MCAT Physical : Physical Chemistry

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

Example Question #3 : 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.

Possible Answers:

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

Entropy of the system must increase

The reaction is strongly endothermic

Correct answer:

More energy is released when hydrated ions form than is absorbed when ionic bonds break

Explanation:

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 #1 : 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 __________.

Possible Answers:

Exothermic, with a positive

Endothermic, with a positive

Endothermic, with a negative

Exothermic, with a negative

Correct answer:

Exothermic, with a negative

Explanation:

Any reaction in which heat is evolved (released) is classified as exothermic. The change in heat content () is negative for this type of reaction.

Example Question #56 : Thermochemistry And Energetics

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 __________.

Possible Answers:

exothermic in this scenario, and nonspontaneous at standard conditions

exothermic in this scenario, and spontaneous at standard conditions

exothermic in this scenario, and nonspontaneous at current conditions

endothermic in this scenario, and nonspontaneous at standard conditions

endothermic in this scenario, and spontaneous at standard conditions

Correct answer:

endothermic in this scenario, and nonspontaneous at standard conditions

Explanation:

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 #5 : Endothermic And Exothermic Reactions

A system experiences the following energetic changes:

Which of the following is true? 

Possible Answers:

The system loses heat

The system loses  of energy

 of energy is input into the system

 of energy is input into the system

Correct answer:

 of energy is input into the system

Explanation:

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,  of energy is input into the system rather than lost to the surroundings.

Example Question #2 : Endothermic And Exothermic Reactions

Is the reaction above endothermic or exothermic?

Possible Answers:

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 right

Endothermic 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

Correct answer:

Exothermic because an increase in temperture would cause the reaction to shift to the left

Explanation:

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?

Possible Answers:

Remove reactant from the reaction

Increase the temperature

Decrease the temperature

Add product to the reaction

Correct answer:

Increase the temperature

Explanation:

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 #12 : 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.

 

 

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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)

Possible Answers:

A cold container from an exothermic reaction

A warm container from an endothermic reaction

A cold container from an endothermic reaction

The products of this reaction have an equal energy level to the reactants

A warm container from an exothermic reaction

Correct answer:

A warm container from an exothermic reaction

Explanation:

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 #91 : Biochemistry, Organic Chemistry, And Other Concepts

Why does a chemical ice pack feel cold? 

Possible Answers:

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.

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 endothermic reaction; this releases heat into the surroundings, making the ice pack feel cold.

Correct answer:

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.

Explanation:

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,  is the mole fraction of the solvent, is the vapor pressure of the pure solvent, and  is the vapor pressure of the solution. When this vapor pressure is equal to the local atmospheric pressure, the solution boils.

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)

Possible Answers:

It always 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 low temperatures

It never forms spontaneously

Correct answer:

It forms spontaneously only at high temperatures

Explanation:

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  and a positive  only when temperature is high.

Example Question #1 : 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  value of .  Which of the following is true?

Possible Answers:

The reaction is never spontaneous

The reaction is spontaneous only at low temperatures

The reaction is always spontaneous

The reaction is spontaneous only at high temperatures

The spontaneity of the reaction cannot be predicted

Correct answer:

The reaction is always spontaneous

Explanation:

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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