MCAT Physical : Thermochemistry and Energetics
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Example Questions
Example Question #81 : Physical Chemistry
Which of the following scenarios describes a reaction in equilibrium?
When 
When Gibbs free energy is negative
When the forward rate is 0M/s
When the enthalpy is equal to the entropy
When
According to the Gibbs free energy equation, a system is at equilibrium when 
Since 
Example Question #1 : Other Thermodynamic Principles
Which of the following is not related to bond dissociation energy?
Energy stored in coordinate covalent bonds
Energy stored in ionic bonds
Energy stored in hydrogen bonds
Energy stored in nonpolar covalent bonds
Energy stored in polar covalent bonds
Energy stored in hydrogen bonds
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 #2 : 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?
Element 1 is in a fixed container, while element 2's container expanded when the compound was heated.
The element in container 2 released more heat to the surroundings.
The elements are in different phases.
The element in container 1 has more mass than the element in container 2.
The element in container 1 has more mass than the element in container 2.
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 
Rewritten as 
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 #3 : Other Thermodynamic Principles
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?
Convection
Transduction
Conduction
The heat transfer does not come from the burning tree
Radiation
Convection
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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