5.77 Ethanol (C₂H₅OH) is currently blended with gasoline as an automobile fuel.

(a) Write a balanced equation for the combustion of liquid ethanol in air.

Yay!! More very good review: this is a combustion, not a formation!

C₂H₅OH(l) + 3O₂(g) --> 2CO₂(g) + 3H₂O(g)

(b) Calculate the standard enthalpy change for the reaction, assuming H₂O(g) as a product.

from Appendix C,

so....

(c) calculate the heat produced by combustion per liter of ethanol under constant pressure. Ethanol has a density of 0.789 g/mL.

Recall that, at constant pressure, the enthalpy change is the same as heat, so we're dealing with enthalpy changes here. For the combustion of ethanol, from the above reaction, we produce 1.235x10³ kJ of heat from 1 mol ethanol. We use the molar mass to put this on a per gram basis, and use the density to find the volume (and convert to liters.) here goes:

molas mass ethanol

so we produce 2.12x10⁴ kJ per liter of ethanol (this is heat liberated - I just left off the negative sign from above.)

(d) calculate the mass of CO₂ produced per kJ of heat emitted.

OK, the enthalpy change from above is -1.235x10³ kJ for production of 2 mol CO₂. Convert this to g CO₂ to get the amount of heat released per g CO₂ formed:

this is kJ/g CO₂; we want g CO₂ per kJ heat, so invert:

DAC 7/9/08