There are 4 laws of thermodynamics:
- The Zeroth law of thermodynamics.
- The First law of thermodynamics.
- The Second law of thermodynamics.
- The Third law of thermodynamics
Zeroth law of thermodynamics
The zeroth law of thermodynamics states that if two thermodynamic systems are each in thermal equilibrium with a third one, then they are in thermal equilibrium with each other. The law is based on temperature measurement and is used to predict whether heat will transfer between objects or not. But overall, this law focuses on the idea that if the systems are in thermal equilibrium, no heat flow will take place.
First law of thermodynamics
The First Law of Thermodynamics states that heat is a form of energy, and thermodynamic processes are therefore subject to the principle of conservation of energy. This indicates that heat energy cannot be created or destroyed. It can, however, be transferred from one location to another and converted to and from other forms of energy, and heat given to the system is used to change the internal energy, while the rest is used to do work by the system.
Second Law of Thermodynamics
The second law of thermodynamics states that the entropy in an isolated system always increases. Any isolated system spontaneously evolves toward thermal equilibrium, the state of maximum entropy. It is the second law of thermodynamics that provides the criterion for the feasibility of any process.
Note: The entropy of the universe only increases and never decreases.
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Entropy
Entropy is defined as the measure of the randomness of the system, or it is the measure of energy within an isolated system. The concept was introduced by physicist Rudolf Clausius in 1865. The entropy of the universe only increases and never decreases. It can be considered as a quantitative index that describes the quality of energy.
Now let us consider two factors that cause an increase in the entropy of a closed system.
- In a closed system, while the mass remains constant, there is an exchange of heat with the surroundings. This change in the heat content creates a disturbance in the system; hence, the entropy of the system increases.
- Internal changes may occur in the movements of the molecules of the system. This leads to disturbances that further cause irreversibilities inside the system, resulting in the increase of entropy.
Thermodynamics
This law was developed by the German chemist Walther Nernst between the years 1906 and 1912. It states that the entropy of a perfect crystal at a temperature of zero Kelvin (absolute zero) is equal to zero.
Entropy is denoted as ’S’; it is a measure of the disorder in a closed system. It is directly related to the number of microstates accessible to the system, which means the greater the number of microstates the closed system can occupy, the greater its entropy. The microstate in which the energy of the system is at its minimum is called the ground state of the system.
Mathematical Explanation of the Third Law
Enthalpy (H)
It is the total heat content of a system. It is considered the sum of internal energy and pressure-volume energy of a system, where
H = U + pV
The unit of enthalpy is kJ/mol. It is a state function with an extensive property. So whenever there is a change in enthalpy, it depends on the initial and final values and not on the path followed.
H2 is the enthalpy in the final state, then the change in enthalpy is
The standard enthalpy of reaction
It is the enthalpy change for a reaction when all the substances are in their standard states. In thermodynamics, the standard state refers to the existence of a substance in its pure form at 1 bar pressure and 298 K temperature.
Enthalpy of phase transition
It is the enthalpy change when one mole of a substance changes from one phase to another phase at a particular temperature. The important types of Enthalpies of phase transition are of 3 types:
Note: Enthalpies of fusion, vaporization, and sublimation are positive, since heat is absorbed during these processes; the magnitude of the enthalpy change depends on the strength of the intermolecular forces in the substance.
Relation between ΔH and ΔU
Or, ΔH = (U2 − U1) + (pV2 − pV1)
Or, ΔH = ΔU + p (V2 − V1)
Or, ΔH = ΔU + pΔV (eq.3)
Endothermic and Exothermic processes
Exothermic and endothermic processes are thermodynamic changes that describe how heat and energy move between a chemical system and its surroundings. Let’s learn more about it:
Hess’s Law of Constant Heat Summation
The law states that the total enthalpy change for a physical or chemical process is the same whether the reaction takes place in a single step or in several steps.
Standard enthalpy of combustion (ΔcHΘ)
The total heat released when exactly one mole of a substance burns completely in excess oxygen under standard conditions. It is always negative since heat always evolves during combustion.
Enthalpy of atomization (ΔaHΘ)
It is the enthalpy change on breaking one mole of bonds completely to obtain atoms in the gas phase. In the case of diatomic molecules, the enthalpy of atomization is the same as the bond dissociation enthalpy.
Bond Enthalpy (ΔbondHΘ)
A chemical reaction that involves the breaking and making of chemical bonds. Energy is required to break a bond, and energy is released when a bond is formed. So enthalpy of bond dissociation is positive, and enthalpy of bond formation is negative.
Bond dissociation enthalpy
The amount of heat required to break one mole of covalent bonds to form products in the gas phase is called the bond dissociation enthalpy. For diatomic molecules, bond dissociation enthalpy and bond enthalpy are the same.
Heat capacity ( C)
- Specific heat capacity(c) is the heat capacity for one unit of mass (usually 1 kg or 1g) of a substance. It is measured in J/(kg .K)
- Molar heat capacity (Cm) is the amount of heat required to raise the temperature of 1 mole of a substance.It is measured in J/(mol. K)
Gibbs Energy
FAQ's
The First Law states that heat is a form of energy and follows the principle of conservation of energy.
Enthalpy is given by H = U + PV, where U is internal energy, P is pressure, and V is volume.
An endothermic process absorbs heat from the surroundings, whereas an exothermic process releases heat to the surroundings.
Hess’s Law states that the total enthalpy change is the same whether a reaction occurs in a single step or in several steps.
The Second Law states that the entropy of an isolated system always increases, and the entropy of the universe never decreases.
It is the total heat released when exactly one mole of a substance burns completely in excess oxygen under standard conditions.
The enthalpy of a reaction is defined as the heat energy change that occurs when reactants become products. ΔH is positive if heat is absorbed during the reaction; if heat is released, ΔH is negative.
This is the conversion of kinetic energy to chemical energy. Heat is absorbed and converted into chemical energy during an endothermic reaction.
The main difference between exothermic and endothermic reactions is that an endothermic reaction absorbs energy in the form of heat from its surroundings, whereas an exothermic reaction releases energy to the surroundings
Bond dissociation enthalpy is the amount of heat required to break one mole of covalent bonds and form products in the gas phase.