Question

P5.) Using the AH value for ethanol (-1368 kJ/mol) and AH value for hexane (-4163 kJ/mol) calculate the expected AH (kJ/g fuel mixture) for E10 and E85 fuel blends. The density of ethanol is 0.789 g/mL and the density of hexane is 0.655 g/mL. You must show ALL work to receive credit. Calculated AH (kJ/g fuel) Fuel Blend E10 E85

Answer 1

$\Delta$H of ethanol is -1368 kJ/mole. Let us first convert $\Delta$H in terms of kJ / g.

For ethanol one mole is equla to its molecular weight = 46.07 g/mole

$\Delta$H of ethanol in KJ/g = -1368/46.07 = -29.7 KJ/g

Similarly,

$\Delta$H of hexane is -4163 kJ/mole. Let us first convert $\Delta$H in terms of kJ / g.

For Hexane one mole is equla to its molecular weight = 86.18 g/mole

$\Delta$H of hexane in KJ/g = -4163/86.18 = -48.30 KJ/g

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E10 blend means 10 % of ethanol by volume.

Out of 100 mL of mixture 10 mL will be ethanol and 90 mL will be hexane.

The number of grams of ethanol in 10 mL is

10 x density of ethanol(g/mL) = 10 x 0.789 = 7.89 g.

The energy released from 10 ml or 7.89 g of ethanol is  

= 7.89 x $\Delta$H of ethanol kJ / g = 7.89 x -29.7 = -234.33 KJ

The number of grams of hexane in 90 mL is

90 x density of Hexane(g/mL) = 90 x 0.655 = 58.95 g.

The energy released from 90 ml or 58.95 g of Hexane is  

= 58.95 x $\Delta$H in kJ / g = 58.95 x -48.30 = 2847.28 KJ

Total energy released from 100 mL of E10 mixture is

= -2847.28 + (-)234.33 = -3081.61 KJ

The total number of grams of ethanol and hexane is

= 7.89 + 58.95 =66.84

The $\Delta$H of E10 blend in KJ/g is

= Total energy / number of grams of fuel mixture = -3081.61/66.84 = -46.10 KJ/g

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E85 blend means 85 % of ethanol by volume.

Out of 100 mL of mixture 85 mL will be ethanol and 15 mL will be hexane.

The number of grams of ethanol in 85 mL is

= 10 x density of ethanol(g/mL) = 85 x 0.789 = 67.06 g.

The energy released from 85 ml or 67.06 g of ethanol is  

= 67.05 x $\Delta$H of ethanol kJ / g = 67.06 x -29.7 = - 1991.68 KJ

The number of grams of hexane in 15 mL is

= 15 x density of Hexane(g/mL) = 15 x 0.655 = 9.825 g.

The energy released from 15 ml or 9.825 g of Hexane is  

= 9.825 x $\Delta$H in kJ / g = 9.825 x -48.30 = - 474.54 KJ

Total energy released from 100 mL of E10 mixture is

= -1991.68 + (-)474.54 = - 2466.22 KJ

The total number of grams of ethanol and hexane is

= 67.06 + 9.825 = 76.875

The $\Delta$H of E85 blend in KJ/g is

= Total energy / number of grams of fuel mixture = -2466.22 / 76.875 = - 32.08 KJ/g