings) m(cold)c(water)deltaT = -m(hot)c(water)delta T (100g)(1)(Tf-300) = -(200g)(1)(Tf-450K) 100Tf-30000=-200Tf+90000 300Tf=120000 Tf = 400K
In the reaction of N(g) + 3H2(g) + heat -><- 2NH3 (g), what would happen if the temperature was decreased?
shift to the left
In the reaction of N(g) + 3H2(g)-><- 2NH3 (g) + Heat, what would happen if the temperature was decreased?
shift to the right
Compare the usage of standard conditions and STP.
standard conditions: kinetics, equilibrium, and thermodynamic problems STP: ideal gas calculations
What is specific heat?
the amount of energy required to raise the temperature of one gram of a substance by one degree Celsius.
What does Hess's law tell us?
the change in enthalpy of the forward reaction is the negative change in enthalpy in the reverse reaction
What is the standard enthalpy of formation of a compound?
the enthalpy required to produce one mole of a compound from its element in their standard states
What is enthalpy?
the heat added to the system
What is the relationship between the activation energy and the rate of the reaction?
the lower the activation energy, the faster the rate of the reaction
What is heat capacity?
the number of heat units needed to raise the temperature of a body by one degree
What is a process function?
the pathway from one equilibrium state to another
What variables are associated to the thermodynamics and kinetics of a reaction?
thermodynamics (delta G) kinetics (activation energy/Ea)
What is the definition of the triple point? The critical point?
triple point: the point at which all phases are in equilibrium critical point: the temperature and pressure at which the phases are nondistinguishable
What are the common 2 process functions?
work and heat
What equation relates Gibbs free energy with redox reactions?
ΔG°cell = −nFE°cell. If E°cell > 0, then the process is spontaneous (galvanic cell). If E°cell < 0, then the process is nonspontaneous (electrolytic cell)
What is the Gibbs free energy equation?
∆G = ∆H - T∆S
Why is it that water in a glass would heat up faster than water in a swimming pool?
Even though they both have the same specific heat, they have different heat capacities, which is mass(specific heat).
True or False. Thermodynamics can determine kinetics.
False. If delta H is less than ), the bonds of the products are more stable than reactants so rate has no relation to stability of both reactions
If there was a reaction of A->B->C, how would Gibbs free energy be calculated?
Find the individual free energies from A to B and B to C and add them
How does reaction involving gases differ from reactions involving liquids/gases in terms of enthalpy?
Gases are compressible so have PV work on the system. Liquids and solids are not compressible so do not have any PV work and delta H ~= delta U (internal energy)
