- The First Law of Thermodynamics states that energy can neither be created nor destroyed but can only be converted from one form to another.
- According to this law, the total energy of the system and surroundings remains constant during any physical or chemical change.
- When a system exchanges heat or work with surroundings, its internal energy changes.
- If heat (Q) is supplied to the system and work (W) is done on the system, the internal energy increases.
- The mathematical expression of the first law is
ΔU = Q + W. - For infinitesimal changes, the first law is written as
dU = dQ + dW. - For special processes:
• Isothermal process: ΔU = 0
• Adiabatic process: Q = 0 and −ΔU = W
• Isochoric process: ΔV = 0 and ΔU = Qᵥ
• Isobaric process: Qₚ = ΔU + PΔV.
Definitions [35]
Define isobaric process.
In isobaric process the pressure remains constant during the transformation.
Define Intensive property.
A property which is independent of the amount of matter in a system is called an intensive property.
e.g., pressure, temperature.
Define a state function.
A property that depends on the state of a system and is independent of the path taken to reach that state is called a state function.
Define the extensive properties.
A property that depends on the amount of matter present in a system is called an extensive property.
Example: mass, volume.
Define Closed system.
A closed system is one that is able to interchange energy but not matter with its surroundings.
Define Adiabatic process.
A process in which there is no exchange of heat between the system and surroundings is an adiabatic process, or Q = 0.
Define the internal energy of the system.
The internal energy of a thermodynamic system is the sum of kinetic and potential energies of all the molecules of the system with respect to the center of mass of the system.
Define the quasi-static process.
A quasi-static process is an infinitely slow process in which the system changes its variables (P, V, T) so slowly such that it remains in thermal, mechanical, and chemical equilibrium with its surroundings throughout.
Define one calorie.
One calorie is defined as the amount of heat energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere.
Define enthalpy of neutralization.
The enthalpy of neutralization is defined as the change in enthalpy of the system when one gram equivalent of an acid is neutralized by one gram equivalent of a base or vice versa in dilute solution.
\[\ce{H^+_{(aq)} + OH^-_{(aq)} -> H2O_{(l)}}\] = 57.32 kJ
Define enthalpy of combustion.
The heat of combustion of a substance is defined as “The change in enthalpy of a system when one mole of the substance is completely burnt in excess of air or oxygen”. It is denoted by ∆HC.
Define enthalpy.
Enthalpy of a system is sum of internal energy of a system and the energy equivalent to PV work.
H = U + PV
Define the Enthalpy of atomization.
The enthalpy change accompanying the dissociation of one mole of gaseous substance into atoms is called enthalpy of atomization.
Define the Enthalpy of sublimation.
Enthalpy of sublimation is the enthalpy change for the conversion of one mole of solid directly into vapour at constant temperature and pressure.
Define the enthalpy of freezing.
The enthalpy change that accompanies the solidification of one mole of a liquid into a solid at constant temperature and pressure is called the enthalpy of freezing.
Define the Enthalpy of vaporization.
Enthalpy of vaporization is the enthalpy change accompanying the vaporization of one mole of liquid without changing its temperature at constant pressure.
Define the Standard enthalpy of combustion.
The standard enthalpy of combustion of a substance is the standard enthalpy change accompanying a reaction in which one mole of the substance in its standard state is completely oxidised.
Define standard enthalpy of formation.
The standard enthalpy of formation of a compound is the enthalpy change that accompanies a reaction in which one mole of pure compound in its standard state is formed from its elements in their standard states.
Define the Bond enthalpy.
The enthalpy change required to break a particular covalent bond in one mole of the gaseous molecule to produce gaseous atoms and/or radicals is called bond enthalpy.
Define the Enthalpy of ionisation.
Enthalpy of ionization is the enthalpy change accompanying the removal of an electron from one mole of a gaseous atom.
Define entropy.
Entropy (S) is defined more precisely as a thermodynamic state function that measures the degree of randomness or disorder of the particles in a system.
Define second law of thermodynamics.
Second law of thermodynamics: In a spontaneous process, the overall entropy of the system and its surroundings grows.
Definition: Enthalpy of vaporization
Enthalpy change accompanying vaporization of one mole of liquid at constant temperature and pressure is called enthalpy of vaporization.
Definition: Enthalpy of sublimation
Enthalpy change for conversion of one mole of solid directly into vapour at constant temperature and pressure is called enthalpy of sublimation.
Definition: Enthalpy of fusion
Enthalpy change accompanying conversion of one mole of solid into liquid at constant temperature and pressure is called enthalpy of fusion.
Definition: Intensive property
A property which is independent of the amount of matter in a system is called intensive property.
Examples: Pressure, temperature, surface tension, viscosity, melting point, boiling point, specific heat.
Definition: Extensive property
A property which depends on the amount of matter present in a system is called an extensive property.
Examples: Mass, volume, internal energy, heat capacity, number of moles.
Definition: Entropy
A measure of molecular disorder or randomness of a system is called entropy.
Definition: Spontaneous process
A process which occurs on its own without any external influence is called spontaneous process.
Definition: Chemical Transformations
Thermodynamics is concerned with the energy changes in physical and chemical transformations.
Definition: State Function
The property which depends only on the state of the system and not on the path followed is called state function.
Definition: Path functions
The properties which depend on the path followed are called path functions.
Definition: Thermodynamic equilibrium
A system is said to be in thermodynamic equilibrium when its state functions do not change with time is called thermodynamic equilibrium.
Definition: Enthalpy
The sum of internal energy and pressure–volume energy of a system is called enthalpy.
Definition: Bond enthalpy
The enthalpy change required ·to break particular covalent bond in one mole of gaseous molecule to produce gaseous atoms and/or radicals, is called bond enthalpy.
Formulae [1]
Write the mathematical equation of the first law of thermodynamics for an isochoric process.
By substituting equation W = −pex . ΔV in the equation ΔU = q + W, we get
ΔU = q − pex . ΔV ...(1)
If the reaction is carried out in a closed container so that the volume of the system is constant, then Δ = 0. In such a case, no work is involved.
The equation (1) becomes ΔU = qv
Equation (1) suggests that the change in internal energy of the system is due to heat transfer. The subscript v indicates a constant volume process. As U is a state function, qv is also a state function. We see that an increase in the internal energy of a system is numerically equal to the heat absorbed by the system in a constant volume (isochoric) process.
Theorems and Laws [3]
Write the mathematical equation of the first law of thermodynamics for an isochoric process.
By substituting equation W = −pex . ΔV in the equation ΔU = q + W, we get
ΔU = q − pex . ΔV ...(1)
If the reaction is carried out in a closed container so that the volume of the system is constant, then Δ = 0. In such a case, no work is involved.
The equation (1) becomes ΔU = qv
Equation (1) suggests that the change in internal energy of the system is due to heat transfer. The subscript v indicates a constant volume process. As U is a state function, qv is also a state function. We see that an increase in the internal energy of a system is numerically equal to the heat absorbed by the system in a constant volume (isochoric) process.
Law: First law of thermodynamics
Law: Second low of thermodynamics
- The Second Law of Thermodynamics states that the total entropy of a system and its surroundings increases in a spontaneous process.
- For a process to be spontaneous, the total entropy change must be positive, given by
ΔSₜₒₜₐₗ = ΔSₛᵧₛ + ΔSₛᵤᵣᵣ > 0. - The entropy change of surroundings is calculated using
ΔSₛᵤᵣᵣ = −ΔH / T (at constant temperature). - If ΔSₜₒₜₐₗ > 0, the process is spontaneous; if ΔSₜₒₜₐₗ < 0, the process is non-spontaneous.
- At equilibrium, the total entropy change is zero, that is
ΔSₜₒₜₐₗ = 0.
Key Points
Key Points: Types of system
| Type of System | Exchange of Energy | Exchange of Matter | Example |
|---|---|---|---|
| Open System | Energy is exchanged with surroundings | Matter is exchanged with surroundings | Example: Open cup of hot coffee. It releases heat to surroundings and water vapour escapes into air. |
| Closed System | Energy is exchanged with surroundings | Matter is not exchanged with surroundings | Example: Hot coffee covered with saucer. It loses heat but water vapour does not escape. |
| Isolated System | No exchange of energy | No exchange of matter | Example: Insulated cup of coffee. Neither heat nor water vapour escapes to surroundings. |
Key Points: Nature of heat and work
- In mechanics, work is defined as force multiplied by displacement, given by the formula W = f × d.
- In thermodynamics, the work involved is pressure–volume work (PV work), which is done when a gas expands or contracts against an external opposing pressure.
- PV work is given by the expression W = −Pₑₓₜ ΔV, where ΔV is the change in volume.
- When a gas expands, it does work on the surroundings; when it is compressed, work is done on the system by the surroundings.
- Heat (Q) is a form of energy that is exchanged between the system and surroundings due to temperature difference.
- According to sign convention, +Q means heat absorbed by the system, −Q means heat released, +W means work done on the system, and −W means work done by the system; both heat and work are path functions.
Key Points: Expression for pressure-volume (PV) work
- Pressure–volume (PV) work is done when a gas expands or compresses against an external pressure in a cylinder fitted with a movable piston.
- The force exerted by the gas on the piston is equal to external pressure multiplied by area, given by
f = −Pₑₓₜ A. - Work done is equal to force multiplied by displacement, therefore
W = f × d. - Since change in volume is equal to area × displacement,
ΔV = A × d,
the expression for PV work becomes
W = −Pₑₓₜ ΔV = −Pₑₓₜ (V₂ − V₁). - During expansion (V₂ > V₁), work is done by the system on the surroundings and W is negative; during compression (V₂ < V₁), work is done on the system and W is positive.
- In free expansion (expansion in vacuum), external pressure is zero (Pₑₓₜ = 0), therefore
W = 0, and no work is done.
Important Questions [25]
- Define Adiabatic process.
- Define the extensive properties.
- Define Intensive property.
- Calculate the work done during the expansion of 2 moles of an ideal gas from 10 dm3 to 20 dm3 at 298 K in a vacuum.
- Write the sign convention of work done during expansion of gas.
- Three moles of an ideal gas are expanded isothermally from 15 dm3 to 20 dm3 at a constant external pressure of 1.2 bar. Calculate the amount of work in Joules.
- 2000 mmol of an ideal gas expanded isothermally and reversibly from 20 L to 30 L at 300 K, calculate the work done in the process (R = 8.314 JK–1 mol–1).
- One mole of an ideal gas is expanded isothermally and reversibly from 10 L to 15 L at 300 K. Calculate the work done in the process.
- Derive an expression for maximum work in isothermal reversible expansion of two moles of an ideal gas.
- Write one statement of first law of thermodyamics and its mathematical expression
- Calculate the internal energy at 298K for the formation of one mole of ammonia, if the enthalpy change at constant pressure is – 42.0 kJ mol-1. (Given: R = 8.314 J K-1 mol-1)
- Write the mathematical equation of the first law of thermodynamics for an isochoric process.
- Write mathematical equation of first law of thermodynamics for Adiabatic process
- Prove that ΔH=ΔU+ΔnRT. what is the condition under which ΔU=ΔH?
- Obtain the relationship between ΔH and ΔU for gas phase reactions.
- The enthalpy change for the chemical reaction HA2OA(s)⟶HA2OA(l) is called enthalpy of ______.
- Calculate the time required to deposit 2.4 g of Cu, when 2.03 A of current passed through CuSOA4, solution. (At. mass of Cu = 63.5 g mol−1)
- Calculate the standard enthalpy of formation of CH3OH(l) from the following data: [\ce{CH3OH_{(l)} + 3/2 O2_{(g)} -> CO2_{(g)} + 2H2O_{(l)}ΔH^° = - 726 kJ mol^{-1}}]
- Calculate the standard enthalpy of the reaction: SiO2(s) + 3C(graphite) → SiC(s) + 2CO(g) from the following reactions: Si(s) + O2(g) → SiO2(s),
- Define the Standard enthalpy of combustion
- Calculate the standard enthalpy of combustion of methane if the standard enthalpy of formation of methane, carbon dioxide and water are −74.8, −393.5 and −285.8 kJmol−1 respectively.
- Write the correct condition for spontaneity in terms of Gibbs energy.
- Answer the following in one or two sentences. State second law of thermodynamics in terms of entropy.
- For a certain reaction ΔH0 is −224 kJ and ΔS0 is −153 J K−1. At what temperature the change over from spontaneous to non-spontaneous will occur?
- Define entropy.
Concepts [12]
- Chemical Thermodynamics
- Terms Used in Thermodynamics
- Nature of Heat and Work
- Expression for Pressure-volume (PV) Work
- Concept of Maximum Work
- Internal Energy (U)
- First Law of Thermodynamics
- Enthalpy (H)
- Enthalpies of Physical Transformations
- Thermochemistry
- Spontaneous (Irreversible) Process
- Overview of Chemical Thermodynamics
