- •2.Topic №2: Bases of the chemical thermodynamics. Thermo-chemical equation.
- •1)Law of conservation of energy - the main law of nature.
- •2) Thermodynamics and bioenergetics. The main concepts and terms of topic
- •3) First law of thermodynamics. Internal energy. Enthalpy.
- •4) Thermo-chemical. Hess’s law. Thermo-chemical calculations.
- •Hess's law.
- •Applications of Hess's law.
- •5)Fuel value of food Calorie content of food.
- •6) Standard condition and thermochemical (enthalpy of formation)
- •7) Second law of thermodynamics. Spontaneous and non spontaneous processes . Gibb's energy. Entropy.
- •8)The united law of the thermodynamics. The criteria and direction of the spontaneous processes. Enthalpy and entropy factors in spontaneous process.
Applications of Hess's law.
The amount of heat of the chemical reaction is equal to sum of standard heat of the reactants minus sum of standard heat of the products pointing out the balance numbers in the chemical reaction.
∆H0 reaction=∆H0 products +∆H0 reactants
∆H0 it is heat of formation of the chemical compounds.
Heat of formation of heat of reaction evolved or absorbed by at formation of 1 mole of complex compound from the simple by standard conditions.
The standard heats of formation are shown in standard charts. Minus sign means that heat evolves system loses heat.
Heat of simple substances formation such as O2, Fe, H2 and so on is always equal to zero.
Example:
4NH3 (g) + 5O2(g) ---> 4NO (g) + 6H2O (g) + ∆H
According to Hess's law heat amount of this reaction is equal to:
∆H°reaction = (4 ∆H° (NO)+6 ∆H° (H2O))-(4 ∆H° (NH3)+5 ∆H°O2)
next step is brackets opening and removing of heat of the simple substances.
∆H0 reaction =4∆H° (NO)+6 ∆H° (H2O) - 4 ∆H° (NH3)
Absolute enthalpy is hard to measure, but enthalpy changes during reactions are easy to measure because there will be an observable energy exchanges between the chemicals and the surroundings.
Heat of Combustion or enthalpy of Combustion – this is the heat released when 1 mole of a substance is completely burnt in oxygen.
This is heat evolved by complete combustion or oxidation of one mole of a substance in air excess or oxygen. All organic compounds are oxidized till CO2 and H2O and inorganic oxides are oxidized to oxides with higher oxidation number.
CH3OH (l) + O2 (g) ---> CO2(g) + 2H2O ∆H°=-715 kJ/mole
C2H5OH (l) + 3O2 (g) ---> 2CO2(g) + 3H2O ∆H°=-1371 kJ/mole
The increase in enthalpy of combustion is regular as you proceed up the homologous series because each increase of a carbon and two hydrogen’s produces the same amount of heat energy.
Heat of Solution or enthalpy of Solution – this is the enthalpy change when 1 mole of substance is completely dissolved in water.
This is heat evolved or absorbed during the complete dissolution of one mole of the substance in a large amount of solvent by 25 degree.
NaCl (s) + (aq) ---> NaCl(aq) ∆H°=+5.0 kJ/mole
CuSO4 (s) + (aq) ---> CuSO4(aq) ∆H°=-66.5 kJ/mole
Heat of Neutralization or enthalpy of Neutralization – this is the heat released when 1 mole of water is formed during the neutralization of strong acid by an alkali.
HCl(aq) + NaOH(aq) ---> NaCl(aq) + H2O(l) ∆H°=-57.1 kJ/mole
The neutralization of any strong acid by any alkali always produces the same final equation and the same value for the change in enthalpy.
Heat of Formation or enthalpy of Formation – this is the enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states (state at room temperature, 250C and at a pressure of 1 atmosphere).
2C(s) + 3H2(g) ---> C2H6(g) ∆H°=-85 kJ/mole