Question

I'm having trouble converting the units and knowing exactly what they want for part B of this questions. I'm not sure what they want after using the STP of 22.4 to find out density of CH4 and the other gasses, so I'm a little confused on how to convert it to Energy density form.

6) a) Which of CH4(g), C2H2(g), and CH3OH(l) provides the most energy per gram upon combustion and which provides the least?

CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) ΔH° = -890 kJ

2 C2H2(g) + 5 O2(g) → 4 CO2(g) + 2 H2O(l) ΔH° = -2599 kJ

2 CH3OH(l) + 3 O2(g) → 2 CO2(g) + 4 H2O(l) ΔH° = -1453 kJ 4

b) Energy density is the amount energy per unit volume.; this is an important consideration in designing automotive fuel systems (how far can I go on a tank of fuel?). Calculate the energy density for each of the fuels at STP (the density of methanol is 0.791 g/mL).

Answer 1

a)
For CH4:
1 mol of CH4 gives 890 KJ
Mass of 1 mol CH4 = 16 g
Energy released per g = 890 KJ/16 g = 55.6 KJ/g

For C2H2:
2 mol of C2H2 gives 2599 KJ
Mass of 2 mol C2H2 = 2* mass of 1 mol = 2*26 g = 52 g
Energy released per g = 2599 KJ/52 g = 50 KJ/g

For CH3OH:
2 mol of CH3OH gives 1453 KJ
Mass of 2 mol CH3OH = 2* mass of 1 mol = 2*32 g = 64 g
Energy released per g = 1453 KJ/64 g = 22.7 KJ/g

Most energy per gram is provided by CH4
Least energy per gram is provided by CH3OH

b)
AT STP 1 mol has a volume of 22.4 L

For CH4:
1 mol of CH4 gives 890 KJ
Volume of 1 mol = 22.4 L
Energy density = 890 KJ/22.4 L = 39.7 KJ/L

For C2H2 :
2 mol of C2H2 gives 2599 KJ
Volume of 2 mol = 2*22.4 = 44.8 L
Energy density = 2599 KJ/44.8 L = 58 KJ/L

For C2H2 :
2 mol of CH3OH gives 1453 KJ
Volume of 2 mol = 2*22.4 = 44.8 L
Energy density = 1453 KJ/44.8 L = 32.4 KJ/L