# Acetylene torches utilize the following reaction: 2 C2H2 (g) + 5 O2 (g) -----> 4 CO2 (g) + 2 H20 (g) How to calculate Delta H in Celsius for this reaction ? Species Delta H C2H2 (g )= +226.7 CO2 (g )= -393.5 H20 (g) = -241.6

##### 1 Answer

To calculate the enthalpy of combustion of acetylene,

More specifically, you need to subtract from the sum of enthalpies of formation of the products the sum of the enthalpies of formation of the reactants.

Keep in mind that each of these enthalpies must be multiplied or divided in accordance to their stoichiometric coefficients from the balanced chemical reaction.

So, the balanced chemical reaction is

You were given the enthalpies of formation for *1 mole* of **zero** (read more on that here: http://socratic.org/questions/why-is-the-enthalpy-of-formation-of-oxygen-zero).

But notice that the balanced chemical reaction has **2 moles** of **2 moles** of **4 moles** of *for 1 mole of acetylene*, divide the balanced equation by **2**

Now the expression for the enthalpy of combustion will be

Therefore, the enthalpy of combustion for acetylene is

Here's a link to a similar answer on this topic:

http://socratic.org/questions/how-do-you-calculate-enthalpy-change-of-combustion

**SIDE NOTE** *I'm not sure what you mean by* *in Celsius*, *since the heat of combustion is expressed as energy per mole; in this case, burning 1 mole of acetylene will release 1253.3 kJ of energy*.