Thermodynamics is a branch of physics that deals with the relationships between heat, work, and energy. It is concerned with the energy exchanges that occur during physical and chemical processes, and the effects these exchanges have on the properties of matter.
The study of thermodynamics is essential for understanding a wide range of phenomena, from the behavior of engines and refrigerators to the dynamics of chemical reactions.
At the heart of thermodynamics are two fundamental laws that describe the behavior of energy and the behavior of entropy (a measure of the disorder or randomness of a system).
These laws provide a framework for understanding how energy is transformed and how it is used to do work, and they have wide-ranging applications in fields as diverse as engineering, chemistry, and biology.
Definition: Thermodynamics is the study of energy and work exchanges that occur during physical and chemical processes.
First Law of Thermodynamics
The law of energy conservation, stating that energy cannot be created or destroyed, only transformed from one form to another.
Second Law of Thermodynamics
The law stating that in any energy exchange, there is always some loss, and the total entropy (a measure of the disorder or randomness of a system) always increases over time.
Basic Concepts:
Work and Heat Transfer
Types of Work
- Pressure-volume work: Work done by a system when its volume changes under constant pressure.
- Electrical work: Work done by an electric current passing through a conductor.
- Other types of work include work done by a moving object against a force, work done by a magnet on a magnetic field, and work done by a spring as it expands or contracts.
Modes of Heat Transfer
- Conduction: Heat transfer through direct contact between objects.
- Convection: Heat transfer through the movement of a fluid (such as a gas or liquid).
- Radiation: Heat transfer through electromagnetic waves, such as light.
In thermodynamics, work is the transfer of energy by using force, and heat is the transfer of energy between systems because of a difference in temperature. To understand how energy behaves in different systems, you need to know about the different kinds of work and how heat moves.
Thermodynamic systems and processes
Thermodynamic systems and processes are central to the study of thermodynamics. A thermodynamic system is a part of the universe that we choose to focus on for the purposes of study, and it is distinguished from its surroundings by a boundary that separates the two.
The type of system (closed, open, or isolated) determines the possible exchanges of matter and energy with the surroundings.
A reversible process is one that can be returned to its initial state by reversing the path of the process, while an irreversible process is one that cannot be returned to its initial state.
Understanding the concept of thermodynamic systems and the nature of reversible and irreversible processes is essential for the study of thermodynamics.
a. Closed Systems
- A closed system is one in which matter cannot enter or leave the system, but energy can be exchanged with the surroundings.
- An example of a closed system is a sealed container of gas, where the gas molecules can move and interact but no new gas can be added or removed.
b. Open Systems
- An open system is one in which both matter and energy can be exchanged with the surroundings.
- An example of an open system is a pot of water on a stove, where the water can evaporate and new water can be added, and heat can be transferred to or from the water.
c. Isolated Systems
- An isolated system is one in which neither matter nor energy can be exchanged with the surroundings.
- An example of an isolated system is a sealed container of gas in outer space, where there are no other objects or energy sources present.
d. Reversible Processes
- A reversible process is one in which a system can be returned to its initial state by reversing the path of the process.
- An example of a reversible process is the expansion and contraction of a gas in a cylinder with a movable piston, where the gas can be returned to its initial state by reversing the direction of the piston.
e. Irreversible Processes
- An irreversible process is one in which a system cannot be returned to its initial state by reversing the path of the process.
- An example of an irreversible process is the mixing of two gases, where the gases cannot be separated by reversing the process of mixing.
In thermodynamics, the concept of a system is important because it allows us to study the exchange of energy and matter with the surroundings. The type of system (closed, open, or isolated) determines the possible energy exchanges, and the reversibility or irreversibility of a process determines the amount of energy lost during the process.
Thermodynamic properties of substances
Thermodynamic properties of substances are fundamental to the study of thermodynamics. These properties describe the energy of a substance and how it changes in response to various influences, such as heat, work, and chemical reactions.
The internal energy of a substance is the total energy of its molecules, including the kinetic energy of their motion and the potential energy of their interactions. Enthalpy is a measure of the energy of a system, including the heat absorbed or released at a constant pressure. Entropy is a measure of the disorder or randomness of a system.
Understanding these thermodynamic properties and how they change during various processes is essential for predicting and controlling the behavior of energy in various systems.
a. Internal Energy
- The internal energy of a substance is the total energy of its molecules, including the kinetic energy of their motion and the potential energy of their interactions.
- The internal energy of a substance can change due to the transfer of heat or work, or due to changes in the substance itself, such as a chemical reaction or a phase change.
b. Enthalpy
- Enthalpy is a measure of the energy of a system, including the heat absorbed or released at a constant pressure.
- The enthalpy of a substance can change due to changes in temperature, pressure, or the chemical composition of the substance.
c. Entropy
- Entropy is a measure of the disorder or randomness of a system.
- The entropy of a substance can increase due to an increase in the number of possible microstates (arrangements of the molecules) available to the system, or due to the transfer of heat from a hotter to a cooler body.
d. Ideal Gases
- An ideal gas is a hypothetical gas that follows the ideal gas law, a mathematical model that describes the behavior of gases under certain conditions.
- The internal energy, enthalpy, and entropy of an ideal gas can be calculated using the ideal gas law and other thermodynamic equations.
e. Tables and Diagrams
- Thermodynamic tables and diagrams, such as the ideal gas law and steam tables, provide a convenient way to look up and compare the thermodynamic properties of different substances.
- These tables and diagrams can be used to calculate the changes in thermodynamic properties that occur during various processes, such as heating or cooling, expansion or contraction, or chemical reactions.
Thermodynamic Tables and Diagrams
| Substance | Ideal Gas Law | Steam Tables |
|---|---|---|
| Substance 1 | Data | Data |
| Substance 2 | Data | Data |
| Substance 3 | Data | Data |
Thermodynamic Processes
| Process | Property Changes |
|---|---|
| Heating | Property 1 |
| Cooling | Property 2 |
| Expansion | Property 3 |
| Contraction | Property 4 |
| Chemical Reactions | Property 5 |
Note: The Data in the table above can be replaced with respective pressure, temperature, density, specific heat, enthalpy, entropy etc. and in the second table can be replaced with the respective property that changes during each process.
Understanding the thermodynamic properties of substances is essential for predicting and controlling the behavior of energy in various systems. The internal energy, enthalpy, and entropy of a substance can be calculated using the appropriate equations and tables, and these properties can be used to predict the outcomes of various processes.
Thermochemistry
Thermochemistry is the study of the heat changes that occur during chemical reactions. It is a branch of thermodynamics that deals with the measurement and calculation of the enthalpy of chemical reactions, and the use of this information to predict the feasibility of reactions and the amount of energy that is released or absorbed.
The enthalpy of a chemical reaction is an important thermodynamic property that can be used to predict the direction of a reaction and the amount of energy that is required or released.
Hess’s law allows us to calculate the enthalpy change of a reaction by summing the enthalpy changes of intermediate reactions.
Understanding the principles of thermochemistry is essential for predicting and controlling the behavior of energy in various systems.
Enthalpy of Chemical Reactions
The enthalpy of a chemical reaction is the change in enthalpy that occurs when the reactants are converted to the products.
The enthalpy of a reaction can be calculated using the enthalpies of formation of the reactants and products, according to the equation:
∆H = ∑H products – ∑H reactants
For example, the enthalpy of the reaction between hydrogen gas (H2) and oxygen gas (O2) to form water (H2O) can be calculated using the enthalpies of formation of H2, O2, and H2O:
∆H = H2O (g) – H2 (g) – O2 (g) = -285.8 kJ/mol
Hess’s Law
Hess’s law states that the enthalpy change of a chemical reaction is the same, regardless of the path taken to get from the reactants to the products. This means that the enthalpy change of a reaction can be calculated by summing the enthalpy changes of a series of intermediate reactions that add up to the final reaction.
For example, consider the reaction between H2 and O2 to form H2O. If we break this reaction down into two intermediate reactions:
H2 (g) + (1/2) O2 (g) -> H2O (l) ∆H1 = -285.8 kJ/mol
H2O (l) -> H2O (g) ∆H2 = +44.0 kJ/mol
The final reaction can be written as:
H2 (g) + O2 (g) -> 2 H2O (g) ∆H = ∆H1 + ∆H2 = -241.8 kJ/mol
Enthalpy Changes in Chemical Reactions
- The enthalpy change of a chemical reaction can be used to predict the feasibility of a reaction and the amount of energy that is released or absorbed during the reaction.
- Exothermic reactions are reactions that release heat, resulting in a negative enthalpy change.
- Endothermic reactions are reactions that absorb heat, resulting in a positive enthalpy change.
Thermochemistry is the study of the heat changes that occur during chemical reactions. The enthalpy of a chemical reaction is an important thermodynamic property that can be used to predict the feasibility of a reaction and the amount of energy that is released or absorbed.
Hess’s law allows us to calculate the enthalpy change of a reaction by summing the enthalpy changes of intermediate reactions. Understanding the enthalpy changes of chemical reactions is essential for predicting and controlling the behavior of energy in various systems.
Phase equilibria and phase diagrams
Phase equilibria and phase diagrams are important tools for understanding the behavior of substances under different conditions. Phase changes involve the transfer of heat, and the enthalpy of the phase change is a measure of this heat transfer.
Phase diagrams provide a graphical representation of the phase changes that a substance can undergo, and can be used to predict the phase of a substance at a given temperature and pressure.
There are several types of phase diagrams, including pressure-temperature (PT) diagrams, pressure-composition (PC) diagrams, and temperature-composition (TC) diagrams. Understanding phase equilibria and phase diagrams is essential for predicting and controlling the behavior of matter in various systems.
Phase Changes
- A phase change is a change in the physical state of a substance, such as the change from a solid to a liquid or a gas.
- Phase changes involve the transfer of heat, and the enthalpy of the phase change is the heat absorbed or released during the change.
- Solid-liquid-gas equilibrium refers to the balance between the three phases of a substance, and the vapor pressure of a substance is the pressure exerted by its vapor in equilibrium with the liquid or solid phase.
Phase Diagrams
- A phase diagram is a graphical representation of the phase changes that a substance can undergo as a function of temperature and pressure.
- There are several types of phase diagrams, including pressure-temperature (PT) diagrams, pressure-composition (PC) diagrams, and temperature-composition (TC) diagrams.
- Phase diagrams can be used to predict the phase of a substance at a given temperature and pressure, and to determine the conditions under which phase changes will occur.
Applications of Thermodynamics
Thermodynamics has a wide range of applications in fields such as power generation, refrigeration and air conditioning, and chemical reactions.
Power cycles convert heat into work, and refrigeration and air conditioning systems transfer heat from a low temperature to a high temperature.
Chemical reactions involve the transfer of energy, and the principles of thermodynamics can be used to predict the feasibility and rate of chemical reactions.
a. Power Cycles
- Power cycles are systems that convert heat into work, such as the Rankine cycle and the Carnot cycle.
- The Rankine cycle is a power cycle that uses water as the working fluid, and it is commonly used in steam power plants.
- The Carnot cycle is a theoretical power cycle that is the most efficient possible cycle for converting heat into work, according to the second law of thermodynamics.
b. Refrigeration and Air Conditioning
- Refrigeration and air conditioning systems use the principles of thermodynamics to transfer heat from a low temperature to a high temperature, and to remove heat from a space or object.
- Common refrigerants include ammonia, Freon, and HFCs.
c. Chemical Reactions and Chemical Equilibrium
- Chemical reactions involve the transfer of energy, and the principles of thermodynamics can be used to predict the feasibility and rate of chemical reactions.
- Chemical equilibrium is the state in which the rates of the forward and reverse reactions are equal, and the concentrations of the reactants and products remain constant.
- The equilibrium constant is a measure of the relative concentrations of the reactants and products at equilibrium, and it can be used to predict the direction of a chemical reaction.
To predict and control how energy acts in different systems, it is important to understand how thermodynamics can be used.
