ERT 108 Physical Chemistry The First Law of Thermodynamics by Miss Anis Atikah binti Ahmad

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 in internal energy of the system Heat flow into the system Work done on the system The first law of thermodynamics relates ∆U to heat (q) and work (w) that flows 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 of forms such as:  The translational the molecules  The molecular vibrations & rotations  The internal energy stored in the form of chemical bonds that can 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 states of the system and independent of the path used to bring the system 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 the initial state : U 2 = U 1 Thus, ∆ U = 0

Example  If an electric motor produced 15kJ of energy each second as mechanical work and lost 2kJ as heat to the surroundings, then the change in the internal energy of the motor each second is

Example  If an electric motor produced 15kJ of energy each second as mechanical work and lost 2kJ as heat to the surroundings, then the change in the internal energy of the motor each second is

Example  Suppose that, when a spring was wound, 100 J of work was done on it but 15 J escaped to the surroundings as heat. The change in internal energy of the spring is:

Example  Suppose that, when a spring was wound, 100 J of work was done on it but 15 J escaped to the surroundings as heat. The change in internal energy of the spring is:

Enthalpy  Enthalpy can be defined as:  Enthalpy is a state function (values depend on current state of the system, not on HOW the system acquired that state, which is independent of path.) Internal energy Pressure Volume

Enthalpy  Energy transferred as heat at constant pressure is equal 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, Constant volume

 For a constant-pressure process;  For a constant-volume process; Recall: Heat Capacities Recall: -Is a measure of energy needed to change the temperature of a substance by a given amount. - Extensive property (doubles as the mass of the system doubled)

Molar heat capacities  At constant P,  At constant V,

Molar heat capacities Molar heat capacities for a number of gases.