MCAT Physical : Biochemistry, Organic Chemistry, and Other Concepts
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Example Questions
Example Question #6 : Thermodynamics Of Phase Changes
The molar heats of fusion and vaporization for water are given below:
Given the following information, which substance would be expected to have an intermolecular force strength similar to that of water?
|
Substance |
ΔHfus |
ΔHvap |
|
Acetone |
5.72 kJ/mol |
29.1 kJ/mol |
|
Ethanol |
4.60 kJ/mol |
43.5 kJ/mol |
|
Hydrogen |
0.12 kJ/mol |
0.90 kJ/mol |
|
Ammonia |
5.65 kJ/mol |
23.4 kJ/mol |
Ethanol and ammonia
Hydrogen
Acetone and ammonia
Ethanol
Ethanol
Intermolecular forces play a key role in determining the energy required for phase changes. Strong intermolecular forces result in more resistance to changes that result in greater distance between molecules (greater entropy), as the forces cause the molecules to "stick" to one another. When a liquid is vaporized, the strength of the intermolecular force is overcome; similar heats of vaporization indicate similar intermolecular forces. A similar concept governs the transition from solid to liquid. If two compounds share similar enthalpies of fusion and vaporization, then they likely have similar intermolecular forces.
In the given table, ethanol enthalpy values are most similar to those of water, meaning it likely has similar intermolecular forces.
Example Question #2 : Thermodynamics Of Phase Changes
The following diagram shows the temperature of a substance as constant heat is supplied. Suppose the substance began in a solid state.
If 
condenses . . . absorbed
condenses . . . released
vaporizes . . . released
solidifies . . . released
condenses . . . released
Plateaus in the graph represent phase changes: period B shows the transitions between solid and liquid, and period D shows the transitions between liquid and gas. When the substance transitions through period D, it undergoes either vaporization (C to E transition) or condensation (E to C transition). If the substances starts out at interval E and ends at interval C, then it has undergone condensation.
Condensation involves transition from a high energy gas to a lower energy liquid, and has a net decrease in heat energy and temperature. This heat release is known as an exothermic process.
Example Question #11 : Thermodynamics Of Phase Changes
The molar heats of fusion and vaporization for water are given below:
Which of the following explains why the heat of vaporization is much greater than the heat of fusion?
More heat is required to vaporize a given quantity of water
Liquids are in constant motion and hold more energy than solids
The difference in kinetic energy of gas and liquid molecules is higher than the difference between solid and liquid molecules
Vaporization is an endothermic process while fusion is an exothermic process
The difference in kinetic energy of gas and liquid molecules is higher than the difference between solid and liquid molecules
Notice that most of these are true statements (the only incorrect statement is that fusion is an exothermic process). The key is finding the statement that best explains why the heat of vaporization is greater than the heat of fusion. Essentially, we are looking for the reason why a transition from liquid to gas requires more energy than a transition from solid to liquid.
It is true that more heat is required to vaporize a given quantity of water, but the reason for this can be found on a molecular level. Gas molecules have a relatively high kinetic energy. The large difference in kinetic energies between the states accounts for the difference in heats of fusion and vaporization. Solids are the lowest energy state, followed by liquids, and then gases. The difference in energy between equal amounts of solid and liquid is given by the heat of fusion, while the difference in energy between equal amounts of liquid and gas is given by the heat of vaporization. The discrepancy in energy is reflected in the difference between these two heat quantities.
Example Question #11 : Phase Changes
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.
In scenario 1, the Gibbs Free Energy and Keq of the Liquid-Solid Water Phase Change Reaction, as the reaction begins, is best characterized as __________.
undefined and decreasing, respectively
positive and increasing, respectively
negative and increasing, respectively
zero and decreasing, respectively
negative and decreasing, respectively
negative and increasing, respectively
As scenario 1 begins, the reaction is spontaneous as written, and so the Gibbs Free Energy is negative. Additionally, the Van't Hoff equation proves that Keq increases with decreasing temperature in exothermic reactions. As this reaction is exothermic and placed in low-temperature conditions, the relative abundance of the products will become the prevailing state. Keq, therefore, increases.
Example Question #21 : Phase Changes
Imagine a graph that shows the amount of heat being added to water. Heat (q) is on the x-axis, and the temperature of the water is on the y-axis. As more heat is added to the water, the temperature will increase, however, there are two parts of the graph that appear to plateau, and it takes a set amount of heat in order to continue raising the water's temperature.
What is the explanation for these plateaus on the graph?
It eventually requires a greater amount of heat to raise the temperature of water every Kelvin unit.
Water expands when heated, and the heat must warm a larger area. This results in a temporary plateau in temperature increase.
Heat is dedicated to breaking the hydrogen bonds between water molecules at these plateaus.
Water gives off heat very quickly, and a great amount of heat must be accumulated before the water temperature will continue to rise.
Heat is dedicated to breaking the hydrogen bonds between water molecules at these plateaus.
There are two plateaus on the graph, at 273K and 373K. These are the melting point and boiling points of water, respectively. At these temperatures, heat is responsible for breaking the hydrogen bonds between molecules in order to cause the phase change. The amount of heat required to melt the ice is called the enthalpy of fusion, and the amount of heat necessary to vaporize water is called the enthalpy of vaporization.
Example Question #22 : Phase Changes
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.
When the scientist moves the frozen water to the stove, the water melts. If the temperature of the stove is kept such that the reaction is at equilibrium, and the water is almost entirely melted, which of the following is a possible Keq value for the Liquid-Solid Water Phase Change Reaction as it is written?
0
1
Once the water is almost entirely melted, the reaction is heavily skewed to the left side of the reaction. Notice the reaction is written with liquid water on the left. The equilibrium constant equation is written with the product (right-side) concentrations over the reactant concentrations; therefore, if there is a relative abundance of the reactants to products, K will be less than 1, but not all the way down to zero.
Example Question #151 : Biochemistry, Organic Chemistry, And Other Concepts
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 best describes the Liquid-Solid Water Phase Change Reaction above?
Physical reaction
Both an aqueous redox and physical reaction
Chemical reaction
Both a chemical and physical reaction
Aqueous redox reaction
Physical reaction
This is an example of a phase change physical reaction at the liquid-solid interface. There is no change in the organization of chemical bonds; therefore, it is not an example of a chemical reaction. There is also no change in the bonding partners of the constituent oxygen and hydrogen atoms of water. As a result, there is no change in oxidation numbers. Without a change in oxidation number values, it is not a redox reaction.
Example Question #1 : Phase Diagrams
Which best describes the significance of a "critical point" on a phase diagram?
The lowest temperature and pressure at which a substance may exist in a liquid phase
The highest temperature and pressure at which a substance may exist in distinct solid and liquid phases
The temperature and pressure at which a substance may exist in equilbrium between the solid, liquid, and gas phases
The lowest temperature and pressure at which a substance may exist in a gaseous phase
The highest temperature and pressure at which a substance may exist in distinct liquid and gas phases
The highest temperature and pressure at which a substance may exist in distinct liquid and gas phases
The critical point is defined by the highest temperature and highest pressure at which a substance exists in distinct liquid and gas phases. Beyond this point, there is no distinction between the liquid and gaseous phases.
The temperature and pressure at which a substance may exist in equilibrium between the solid, liquid, and gas phases is known as the triple-point.
Example Question #3 : Phase Diagrams
Phase diagrams are used to depict changes in the properties of a solution at different temperatures and pressures. Below is a phase diagram of a polar solution.
What is the name of Point D?
Freezing point
Critical point
Triple point
Boiling point
Triple point
Point D occurs at the intersection of the three segments, and is called the triple point. All three phases (solid, liquid, and gas) co-exist at the triple point.
Point B corresponds to the critical point. The freezing and boiling points are dependent on pressure, and are depicted as the lines between regions 1 and 2 (freezing) and between regions 2 and 3 (boiling).
Example Question #4 : Phase Diagrams
Phase diagrams are used to depict changes in the properties of a solution at different temperatures and pressures. Below is a phase diagram of a polar solution.
What processes are occurring at Point D?
Freezing (and melting)
Condensation (and vaporization)
Deposition (and sublimation)
All of the above
All of the above
Point D is called the triple point. At the triple point, all three phases (solid, liquid, and gas) co-exist. The processes that are occurring are condensation and vaporization, sublimation and deposition, and freezing and melting.
Condensation and vaporization allow liquid and gas to co-exist. Sublimation and deposition allow solid and gas to co-exist. Freezing and melting allow solid and liquid to co-exist. In these terms, it makes sense that all six processes occur at the triple point.
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