The enthalpy of combustion of H2, cyclohexene (C6H10) and cyclohexene (C6H12) are -241, -3800 and -3920 kJ per mol respectively. Heat of hydrogenation of cyclohexene is:
- -121 kJ per mol
- +121 kJ per mol
- +484 kJ per mol
- -484 kJ per mol
The Correct Option is A
Solution and Explanation
Given the equations:
H₂ + \(\frac 12\)O₂ \(→\) H₂O, ΔH = -241 kJ/mol ……..(1)
C₆H₁₀ + \(\frac {17}{2}\)O₂ \(→\) 6CO₂ + 5H₂O, ΔH = -3800 kJ/mol ……..(2)
C₆H₁₂ + 9O₂ \(→\) 6CO₂ + 6H₂O, ΔH = -3920 kJ/mol ……..(3)
Equation (1) and equation (2) are added, and then equation (3) is subtracted to yield the following result:
C₆H₁₂ + H₂ \(→\) C₆H₁₂
Calculation of Enthalpy Change for Hydrogenation:
The following formula is used to calculate the enthalpy change for hydrogenation, abbreviated as H Hydrogen:
ΔH Hydrogen = (-241 kJ/mol) + (-3800 kJ/mol) - (-3920 kJ/mol) = -121 kJ/mol
So, the correct option is (A): -121 kJ per mol
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Concepts Used:
Thermodynamics
Thermodynamics in physics is a branch that deals with heat, work and temperature, and their relation to energy, radiation and physical properties of matter.
Important Terms
System
A thermodynamic system is a specific portion of matter with a definite boundary on which our attention is focused. The system boundary may be real or imaginary, fixed or deformable.
There are three types of systems:
- Isolated System – An isolated system cannot exchange both energy and mass with its surroundings. The universe is considered an isolated system.
- Closed System – Across the boundary of the closed system, the transfer of energy takes place but the transfer of mass doesn’t take place. Refrigerators and compression of gas in the piston-cylinder assembly are examples of closed systems.
- Open System – In an open system, the mass and energy both may be transferred between the system and surroundings. A steam turbine is an example of an open system.
Thermodynamic Process
A system undergoes a thermodynamic process when there is some energetic change within the system that is associated with changes in pressure, volume and internal energy.
There are four types of thermodynamic process that have their unique properties, and they are:
- Adiabatic Process – A process in which no heat transfer takes place.
- Isochoric Process – A thermodynamic process taking place at constant volume is known as the isochoric process.
- Isobaric Process – A process in which no change in pressure occurs.
- Isothermal Process – A process in which no change in temperature occurs.
Laws of Thermodynamics
Zeroth Law of Thermodynamics
The Zeroth law of thermodynamics states that if two bodies are individually in equilibrium with a separate third body, then the first two bodies are also in thermal equilibrium with each other.
First Law of Thermodynamics
The First law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic processes, distinguishing three kinds of transfer of energy, as heat, as thermodynamic work, and as energy associated with matter transfer, and relating them to a function of a body's state, called internal energy.
Second Law of Thermodynamics
The Second law of thermodynamics is a physical law of thermodynamics about heat and loss in its conversion.
Third Law of Thermodynamics
Third law of thermodynamics states, regarding the properties of closed systems in thermodynamic equilibrium: The entropy of a system approaches a constant value when its temperature approaches absolute zero.