Using enthalpies of formation, calculate the quantity of heat produced when 14.0 g of propane is completely combusted in air under standard conditions.
Write the equation and balance it.
delta Hrxn = (n*DHproducts)-(n*DHproducts)
Then delta Hrxn x 14.0/ = q
To calculate the quantity of heat produced when propane is completely combusted, we need to use the enthalpies of formation for propane and the reactants involved in the combustion reaction.
Step 1: Write down the balanced chemical equation for the combustion of propane:
C3H8 + 5O2 -> 3CO2 + 4H2O
Step 2: Find the enthalpies of formation for each compound in the balanced equation. The enthalpy of formation (ΔHf) is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states. Here are the values:
ΔHf(C3H8) = -104.7 kJ/mol
ΔHf(O2) = 0 kJ/mol (since oxygen is in its standard state)
ΔHf(CO2) = -393.5 kJ/mol
ΔHf(H2O) = -285.8 kJ/mol
Step 3: Calculate the heat produced in the combustion reaction.
Since we have the enthalpies of formation for each compound involved, we can use the following equation:
ΔHrxn = Σ(nΔHf(products)) - Σ(nΔHf(reactants))
First, calculate the moles of propane used:
Molar mass of propane (C3H8) = 44.1 g/mol
moles of propane (C3H8) = mass / molar mass = 14.0 g / 44.1 g/mol ≈ 0.317 mol
Now, substitute the values into the equation:
ΔHrxn = (3 mol CO2 x -393.5 kJ/mol) + (4 mol H2O x -285.8 kJ/mol) - (0.317 mol C3H8 x 104.7 kJ/mol) - (5 mol O2 x 0 kJ/mol)
ΔHrxn = -1177.05 kJ + (-1143.2 kJ) - (33.2399 kJ) - 0 kJ
ΔHrxn = -2353.4899 kJ/mol
Step 4: Calculate the heat produced for the given mass of propane.
Since 0.317 mol of propane is combusted, we need to convert it to grams using the molar mass of propane:
Heat produced = ΔHrxn x moles of propane
Heat produced = -2353.4899 kJ/mol x 0.317 mol
Heat produced = -745.7857 kJ
The quantity of heat produced when 14.0 g of propane is completely combusted under standard conditions is approximately -745.8 kJ. Note that the negative sign indicates that the process is exothermic (heat is released).