Question

Given Hf (CO2(g)) = -393.5 kJ/mole, Hf (H2O(g)) = -241.8 kJ/mole and Hf (C5H12(g)) = -146.4 kJ/mole (a) (10 points) How much heat is evolved if 36.0 g C5H12 undergoes combustion at constant pressure?

Please show how you got it.

Answer 1

Answer:

The amount of heat evolved when 36g of pentane undergoes combustion is 1635.95kJ

Explanation:

Let us write the combustion equation for pentane:

$C_{5}H_{12}(g)+8O_2}--->5CO_{2}+6H_{2}O$

The enthalpy of reaction will be:

$\Delta H_{rxn}=\Delta H_{f}[products]-\Delta H_{f}[reactants]$

Thus the enthalpy of combustion of above reaction will be:

$\Delta H_{comb}= [6X \Delta H_{f} [H_{2}O]+5X \Delta H_{f}[CO_{2}]-[\Delta H _{f}[C_{5}H_{12}]]\\\Delta H_{comb}= [6X(-241.8) +5(-393.5)]-[-146.4]=-3271.9kJ/mol$

this much heat will be evolved if one mole of petane will undergo combustion. The molar mass of pentane is = 72g/mol

The given mass is 36 grams.

Thus the number of moles of pentane undergoing combustion is =$\frac{mass}{molarmass}=\frac{36}{72}=0.5mol$

The heat evolved will be =$0.5X(3271.9)=1635.95 kJ$