MCAT Physical : Other Thermodynamic Principles

Study concepts, example questions & explanations for MCAT Physical

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Example Questions

Example Question #72 : Thermochemistry And Energetics

Which of the following is not related to bond dissociation energy?

Possible Answers:

Energy stored in hydrogen bonds

Energy stored in polar covalent bonds

Energy stored in ionic bonds

Energy stored in nonpolar covalent bonds

Energy stored in coordinate covalent bonds

Correct answer:

Energy stored in hydrogen bonds

Explanation:

Bond dissociation energy is the energy associated with a bond within a molecule. This means that bond dissociation energy is measured for intramolecular bonds. All covalent and ionic bonds are considered intramolecular bonds, and are generally quite permanent. Ionic bonds and polar covalent bonds can help develop dipoles in a molecule, which later facilitate intermolecular interactions.

Hydrogen bonds are intermolecular bonds, which do not have associated bond dissociation energies because these type of bonds are temporary, and are formed between different molecules.

Example Question #1 : Other Thermodynamic Principles

A specific element is kept in two separate containers. The samples are both heated by the same amount of heat, but the element in container 1 experiences a greater temperature change than the element in container 2.

Which of the following could NOT be an explanation for this difference in temperature change?

Possible Answers:

The elements are in different phases.

The element in container 1 has more mass than the element in container 2.

The element in container 2 released more heat to the surroundings.

Element 1 is in a fixed container, while element 2's container expanded when the compound was heated.

Correct answer:

The element in container 1 has more mass than the element in container 2.

Explanation:

If there was more mass (in grams) of the element in container 1 than there was in container 2, we would expect container 1 to undergo a smaller temperature change compared to container 2. This is justified by the equation , where m is the mass.

Rewritten as , we can see that a larger mass would result in a smaller temperature change. Remember that q is held constant between the two containers.

If element 2 expelled some of its energy to the surroundings in the form of work (expanding the container), it could explain why it had a smaller temperature change. In addition, different phases of an element have different specific heat capacities. This can also explain the difference in temperature between the two element samples.

Example Question #74 : Thermochemistry And Energetics

A burning tree in a forest causes another tree that is twenty inches away to also start burning, without touching it. How is the heat being transferred from the burning tree to the other?

Possible Answers:

Transduction

The heat transfer does not come from the burning tree

Conduction

Convection

Radiation

Correct answer:

Convection

Explanation:

Convection describes heat transfer through a fluid medium, such as a gas or liquid. In this case, the burning tree transfers heat to the air, which transfers the heat to the other tree.

Conduction requires direct contact, which would occur if a burning tree fell into another tree. Radiation is the electromagnetic transfer of heat, such as the sun's heat that travels to Earth, and does not require matter to transfer. Transduction is not a mechanism for heat transfer.

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