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Wykłady z Chemii o Termodynamice Wydział Budownictwa sem. 1 (2008/2009)

Termochemistry and Chemical Thermodynamics

1. First Law of Thermodynamics

■N A SIMPLE CHEMICAL OR PHYSICAL PROCESS, ENERGY CAN BE NEITHER CREATED NOR DESTROYED, IT ONLY CHANGES IT FORM FORM WORK TO HE AT OR THE OPPOSITE

From the point ofview of Einstein’s theory, the energy to mass ratio is constant. This, however, have to be considered when considering nuclear reactions.

Intemal Energy, E, is said to be State function - property that is deteimined by the State the system but not by the way it increases. Ie. the same increase in energy might be obtained by the increase in gas temperaturę (heating) or volume (compression).

There are three distinct systems possible when considering thermodynamic changes:

-    An isolated system that doesn't exchange any form of energy nor mass with the

surrounding,

-    The closed system that doesn't exchange mass with the surrounding while energy

exchange is possible,

-    The open system capabłe of exehanging both mass and energy with the surrounding.

Into an closed system, energy changes as a result of heat transferred to the system or the work done on or by the system, therefore:

AE = Q + W = Q- pAV

The heat, Q, was set to be eąual to enthalpy change:

AE = AH - pdV

And enthalpy is another state fiinction that corresponds to intemal energy at constant pressure (under isobaric condition).

When integrating the above expression, the defmition of enthalpy may be derived:

H = E + pW

The heat, enthalpy change, intemal energy change, produced in the system is considered to be negative, the heat absorbed by the system is positive (sińce it increase overall energy of the system).

The work done by the system is considered to be negative (sińce it decrease the system intemal energy) while the work done on the system is considered to be positive sińce it increase the system intemal energy.

At constant pressure, the enthalpy is eąual to the heat involved into process,

At constant volume, the heat of the process is eąual to the intemal energy, E, change.