Determine the heat of combustion of acetylene (C2H2).
C2H2 +5/2 O2 ==> 2CO2 + H2O
I would look up the deltaHf for CO2 and for H2O as well as C2H2. Then
delta Hrxn = (n*DHfCO2)+(n*DHfH2O) - (n*DHfC2H2)
To determine the heat of combustion of acetylene (C2H2), you can use the concept of Hess's law. Hess's law states that the overall enthalpy change of a reaction is independent of the pathway between the initial and final states.
In this case, we can use the following steps to calculate the heat of combustion of acetylene:
1. Write the balanced equation for the combustion of acetylene:
C2H2 + (5/2) O2 -> 2 CO2 + H2O
2. Look up the standard enthalpies of formation (∆Hf°) for each of the compounds involved in the equation. The standard enthalpy of formation is the heat change when one mole of a compound is formed from its elements, with all substances at standard conditions (25°C and 1 atm).
∆Hf° (C2H2) = 227 kJ/mol
∆Hf° (CO2) = -394 kJ/mol
∆Hf° (H2O) = -286 kJ/mol
∆Hf° (O2) = 0 kJ/mol (since it is an element in its standard state)
3. Use Hess's law to calculate the heat of combustion (∆Hc) of acetylene:
∆Hc = ∑(∆Hf° products) - ∑(∆Hf° reactants)
∆Hc = [2 × ∆Hf°(CO2)] + [1 × ∆Hf°(H2O)] - [1 × ∆Hf°(C2H2)] - [(5/2) × ∆Hf°(O2)]
∆Hc = [2 × (-394 kJ/mol)] + [1 × (-286 kJ/mol)] - [1 × 227 kJ/mol] - [(5/2) × 0 kJ/mol]
∆Hc = -787 kJ/mol - 286 kJ/mol - 227 kJ/mol
∆Hc = -1300 kJ/mol
Therefore, the heat of combustion of acetylene (∆Hc) is -1300 kJ/mol.
Note: The negative sign indicates that the reaction is exothermic and releases energy.