MCAT Physical : Thermochemistry and Energetics
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Example Question #5 : Gibbs Free Energy
A galvanic cell is set up, and is determined to have a standard cell potential of 
Electrons will flow toward the anode
Gibb's free energy is negative and the equilibrium constant is greater than one
The equilibrium constant is less than one
The reaction is not spontaneous
Gibb's free energy is negative and the equilibrium constant is greater than one
There is an inherent relationship between cell potential, Gibb's free energy, and the equilibrium constant. Under standard conditions, a positive cell potential will signify a spontaneous reaction. This indicates that the Gibb's free energy (
Electrons will always travel from the site of oxidation (anode) toward the site of reduction (cathode), regardless of reaction spontaneity.
Example Question #6 : Gibbs Free Energy
A scientist prepares an experiment to demonstrate the second law of thermodynamics for a chemistry class. In order to conduct the experiment, the scientist brings the class outside in January and gathers a cup of water and a portable stove.
The temperature outside is –10 degrees Celsius. The scientist asks the students to consider the following when answering his questions:
Gibbs Free Energy Formula:
ΔG = ΔH – TΔS
Liquid-Solid Water Phase Change Reaction:
H2O(l) ⇌ H2O(s) + X
The scientist prepares two scenarios.
Scenario 1:
The scientist buries the cup of water outside in the snow, returns to the classroom with his class for one hour, and the class then checks on the cup. They find that the water has frozen in the cup.
Scenario 2:
The scientist then places the frozen cup of water on the stove and starts the gas. The class finds that the water melts quickly.
After the water melts, the scientist asks the students to consider two hypothetical scenarios as a thought experiment.
Scenario 3:
Once the liquid water at the end of scenario 2 melts completely, the scientist turns off the gas and monitors what happens to the water. Despite being in the cold air, the water never freezes.
Scenario 4:
The scientist takes the frozen water from the end of scenario 1, puts it on the active stove, and the water remains frozen.
Which of the following statements is true of scenario 3?
It is never spontaneous.
It is spontaneous at all temperatures.
It can be used to drive unfavorable reactions.
It is spontaneous only at high temperatures.
It is spontaneous only at low temperatures.
It is never spontaneous.
The reaction described in scenario 3 is the persistence of fully liquid water in the face of low temperatures. In other words, the Liquid-Solid Water Phase Change Reaction is far to the left. As X must be heat, and there is a shortage of heat in the cold air, the reaction would have to be driven to the right via Le Chatlier's Principle. It could never be spontaneous to the left.
Example Question #7 : Gibbs Free Energy
A scientist prepares an experiment to demonstrate the second law of thermodynamics for a chemistry class. In order to conduct the experiment, the scientist brings the class outside in January and gathers a cup of water and a portable stove.
The temperature outside is –10 degrees Celsius. The scientist asks the students to consider the following when answering his questions:
Gibbs Free Energy Formula:
ΔG = ΔH – TΔS
Liquid-Solid Water Phase Change Reaction:
H2O(l) ⇌ H2O(s) + X
The scientist prepares two scenarios.
Scenario 1:
The scientist buries the cup of water outside in the snow, returns to the classroom with his class for one hour, and the class then checks on the cup. They find that the water has frozen in the cup.
Scenario 2:
The scientist then places the frozen cup of water on the stove and starts the gas. The class finds that the water melts quickly.
After the water melts, the scientist asks the students to consider two hypothetical scenarios as a thought experiment.
Scenario 3:
Once the liquid water at the end of scenario 2 melts completely, the scientist turns off the gas and monitors what happens to the water. Despite being in the cold air, the water never freezes.
Scenario 4:
The scientist takes the frozen water from the end of scenario 1, puts it on the active stove, and the water remains frozen.
If a spontaneous reaction is harnessed to do work, how much useful work can the reaction do?
Exactly its Gibbs Free Energy value
Spontaneous reactions typically proceed too quickly to do useful work
Greater than its Gibbs Free Energy value, as this value does not consider enthalpy contributions
Less than its Gibbs Free Energy value, as energy some must go to increase the entropy of the universe
Only nonspontaneous reactions coupled to secondary reactions can do useful work
Exactly its Gibbs Free Energy value
The definition of Gibbs Free Energy is the amount of useful work that a reaction is able to do under defined conditions. It considers the amount of energy that is lost forever to the universe to comply with the Second Law of Thermodynamics.
Example Question #8 : Gibbs Free Energy
ATP stores energy through its ability to lose a phosphate group to form ADP in the following reaction
At body temperature, the thermodynamic values for ATP hydrolysis are as follow.
ΔG = –30kJ/mol
ΔH = –20kJ/mol
ΔS = 34kJ/mol
Glycolysis is an energy-liberating process (ΔG = –218kJ/mol, ΔH = –20 kJ/mol) that is coupled with the conversion of two ADP molecules into two ATP molecules according to the following reaction.
What is the value of ΔG for the reaction below?
–278kJ/mol
218kJ/mol
–158kJ/mol
–218kJ/mol
–158kJ/mol
First, recognize that if ΔG for the formation of ADP is –30 kJ/mol, the ΔG for the formation of ATP must be +30 kJ/mol. Next, we can combine the ΔG of glycolysis with the ΔG of the formation of two ATP molecules (notice the coefficient of 2 in order to balance ATP in the equation).
ΔGglucose = –218kJ/mol
ΔGATP = 30kJ/mol
ΔGtotal = –218 kJ/mol + 2(30kJ/mol) = –158kJ/mol
Example Question #9 : Gibbs Free Energy
For any given chemical reaction, one can draw an energy diagram. Energy diagrams depict the energy levels of the different steps in a reaction, while also indicating the net change in energy and giving clues to relative reaction rate.
Below, a reaction diagram is shown for a reaction that a scientist is studying in a lab. A student began the reaction the evening before, but the scientist is unsure as to the type of the reaction. He cannot find the student’s notes, except for the reaction diagram below.
Instead of "Energy" on the Y-axis, the graduate student in the passage instead labels the axis "Heat." In this case, what can we say for sure about the reaction in question?
(Note: consider the reaction in the vessel to be the system, and the remainder of the universe as the surroundings)
The surroundings must lose entropy
The system must lose entropy
The system may either lose or gain entropy
The system must gain entropy
There is no net entropy change in the system or the surroundings
The system may either lose or gain entropy
With the change to the chart described in the passage, we know that the system is losing heat from reactants to products (5 is lower than 1). This heat is going out of the system to the surroundings, increasing entropy of the universe. Because of this, the system may either gain or lose entropy.
Any entropy losses in the system, however, must be more than balanced by the entropy gained via heat in the surroundings. The universe (system + surroundings) always has a net gain of entropy. Essentailly, since the value for the surroundings will be positive, the value for the system may be either positive or negative, as long at their sum remains positive.
Example Question #1 : Gibbs Free Energy
Imagine there is an endothermic reaction where entropy is increased in the system. This reaction is not spontaneous at room temperature. What can be done in order to make the reaction spontaneous?
Increase the temperature
Decrease the reactant concentrations
Increase the concentration of the products
Add a catalyst
Increase the temperature
The Gibbs free energy equation is written as 
Since a negative 
In order to be spontaneous, in this case, 
Example Question #11 : Gibbs Free Energy
Imagine the following reaction: 
The reaction has an equilibrium constant of 45.8. The initial concentrations of A, B, and C result in a reaction quotient of 35.6.
Which of the following is true at the beginning of the reaction?
Keq and 
The reaction is not spontaneous
When a reaction is spontaneous, 
Example Question #12 : Gibbs Free Energy
Imagine a galvanic cell which uses solid zinc and aqueous iron ions to produce a voltage.
Based on the above reaction, which of the following statements is false?
The reaction is spontaneous
Electrons will travel from the anode to the cathode
A positive cell potential means that the reaction is spontaneous, and is true of all galvanic cells. This is seen in the equation 
A negative 
Example Question #11 : Thermochemistry And Energetics
The combustion of liquid hexane in air at 298K gives gaseous carbon dioxide and liquid water, as shown in this reaction.
The 
At 298K, the 
negative . . . negative . . . spontaneous
negative . . . negative . . . non-spontaneous
positive . . . negative . . . spontaneous
negative . . . positive . . . non-spontaneous
positive . . . positive . . . non-spontaneous
negative . . . negative . . . spontaneous
To determine the 
The reaction is exothermic because we are giver that the enthalpy is negative.
The entropy, 
This makes our equation 
Remember that temperature is given in Kelvin, and will never be negative. At very low temperatures, the 
Example Question #14 : Gibbs Free Energy
If the overall reaction has a positive change in entropy, which of the following statements about the reaction spontaneity is true?
The reaction is spontaneous at all temperatures
The reaction is spontaneous only at low temperatures
The reaction is spontaneous only at high temperatures
The reaction is spontaneous at room temperature
The reaction is not spontaneous at any temperature
The reaction is spontaneous at all temperatures
A reaction is spontaneous if the change of Gibbs free energy 
The total reaction is the change 
For this question, the change of enthalpy 
Both terms on the right side are always negative, and 
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