First+Law

**First Law of Thermodynamics: Conservation of Energy**

**Thermodynamics** is essentially the study of energy transformations from one system to another, or energy transfers from a system to its surroundings.

This energy can be in the form of chemical energy, work, or heat, or it could be a measure of spontaneity. Either way, it is assumed that these changes in energy occur without creating new energy, or destroying existing energy. This is the principle of the **First Law of Thermodynamics.**



The first law of thermodynamics is primarily used in calculations of reaction energetics, and in calorimetry (heat exchanges).

Enthalpy
Heat energy is referred to as enthalpy, and is given the symbol H.

All elements in their natural state are assigned enthalpy values of zero. However, compounds (at standard conditions: 25 o C and 1.00 atm) will have **//enthalpy of formation//** values (H f o ).

Values of enthalpy can be found here.


 * Calculating** **H f o values**

The first law of thermodynamics, conservation of energy, means that all of the chemical energy of the reactants will be equal to all of the chemical energy of the products.

Therefore, during a chemical change, some energy will be converted to heat (or the reaction will require heat to proceed). This can be calculated using the following equation:

**Example 1:** Calculate the enthalpy of formation of propane, C 3 H 8, from its constituent elements. First, derive a chemical equation for this process: ____C (s) + ____H 2 (g) --> ____C 3 H 8 (l) Then balance it: ____3___C (s) + ____4___H 2 (g) --> ____1___C 3 H 8 (l) Next, look up the H f o values for each substance in a table of thermodynamic values (in kJ/mol). Write them down.

____3___C (s) + ____4___H 2 (g) --> ____1___C 3 H 8 (l) H f o 0 0 -104.6