# Thread: how much energy is that?

1. CH4 + 2O2----> CO2 + 2H20 + energy

can anyone tell me exactly how much energy that is and how we figure that?

2.

3. Google can tell you. Heat of combustion and/or heat of formation would be a good place to start.

4. Originally Posted by doggy
CH4 + 2O2----> CO2 + 2H20 + energy

can anyone tell me exactly how much energy that is and how we figure that?
This is a combustion reaction, and the energy will be in the form of heat. So, what you want to do is calculate the standard enthalpy change for the reaction:

Enthalpy change = (sum of the standard enthalpies of formation of the products) - (sum of the standard enthalpies of formation of the reactants)

That will give you the energy change of the reaction, in kJoules.

Be sure to multiply each enthalpy of formation by its stoichiometric coefficient in the balanced equation. For example, 2 moles of H2O are formed - so, multiply the enthalpy of formation for water in your equation by 2. Also, be sure to distinguish between liquid and gaseous forms of the molecules in the reaction; each has a different value for the enthalpy of formation. For example, in this reaction the water formed is a liquid, and the enthalpy of formation for liquid H2O is different from gaseous H2O.

Here is your reaction with the states (gas vs liquid) of the reactants and products written in. It is a good idea to get into the habit of including this info in every chemical equation you deal with or write down, because the physical states of the reactants or products can make a difference:

CH4(g) + 2 O2(g)----> CO2(g) + 2 H20(l) + energy

If you have a general chemistry textbook the formula for the standard enthalpy change of a reaction should be in there, along with a more complete explanation. Also, there is likely to be a table in the back which lists the standard enthalpies of formation for the molecules involved in the reaction.

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