Chemistry - Thermochemistry Question with Solution | TestHub
The enthalpies of combustion of formaldehyde and paraformaldehyde (a polymer of formaldehyde) are -134 and -732 kcal/mol, respectively. The enthalpy of polymerization per mole of paraformaldehyde is -72 kcal. The molecular formula of paraformaldehyde is
Options:
Answer:
Solution:
To determine the formula of paraformaldehyde, we can analyze the energy changes during its depolymerization and subsequent combustion.
First, consider the depolymerization of 1 mol of paraformaldehyde into n moles of HCHO. This process requires an energy input of +72 kcal.
1 mol paraformaldehyde tarrow n mol HCHO (Δ H = +72 kcal)
Next, the combustion of n moles of HCHO releases -134n kcal.
n mol HCHO tarrow combustion products (Δ H = -134n kcal)
The direct combustion of 1 mol of paraformaldehyde releases -732 kcal. We can equate the total enthalpy change from the two-step process to the direct combustion enthalpy:
-732 = 72 - 134n
Now, we solve for n:
-732 - 72 = -134n
-804 = -134n
n = 804/134
n = 6
Since n=6, the formula for paraformaldehyde is (CH₂O)₆, which simplifies to C₆H₁₂O₆. This corresponds to option (b).
