thermochemistry


 * Thermochemistry** is the study of energy changes in chemical reactions or processes.

Energy changes in chemical reactions can be estimated in a number of ways:
 * 1) using standard enthalpies, entropies, or free energies of formation (tabular data in textbooks)
 * 2) using bond energy values (need Lewis diagrams)
 * 3) Hess's Law (need other chemical equations, such as a reaction mechanism, with energy values)

__Standard Enthalpy of Formation__:

Note: enthalpy of formation (H) can be substituted with entropy (S) or free energy (G) in any f these equations.

These problems require a chemical equation and H values from a table.

The standard enthalpy of reaction (Hrxn) is calculated using this equation: **Hrxn = (sum of enthalpy of formation for //products//) - (sum of enthalpy of formation for //reactants//)**

__Example 1:__

Calculate the enthalpy of reaction for the combustion of methane: CH4 (g) + 2 O2 (g) --> CO2 (g) + 2 H2O (g)


 * Important note: take coefficients from the balanced equation into account in the calculation.
 * Use the table of thermodynamic data from a textbook (usually found in the appendix).
 * Note that elements in their standard states have H values of zero.

Hrxn = (CO2 + 2 x H2O) - (CH4 + 2 O2) Hrxn = [-393.5 + 2(-242)] - [-74.9 + 2(0)] Hrxn = (-877.5) - (-74.9)
 * Hrxn = -802.6 kJ/mol**

__Bond Energies:__

These calculations require bond energy values from a text, and a balanced chemical equation that Lewis diagrams can be drawn from.

In these problems, the equation to use is: **Hrxn = (sum of bonds broken in //reactants//) - (sum of bonds formed in //products//)**

__Example 2:__

Use bond energy values to calculate the enthalpy of reaction for the combustion of methane.

Here are the Lewis diagrams:



You can refer to this table for bond energy values.

Hrxn = (4 C-H + 2 O=O) - (2 C=O + 4 O-H) Hrxn = [4(411) + 2(494)] - [2(799) + 4(459)] Hrxn = (1644 + 988) - (1598 + 1836) Hrxn = 2632 - 3434
 * Hrxn = 802 kJ/mol**

Note the similarity to the value calculated using standard enthalpies of formation! (This is not always the case, but the two values should be similar.)