# Acetylene is used in blow torches, and burns according to the following equation: 2 C2H2(g) + 5 O2(g) → 4 CO2(g) + 2 H2O(g) Use the following information to calculate the heat of reaction:?

## Acetylene is used in blow torches, and burns according to the following equation: 2 C2H2(g) + 5 O2(g) → 4 CO2(g) + 2 H2O(g) Use the following information to calculate the heat of reaction: Hfo (H2O(g))= -241.82 kJ/mol Hfo (CO2(g))= -393.5 kJ/mol Hfo (C2H2(g))=226.77 kJ/mol

May 5, 2016

$\Delta \text{H"_(rxn)=-2511.04" ""kJ}$

#### Explanation:

Hess' Law states that the overall enthalpy change of a reaction is independent of the route taken.

Thermodynamics is concerned with initial and final states and the law is a consequence of the conservation of energy.

You can solve this problem by constructing a Hess Cycle.

Write down the reaction you are interested in. Below this write down the elements from which the reactants and products are made.

Then complete the cycle as shown:

Notice I have multiplied the $\Delta \text{H"_"f}$ values by the relevant stoichiometric numbers.

In energy terms the $\textcolor{b l u e}{\text{BLUE}}$ route must equal the $\textcolor{red}{\text{RED}}$ route since the arrows start and finish in the same place.

So we can write:

$\left(2 \times 226.77\right) + \Delta \text{H} = \left(4 \times - 393.5\right) + \left(2 \times - 241.82\right)$

$\therefore 453.4 + \Delta \text{H} = - 1574 - 483.64$

$\Delta \text{H"=-2511.04" ""kJ}$