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Lavoisier and Laplace’s law of Thermochemistry

Two laws of thermochemistry are very important in thermochemical calculations. Both the laws preceded the first law of thermodynamics (1850). It will, however, be shown that the laws of Thermochemistry are a direct consequence of the first law of thermodynamics.

Lavoisier and Laplace’s law (1782): This law may be stated in the general form as the heat change accompanying a chemical reaction in one direction is exactly equal in magnitude, but opposite in sign, to that associated with the same reaction in the reverse direction. This law states that; the heat change (or enthalpy change) of a chemical reaction is exactly equal but opposite in sign for the reverse reaction.

This is evident from the following two fractions:

(a) CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)     [here, ∆H0 = -890.3 kJ mol-1]

(b) CO2 (g) + 2H2O (l) → CH4 (g) + 2O2 (g)      [here, ∆H0 = +890.3 kJ mol-1]

Thus, it can be concluded that ∆Hforward reaction = ∆Hbackword reaction

A chemical equation, when written along with enthalpy change for the reaction, is termed a thermochemical equation. The two equations are examples of thermochemical equations. Laplace’s law allows us to use thermochemical equations as algebraic equations.

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