# Can someone please explain how to find the amount of energy released in a chemical reaction? What formulas do I use and how do I use them?

Apr 3, 2015

More information is required as there are a few different ways. I am guessing you are looking at either
1) measuring by experiment using calorimetry
2) calculating using bond energies
3) calculating using standard heats of formation

Based on your comment, it looks like bond energies. In this case, the heat released (or the enthalpy of reaction $\Delta H$) can be calculated by:

$\Delta H = \sum \left(\text{reactant bond energies")-sum("product bond energies}\right)$

A couple of things you should know
- breaking bonds requires energy (endothermic) and making bonds releases energy (exothermic) : if the $\Delta H$ value is negative, the overall reaction is exothermic; if positive, the overall reaction is endothermic
- you need a table of bond energy (enthalpy) values to solve problems
- you need to be able to determine bonding in molecules to solve problems

Example
$2 {H}_{2} + {O}_{2} \rightarrow 2 {H}_{2} O$

Reactants contain TWO $H - H$ bonds @ $436 \frac{k J}{m o l}$ and
ONE $O = O$ bond @ $498 \frac{k J}{m o l}$

Products contain FOUR $H - O$ bonds @ $463 \frac{k J}{m o l}$

$\Delta H = \sum \left(\text{reactant bond energies")-sum("product bond energies}\right)$
$\Delta H = 2 \left(436\right) + 498 - 4 \left(463\right) = - 482 \frac{k J}{m o l}$