Thermodynamics - Laws & Energy Transfer Flashcards

Question 1

What is Thermodynamics?

Accepted Answer

Thermodynamics is the branch of physics and chemistry that deals with the relationships between heat and other forms of energy and work. It describes how thermal energy is converted to and from other forms of energy and how it affects matter.

Question 2

Define System and Surroundings in Thermodynamics.

Accepted Answer

A **system** is the specific part of the universe under study (e.g., a chemical reaction, a gas in a container). The **surroundings** are everything else in the universe outside the system that can exchange energy or matter with it.

Question 3

Differentiate between Open, Closed, and Isolated Systems.

Accepted Answer

An **open system** can exchange both matter and energy with its surroundings. A **closed system** can exchange energy but not matter. An **isolated system** cannot exchange either matter or energy with its surroundings.

Question 4

State the Zeroth Law of Thermodynamics.

Accepted Answer

The Zeroth Law states that if two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. This law establishes the concept of temperature as a measurable property.

Question 5

State the First Law of Thermodynamics.

Accepted Answer

The First Law of Thermodynamics, also known as the Law of Conservation of Energy, states that energy cannot be created or destroyed in an isolated system; it can only be transferred or changed from one form to another. Mathematically, it's expressed as $\Delta U = Q - W$.

Question 6

What is Internal Energy ($U$ or $E$)?

Accepted Answer

Internal energy ($U$) is the total energy contained within a thermodynamic system, including the kinetic and potential energies of its molecules. It is a state function, meaning its value depends only on the current state of the system, not on the path taken to reach that state.

Question 7

Define Heat ($Q$) in thermodynamics and its sign convention.

Accepted Answer

Heat ($Q$) is the transfer of thermal energy between a system and its surroundings due to a temperature difference. By convention, $Q > 0$ when heat is absorbed by the system (endothermic), and $Q < 0$ when heat is released by the system (exothermic).

Question 8

Define Work ($W$) in thermodynamics, especially pressure-volume work, and its sign convention.

Accepted Answer

Work ($W$) is energy transferred when a force acts over a distance. In thermodynamics, it often refers to pressure-volume work, where $W = P\Delta V$. By convention, $W > 0$ when work is done *by* the system on the surroundings (e.g., expansion), and $W < 0$ when work is done *on* the system by the surroundings (e.g., compression).

Question 9

Explain the First Law equation: $\Delta U = Q - W$.

Accepted Answer

This equation states that the change in a system's internal energy ($\Delta U$) is equal to the heat ($Q$) added to the system minus the work ($W$) done by the system. It quantifies the conservation of energy for a thermodynamic process.

Question 10

Define Enthalpy ($\Delta H$).

Accepted Answer

Enthalpy ($\Delta H$) is a thermodynamic property representing the total heat content of a system at constant pressure. It is defined as $H = U + PV$, where $U$ is internal energy, $P$ is pressure, and $V$ is volume. $\Delta H$ represents the heat absorbed or released during a process at constant pressure.

Question 11

Differentiate between Exothermic and Endothermic processes.

Accepted Answer

**Exothermic processes** release heat to the surroundings, resulting in a negative change in enthalpy ($\Delta H < 0$). The system's energy decreases. **Endothermic processes** absorb heat from the surroundings, resulting in a positive change in enthalpy ($\Delta H > 0$). The system's energy increases.

Question 12

Define Specific Heat Capacity ($c$).

Accepted Answer

Specific heat capacity ($c$) is the amount of heat energy required to raise the temperature of one unit mass (e.g., 1 gram or 1 kg) of a substance by one degree Celsius or one Kelvin. Its units are typically $J/(g \cdot ^\circ C)$ or $J/(kg \cdot K)$.

Question 13

Define Heat Capacity ($C$).

Accepted Answer

Heat capacity ($C$) is the amount of heat energy required to raise the temperature of a given quantity of a substance (e.g., an entire object) by one degree Celsius or one Kelvin. It is an extensive property and is related to specific heat capacity by $C = mc$, where $m$ is mass. Its units are typically $J/^\circ C$ or $J/K$.

Question 14

How do you calculate heat transfer ($Q$) when there is a temperature change?

Accepted Answer

Heat transfer ($Q$) associated with a temperature change ($\Delta T$) can be calculated using the formula: $Q = mc\Delta T$, where $m$ is the mass of the substance, $c$ is its specific heat capacity, and $\Delta T$ is the change in temperature. Alternatively, $Q = C\Delta T$ if the heat capacity ($C$) of the entire substance is known.

Question 15

What is Latent Heat?

Accepted Answer

Latent heat is the heat absorbed or released by a substance during a phase change (e.g., melting, freezing, boiling, condensation) at a constant temperature. It does not cause a temperature change but rather changes the state of matter.

Question 16

How do you calculate heat transfer ($Q$) during a phase change?

Accepted Answer

Heat transfer ($Q$) during a phase change is calculated using the formula: $Q = mL$, where $m$ is the mass of the substance undergoing the phase change, and $L$ is the specific latent heat for that particular phase transition (e.g., latent heat of fusion for melting/freezing, latent heat of vaporization for boiling/condensation).

Question 17

State the Second Law of Thermodynamics.

Accepted Answer

The Second Law of Thermodynamics states that the total entropy of an isolated system can only increase over time, or remain constant in ideal cases where the system is in a steady state or undergoing a reversible process. It implies that spontaneous processes tend towards a state of greater disorder or randomness.

Question 18

Define Entropy ($\Delta S$).

Accepted Answer

Entropy ($\Delta S$) is a thermodynamic property that measures the degree of disorder or randomness in a system. A positive change in entropy ($\Delta S > 0$) indicates an increase in disorder, while a negative change ($\Delta S < 0$) indicates a decrease in disorder. Its units are typically $J/K$.

Question 19

What does $\Delta S_{univ} \ge 0$ imply for spontaneity?

Accepted Answer

The Second Law states that for any spontaneous process, the total entropy of the universe (system + surroundings) must increase or remain constant: $\Delta S_{univ} = \Delta S_{sys} + \Delta S_{surr} \ge 0$. If $\Delta S_{univ} > 0$, the process is spontaneous. If $\Delta S_{univ} = 0$, the process is at equilibrium and reversible. If $\Delta S_{univ} < 0$, the process is non-spontaneous in that direction.

Question 20

Define Gibbs Free Energy ($\Delta G$).

Accepted Answer

Gibbs Free Energy ($\Delta G$) is a thermodynamic potential that measures the 'useful' or process-initiating work obtainable from an isothermal, isobaric thermodynamic system. It is a key indicator of the spontaneity of a process at constant temperature and pressure.

Question 21

State the Gibbs Free Energy equation.

Accepted Answer

The Gibbs Free Energy equation is: $\Delta G = \Delta H - T\Delta S$, where $\Delta G$ is the change in Gibbs Free Energy, $\Delta H$ is the change in enthalpy, $T$ is the absolute temperature (in Kelvin), and $\Delta S$ is the change in entropy.

Question 22

How does $\Delta G$ relate to spontaneity at constant temperature and pressure?

Accepted Answer

For a process occurring at constant temperature and pressure:
*   If $\Delta G < 0$, the process is spontaneous (exergonic).
*   If $\Delta G > 0$, the process is non-spontaneous (endergonic), but the reverse process is spontaneous.
*   If $\Delta G = 0$, the system is at equilibrium.

Question 23

Summarize the conditions for spontaneity based on $\Delta H$ and $\Delta S$.

Accepted Answer

The spontaneity of a reaction depends on $\Delta H$, $\Delta S$, and temperature ($T$): * **$\Delta H < 0$, $\Delta S > 0$**: Always spontaneous (exergonic). * **$\Delta H > 0$, $\Delta S < 0$**: Never spontaneous (endergonic), reverse is always spontaneous. * **$\Delta H < 0$, $\Delta S < 0$**: Spontaneous at low temperatures (when $|T\Delta S| < |\Delta H|$). * **$\Delta H > 0$, $\Delta S > 0$**: Spontaneous at high temperatures (when $|T\Delta S| > |\Delta H|$).

Question 24

State the Third Law of Thermodynamics.

Accepted Answer

The Third Law of Thermodynamics states that the entropy of a perfect crystal at absolute zero (0 Kelvin or $-273.15 ^\circ C$) is exactly zero. This law provides a reference point for determining absolute entropies.

Thermodynamics - Laws & Energy Transfer

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