When 1.560 g of liquid hexane (C6H14) undergoes combustion in a bomb calorimeter, the temperature rises from 25.76 ∘C to 38.31 ∘C. Find ΔErxn for the reaction in kJ/mol hexane. The heat capacity of the bomb calorimeter, determined in a separate experiment, is 5.80 kJ/∘C.
How many mols is 1.560 g hexane? That's 1.560/about 86 = approx 0.02 mols but that's just an estimate.
The liquid hexane gave off q heat. How much? That's Ccal = 5.80 kJ/C x (dT) = estd 70 kJ
So heat combustion = approx 70 kJ/0.02 mols = about 3500 kJ/mol
Post your work if you get stuck.
To find ΔErxn for the reaction in kJ/mol hexane, we need to use the heat capacity of the bomb calorimeter and the temperature change during the combustion.
First, we need to calculate the heat transferred to the bomb calorimeter (qcal) during the combustion reaction. This can be done using the formula:
qcal = Ccal * ΔT
where qcal is the heat transferred to the calorimeter, Ccal is the heat capacity of the bomb calorimeter, and ΔT is the change in temperature.
In this case, Ccal is given as 5.80 kJ/∘C and ΔT is found by subtracting the initial temperature (25.76 ∘C) from the final temperature (38.31 ∘C):
ΔT = 38.31 ∘C - 25.76 ∘C = 12.55 ∘C
Now we can calculate qcal:
qcal = 5.80 kJ/∘C * 12.55 ∘C = 72.29 kJ
Next, we need to calculate the number of moles of hexane used in the combustion reaction. This can be done using the molecular weight of hexane, which is the sum of the atomic weights of carbon (C) and hydrogen (H):
Molecular weight of C6H14 = (6 * atomic weight of C) + (14 * atomic weight of H)
Using the atomic weights from the periodic table (12.01 g/mol for C and 1.008 g/mol for H), we can calculate the molar mass:
Molecular weight of C6H14 = (6 * 12.01 g/mol) + (14 * 1.008 g/mol) = 86.18 g/mol
Given that 1.560 g of hexane was used, we can convert this mass to moles:
moles of hexane = 1.560 g / 86.18 g/mol ≈ 0.0181 mol
Finally, we can calculate the ΔErxn for the reaction using the equation:
ΔErxn = qcal / moles of hexane
Plugging in the values, we get:
ΔErxn = 72.29 kJ / 0.0181 mol ≈ 3999 kJ/mol hexane
Therefore, the ΔErxn for the reaction is approximately 3999 kJ/mol hexane.