HWW Item 26 Part A The change in internal energy for the combustion of 1.0 mol o

Question

HWW Item 26

Part A

The change in internal energy for the combustion of 1.0 mol of octane at a pressure of 1.0 atm is -5084.1 kJ .

If the change in enthalpy is -5074.2 kJ , how much work is done during the combustion? W=_____kJ

Part B

Consider the following thermochemical equation for the combustion of acetone (C3H6O), the main ingredient in nail polish remover.
C3H6O(l)+4O2(g)3CO2(g)+3H2O(g)Hrxn=1790kJ

If a bottle of nail polish remover contains 179 mL of acetone, how much heat would be released by its complete combustion? The density of acetone is 0.788 g/mL. |H| =________ kJ

Explanation / Answer

PART A SOLUTION

Internal energy = heat + work
dU = Q + W
Work = Internal energy - heat

heat = -5074.1kJ (combustion is always exothermic, so negative)

work = -5084.3 kJ - (-5074.2) kJ = -10.1 kJ

PART B SOLUTION

179 mL x (0.788 g / mL) x (1 mole / 58.08 g) = 2.43 moles acetone

from the balanced equation.

1 mole of acetone releases +1790 kJ of heat upon combustion.

2.43 moles x 1790 kJ/mole = 4350 kJ

********************
that value is +... not negative. the negative sign is for the reaction and indicates that 1790 kJ are "released" by the combustion of 1 mole of acetone. if you say -4350 kJ are released, that means 4350 kJ are absorbed. That wasn't the case.

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