ERT 108 Physical ChemistryThe First Law of Thermodynamics
by
Miss Anis Atikah binti Ahmad
anisatikah@unimap.edu.my
OUTLINE
The First Law of Thermodynamics
Enthalpy
Heat Capacities
The Joule and Joule – Thomson Experiments
Perfect gases and The First Law
Calculation of First Law Quantities
The First Law of Thermodynamics
Eventually the rock and ball come to rest.
What happened to its energy of motion??
The First Law of Thermodynamics
Total energy of a body:
The First Law of Thermodynamics
The internal energy of an isolated system is constant
Change ininternalenergy ofthe system
Heat flowinto thesystem
Work doneon thesystem
The first law of thermodynamics relates ∆U to heat (q) and work (w) thatflows across the boundary between the system & surroundings.
(closed system)
The First Law of Thermodynamics
Heat flow into the system from surroundings during a process
Outflow of heat from the system to the surroundings
Work done on the system by the surroundings
Work done by the system on its surroundings
The First Law of Thermodynamics
What is internal energy??
The First Law of Thermodynamics
Internal energy is the energy that take on number offorms such as:
The translational the molecules
The molecular vibrations & rotations
The internal energy stored in the form of chemical bonds thatcan be released through a chemical reaction
The potential energy of interaction between molecules.
The First Law of Thermodynamics
The internal energy, U is a state function.
For any process, ∆U depends only on the final and initial statesof the system and independent of the path used to bring thesystem from initial state to the final state.
If the system goes from state 1 to state to by any process,then:
The First Law of Thermodynamics
Cyclic process:
A process in which the final state of the system is the same as theinitial state :
U2 = U1
Thus,
∆ U = 0
Example
If an electric motor produced 15kJ of energy each secondas mechanical work and lost 2kJ as heat to thesurroundings, then the change in the internal energy ofthe motor each second is
Example
If an electric motor produced 15kJ of energy each secondas mechanical work and lost 2kJ as heat to thesurroundings, then the change in the internal energy ofthe motor each second is
Example
Suppose that, when a spring was wound, 100 J of workwas done on it but 15 J escaped to the surroundings asheat. The change in internal energy of the spring is:
Example
Suppose that, when a spring was wound, 100 J of workwas done on it but 15 J escaped to the surroundings asheat. The change in internal energy of the spring is:
Enthalpy
Enthalpy can be defined as:
Enthalpy is a state function (values depend on current stateof the system, not on HOW the system acquired that state,which is independent of path.)
Internalenergy
Pressure
Volume
Enthalpy
Energy transferred as heat at constant pressure isequal to the change in enthalpy of the system.
HOW?
Enthalpy
Rearranging,
For an infinitesimal change,
For a measurable change
Enthalpy
Consider a constant-volume process:
For an infinitesimal change,
Constantvolume
For a constant-pressure process;
For a constant-volume process;
Recall:
Heat Capacities
Recall:
-Is a measure of energy needed to change thetemperature of a substance by a given amount.
- Extensive property (doubles as the mass of thesystem doubled)
Molar heat capacities
At constant P,
At constant V,
Molar heat capacities
Molar heat capacities for anumber of gases.