3) First law of thermodynamics. Internal energy. Enthalpy.

Each system possesses definite stick of energy. This energy is a sum of potential and kinetic energies of all the particles of a system. It is called internal energy of a system.

U = E potential ⁺ E kinetic, where U is internal energy of all the particles.

Internal energy depends upon:

U = c x m x T, where m-mass of the substance.

T-temperature in Kelvin

c-specific heat of a system. It is amount

of heat that necessary to change the

temperature of 1 kilogram body in one

degree centigrade.

Thermodynamic process which takes place by constant pressure is called isobar, by constant volume isochors, by constant temperature isothermal.

Internal energy of any system may be used for making a useful work (A).

A = p x ∆V, where p-pressure, ∆V - change of volume

Q = A +∆U, where Q - heat of a system, ∆U - change of internal energy.

Q = ∆U + p∆V

This formula expresses that all the energy cannot be used for work – this is law of energy conservation. Heat of a system does not change into work completely, part of this energy is used for internal energy change (U).

Q = A +∆U

This formula and the definition above it is First Law of Thermodynamics.

Q = A +∆U = ∆H it is enthalpy or internal energy of a system.

When Q heat is applied to a system some part of this energy remains in a system in a view of internal energy or enthalpy. The First Low of thermodynamics it is low of energy transforming by constant pressure.

There is another definition of this law:

This law is also called as law of energy conservation.

Energy can either be created nor be destroyed as a result of any chemical or physical change it may be converted from one form into another.

4) Thermo-chemical. Hess’s law. Thermo-chemical calculations.

Thermo-chemistry is the study of the energy changes taking place during chemical reactions. Every substance contains stored chemical energy, called enthalpy, mainly by virtue of chemical bonds.

This is branch chemistry which deals with heat of the chemical reactions.

Heat of the chemical reaction it is enthalpy change of a system.

Depending on change of enthalpy of a system there are two kinds of the chemical reaction.

Endothermic - is reaction with energy or heat absorbing. ∆H enthalpy increases because system gains energy. ∆H > 0

Exothermic - is reaction with Q energy evolving. System loses energy hence ∆H enthalpy change decreases. ∆H < 0

Every measuring in thermo-chemistry is made by standard conditions.

So first law of thermodynamic may be applied in thermo-chemistry and it is called as Hess's law.

Hess's law.

Heat effect of the chemical reaction does not depend upon the way of reaction.

The amount of heat evolved or absorbed in a process is the same whether the process is carried out in one step or in several steps.

For example:

C(g) + O₂ --> CO₂ (g) + AH + 393.5 kJ

C (s) + 1/2 O₂ (g) --> CO (g) + AH, + 110.5 kJ

CO (g) + 1/2 O₂ (g) --> CO₂ (g) + AH₂ + 283.0 kJ

∆H = ∆H₁ + ∆H₂ the sum of heat of each step is equal to heat of a general equation of the reaction. Graphically Hess's law is describes by

Hess' s triangle

∆H₁ ∆H₂

reactants _____________ final products

∆H

Thus we are interested only in primary state and products but not the intermediate steps.