16. From the standard enthalpies of formation, calculate the Horxn for the reaction: (Hof = -151.9 kJ/mol for C6H12.)
C6H12(l) + 9 O2(g) 6 CO2 (g) + 6 H2O(l)
dHrxn = (n*dHf products) - (n*dHf reactants)
To calculate the ΔHorxn for the reaction, we need to use the standard enthalpies of formation (ΔHof) of the reactants and products.
Given:
ΔHof for C6H12(l) = -151.9 kJ/mol
The standard enthalpies of formation for the other substances involved in the reaction are:
ΔHof for O2(g) = 0 kJ/mol
ΔHof for CO2(g) = -393.5 kJ/mol
ΔHof for H2O(l) = -285.8 kJ/mol
Using the equation:
ΔHorxn = Σ(nΔHof products) - Σ(nΔHof reactants)
where n is the stoichiometric coefficient, we can calculate ΔHorxn.
Substituting the values into the equation, we have:
ΔHorxn = [6(ΔHof CO2) + 6(ΔHof H2O)] - [1(ΔHof C6H12) + 9(ΔHof O2)]
ΔHorxn = [6(-393.5 kJ/mol) + 6(-285.8 kJ/mol)] - [-151.9 kJ/mol + 9(0 kJ/mol)]
ΔHorxn = [-2361 kJ/mol + (-1715 kJ/mol)] - [-151.9 kJ/mol]
ΔHorxn = -4076 kJ/mol + 151.9 kJ/mol
ΔHorxn = -3924.1 kJ/mol
Therefore, the ΔHorxn for the given reaction is approximately -3924.1 kJ/mol.
To calculate the change in enthalpy (ΔH) for a reaction using standard enthalpies of formation, you need to follow these steps:
Step 1: Write the balanced chemical equation for the reaction.
In this case, the balanced equation is:
C6H12(l) + 9 O2(g) → 6 CO2(g) + 6 H2O(l)
Step 2: Determine the standard enthalpy of formation (ΔH°f) for each compound involved in the reaction.
Given that ΔH°f for C6H12 is -151.9 kJ/mol.
Step 3: Calculate the heat absorbed or released for each compound using the equation:
ΔH = Σ(n × ΔH°f)
where ΔH is the change in enthalpy, Σ is the summation symbol, and (n × ΔH°f) is the product of the stoichiometric coefficient (n) and the standard enthalpy of formation (ΔH°f) for each compound.
For the reactants:
ΔH = (1 × -151.9 kJ/mol) + (9 × 0 kJ/mol) ... Since O2 is in its standard state, its ΔH°f is 0.
For the products:
ΔH = (6 × -393.5 kJ/mol) + (6 × -285.8 kJ/mol)
Step 4: Calculate the overall change in enthalpy (ΔHorxn) by subtracting the sum of the heat absorbed by the reactants from the sum of the heat released by the products:
ΔHorxn = Σ(heat of products) - Σ(heat of reactants)
ΔHorxn = [(6 × -393.5 kJ/mol) + (6 × -285.8 kJ/mol)] - [(1 × -151.9 kJ/mol) + (9 × 0 kJ/mol)]
Simplify the equation to find the numerical value of ΔHorxn.
Note: Make sure to use the correct signs for the heat values. The signs indicate whether heat is released (-) or absorbed (+) during a reaction.