Test: Thermodynamics - MCAT MCQ

When a substance absorbs heat, it can either use this to increase its temperature or undergo a phase change. It cannot do both.

A solution at boiling point will use its continued applied heat energy to complete its phase change.

The heat of vaporization (ΔH_vaporization) is only used to determine the amount of heat energy used during the vaporization phase change. It does not indicate any information about the temperature change for a given heat energy input. As heat increases a solution’s temperature, the velocity of the solution’s molecules will increase.

The specific heat capacity refers to the amount of heat required to cause a unit of mass to change its temperature by 1°C.

The specific heat capacity does not refer to a material’s resistance to melting at its melting point. This is value is referred to as the heat of fusion, ΔH_fusion​

The specific heat capacity is a proportion that affects how a material’s absorption or release of heat changes its temperature. It is detailed by the equation: q = mcΔT, where q is heat, c is the specific heat capacity, and ΔT is the change in temperature.

According to the q = mcΔT equation, as long as the provided masses of metal solids are the same, the metal with the greatest specific heat will undergo the smallest change in temperature for a given heat energy value.

This is an adiabatic process which means there is no heat transfer, q = 0.
According to the first law of thermodynamics, internal energy is equal to the sum of the heat within the system and the work done on the system. ΔU = q + w 
Following the first law of thermodynamics equation:

The molar concentration of gas is increasing as more moles of gas is added into the rigid container.

A rigid container does not change in volume, it is an isochoric system.

Pressure change is unknown in this system. Using PV = nRTP, for ideal gases, we expect n, moles of gas to increase and V, volume to stay constant. However, we do not know how pressure and temperature are being affected in this system.

W = PΔV

No work is done on or by the container since the container is isochoric.

The first law of thermodynamics states that energy and matter cannot be created or destroyed, but only converted from one form to another.

The creation of matter and energy in a closed system is a violation of this law.

The spring, the machine, and the wind turbine examples all deal with the conversion of energy from one form to another and do not violate the first law of thermodynamics. However, the electrochemical cell that will indefinitely generate electricity is an example where energy is being created in a closed system without external energy input. This is a direct violation of the conservation of energy.

Two objects are only in thermal equilibrium if they are physically touching each other (or have a third intermediary object touching both of them) and experience no heat flow between them. This is not the case in this situation as heat will always transfer from hot objects to cold objects.

Temperature is related to speed of molecules. During collisions between molecules, the speeds tend to average out, increasing the speed of the slower molecules, so the kinetic energy and the related temperature of the molecules tends to even out over time, causing the heat to always transfer from hot to cold as long as there is a conducting path.

Heat flows from hot to cold but internal energy does not necessarily get transferred from high internal energy to lower internal energy. Internal energy also encompasses a system’s stored potential energy that does not immediately alter the speed of molecules and is not easily transferred through a conducting path.

According to the given statement, atmospheric gases absorb more energy than they emit. In thermodynamics, heat is defined as the transfer of energy between a system and its surroundings due to a temperature difference. When a gas absorbs heat, it means that energy is entering the gas from its surroundings, resulting in an increase in the internal energy of the gas. Therefore, the heat absorbed by the gas is positive.

Since the gas is kept at a constant pressure, we can calculate work with W = PΔV.

The volume of the gas is increased so the value of work done on the gas is negative.

The magnitude of work will be the area of under the function from A to B. Which is 200 in the case. The net work done on the gas as it goes from point A to B is −200 J.

We can calculate work as the area under the line graph in the diagram.

The volume of the gas is decreased, so the value of work done on the gas is positive.

The magnitude of work will be the area under the function from C to D. The path from D to E is vertical, so there is no work being done during this portion.

250 Pa x 2 m³ = 500 J.
And 500 J + 0J = 500 J.

An exothermic reaction is a chemical reaction that releases heat energy to the surroundings. It is characterized by a negative change in enthalpy, indicating a decrease in the energy content of the system.