MCAT Physical : Physical Chemistry
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Example Question #6 : Endothermic And Exothermic Reactions
Electronegativity is an important concept in physical chemistry, and often used to help quantify the dipole moment of polar compounds. Polar compounds are different from those compounds that are purely nonpolar or purely ionic. An example can be seen by contrasting sodium chloride, NaCl, with an organic molecule, R-C-OH. The former is purely ionic, and the latter is polar covalent.
When comparing more than one polar covalent molecule, we use the dipole moment value to help us determine relative strength of polarity. Dipole moment, however, is dependent on the electronegativity of the atoms making up the bond. Electronegativity is a property inherent to the atom in question, whereas dipole moment is a property of the bond between them.
For example, oxygen has an electronegativity of 3.44, and hydrogen of 2.20. In other words, oxygen more strongly attracts electrons when in a bond with hydrogen. This leads to the O-H bond having a dipole moment.
When all the dipole moments of polar bonds in a molecule are summed, the molecular dipole moment results, as per the following equation.
Dipole moment = charge * separation distance
A scientist decides that the polar nature of water means that it will be useful to help him investigate the solution chemistry of a salt. He dissolves the salt in water, and is surprised to find that the beaker becomes very hot to the touch. Which of the following is likely true of this reaction?
Assume the system is defined as the beaker with the solute and solvent.
The reaction is strongly endothermic
More energy is released when hydrated ions form than is absorbed when ionic bonds break
Entropy of the system must increase
Equal energy is released when hydrated ions form than is absorbed when ionic bonds break
Less energy is released when hydrated ions form than is absorbed when ionic bonds break
More energy is released when hydrated ions form than is absorbed when ionic bonds break
Energy is always released when bonds form, and always absorbed when bonds break. In the situation described in the question, heat is released when the salt dissolves because energy is released when the water forms dipole-based bonds with the newly dissolved ions. This energy is larger in magnitude than the energy that is absorbed to break the ionic bonds in the salt, thus, there is a net flow of heat out of the system.
The entropy of the system must not necessarily increase in this case, because the entropy of the universe is increasing owing to the release of heat. This could be enough to offset a local decrease in entropy in the system.
Example Question #7 : Endothermic And Exothermic Reactions
5.6g of manganese reacts with 650mL of 6.0M hydrochloric acid to form manganese (V) chloride and hydrogen gas. Along with the products, a large amount of heat is evolved.
This reaction is classified as __________.
Endothermic, with a positive
Exothermic, with a negative
Endothermic, with a negative
Exothermic, with a positive
Exothermic, with a negative
Any reaction in which heat is evolved (released) is classified as exothermic. The change in heat content (
Example Question #1 : Endothermic And Exothermic Reactions
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 2, the reaction that water undergoes would best be characterized as __________.
endothermic in this scenario, and nonspontaneous at standard conditions
exothermic in this scenario, and nonspontaneous at current conditions
exothermic in this scenario, and nonspontaneous at standard conditions
endothermic in this scenario, and spontaneous at standard conditions
exothermic in this scenario, and spontaneous at standard conditions
endothermic in this scenario, and nonspontaneous at standard conditions
The "reaction that water undergoes" is melting. Melting is endothermic, as the reaction written in the pre-question passage illustrates. Standard conditions are defined as 0C and 1 atm of pressure. Water freezes at 0C, and thus the melting reaction is nonspontaneous at these conditions.
Example Question #61 : Thermochemistry And Energetics
A system experiences the following energetic changes:
Which of the following is true?
The system loses 
The system loses heat
Remember that a negative sign denotes an exothermic process (heat is released into the surroundings), while a positive energy change denotes an endothermic process (heat is absorbed).
Using the given values we can calculate the total energy change of the system:
Since the overall energy change is positive, 
Example Question #1 : Endothermic And Exothermic Reactions
Is the reaction above endothermic or exothermic?
Exothermic because an increase in temperture would cause the reaction to shift to the right
Endothermic because an increase in temperture would cause the reaction to shift to the left
Endothermic because an increase in temperture would cause the reaction to shift to the right
Exothermic because an increase in temperture would cause the reaction to shift to the left
Exothermic because an increase in temperture would cause the reaction to shift to the left
The reaction is an exothermic reaction, since the heat is added on the products side. An endothermic reaction would have the heat on the reactants side. An increase in heat in this reaction would cause the Keq to decrease, and there will be a shift to the left of the reaction (toward the reactants).
Remember that Keq is dependent on temperature, and can be affected by changes in heat. Keep in mind, also, that the reverse reaction that occurs during the leftward shift will be endothermic, using the additional heat as a reactant rather than a product.
Example Question #11 : Endothermic And Exothermic Reactions
A student is performing a reaction with unknown compounds in his chemistry lab. The only information the student knows about the reaction is that it is endothermic and reversible. Using this knowledge alone, how can the student increase the yield of his product?
Add product to the reaction
Remove reactant from the reaction
Increase the temperature
Decrease the temperature
Increase the temperature
To answer this question, we need to have a solid understanding of Le Chatelier's principle.
In an endothermic reaction, heat is needed to facilitate the reaction. To increase the products, we want to shift the reaction to the right.
We should already know that adding product or removing reactant shifts the equilibrium to the left, and yields more starting material rather than product. In an endothermic reaction, we can consider heat as a reactant; thus, adding heat (increasing temperature) would allow us to shift the reaction to the right.
Decreasing the temperature, removing reactant, or adding product would all increase the yield of the starting materials.
Example Question #11 : Endothermic And Exothermic Reactions
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.
Growing frustrated by his inability to decipher what chemical reaction the student had started, the scientist decides to measure the reaction vessel's temperature. Based only on the above reaction diagram, what is he most likely to find if the reaction is ongoing?
(Assume that the reaction vessel is defined as the system, and that the entropy of the system decreases)
A cold container from an exothermic reaction
A warm container from an endothermic reaction
A cold container from an endothermic reaction
A warm container from an exothermic reaction
The products of this reaction have an equal energy level to the reactants
A warm container from an exothermic reaction
Point 5 is the energy level of the products of the reaction, while point 1 is the energy level of the reactants. Point 5 is lower than is point 1, indicating that the products of this reaction contain lower overall energy than do the reactants. This energy must be released in some form, likely as heat, characteristic of an exothermic reaction.
The question further specifies that there is a local decrease in entropy of the system, thus, the only way that entropy of the universe can increase is to release heat and increase the entropy of the surroundings.
Example Question #12 : Endothermic And Exothermic Reactions
Why does a chemical ice pack feel cold?
The chemicals in the ice pack undergo an endothermic reaction; this pulls heat from its surroundings for the reaction, making the ice pack feel cold.
The chemicals in the ice pack undergo an endothermic reaction; this releases heat into the surroundings, making the ice pack feel cold.
The chemicals in the ice pack undergo an exothermic reaction; this pulls heat from the surroundings, making the ice pack feel cold.
The chemicals in the ice pack undergo an exothermic reaction; this releases heat into the surroundings, making the ice pack feel cold.
The chemicals in the ice pack undergo an endothermic reaction; this pulls heat from its surroundings for the reaction, making the ice pack feel cold.
Endothermic reactions by definition, require heat as a reactant. By drawing in heat from the surrounding, the surrounding will have a lower temperature compared to air temperature, which is why the ice pack feels cold. If it were an exothermic reaction, the pack would release heat, making the pack feel warm.
Example Question #1 : Endergonic And Exergonic Reactions
Boiling point is the temperature a liquid needs to achieve in order to begin its transformation into a gaseous state. Campers and hikers who prepare food during their trips have to account for differences in atmospheric pressure as they ascend in elevation. During the ascent, the decrease in atmospheric pressure changes the temperature at which water boils.
Further complicating the matter is the observation that addition of a solute to a pure liquid also changes the boiling point. Raoult’s Law can be used to understand the changes in boiling point if a non-volatile solute is present, as expressed here.
In this law, 
A scientist is studying solution chemistry to better understand vapor pressure. He finds that, for one solution he creates, the beaker is cool to the touch after the solute is fully dissolved. Which of the following is true of this solution? (Note: The beaker, solute, and solvent are the system, the remainder of the universe is the surroundings)
It always forms spontaneously
It forms spontaneously only at low temperatures
It never forms spontaneously
It forms spontaneously only if dissolution decreases entropy of the system
It forms spontaneously only at high temperatures
It forms spontaneously only at high temperatures
The act of dissolving a solute in a solvent is a local increase in entropy, converting a single molecule to multiple ions. The absorption of heat from the surroundings (cool beaker) indicates that this is an endothermic dissolution. We can look at the equation for Gibbs free energy to evaluate the possible answers.
In order to be spontaneous, the reaction must have a negative Gibbs free energy. To accomplish this, a reaction may have a negative enthalpy (exothermic) and positive entropy, however we know that our reaction has a positive enthalpy (endothermic) and positive entropy. A reaction will be spontaneous if it has a positive 
Example Question #2 : Endergonic And Exergonic Reactions
A scientist is studying a reaction, and places the reactants in a beaker at room temperature. The reaction progresses, and she analyzes the products via NMR. Based on the NMR readout, she determines the reaction proceeds as follows:
In an attempt to better understand the reaction process, she varies the concentrations of the reactants and studies how the rate of the reaction changes. The table below shows the reaction concentrations as she makes modifications in three experimental trials.
Assume that the forward reaction in the passage is exothermic, and has a 
The reaction is always spontaneous
The reaction is spontaneous only at low temperatures
The reaction is never spontaneous
The reaction is spontaneous only at high temperatures
The spontaneity of the reaction cannot be predicted
The reaction is always spontaneous
The question specifies that this reaction is exothermic, and that it has a positive local increase in entropy as it progresses. This means that the change in enthaply is negative and the change in entropy is positive.
For a reaction to be spontaneous, Gibbs free energy must be negative.
If enthalpy is negative and entropy is positive, the temperature is irrelevant and Gibbs free energy will always be negative.
The reaction is spontaneous at any temperature.
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