Question

Reasonable efficiency for converting chemical energy to electrical energy via combustion is 30%. Calculate how many pounds of coal must be burned to run your reading lamp for two hours of studying for a chemistry exam. A typical reading lamP has a 75 W bulb. Assume that coal is pure graphite. (1 W = 1 J/s.)

_______lbs

Answer 1

Solution :-

Balanced reaction equation is as follows

C(s) + O2(g) -------> CO2(g)

the standard enthalpies of the C and O2 are zero kJ

and the CO2 = -393.5 kJ per mol

delta H reaction = Delta Hf product - Delta Hf reactant

                         = -393.5 kJ - (0 +0 )

                         = -393.5 kJ

therefore enthalpy of coimbustion of the Coal is -393 kJ per mol

Now lets calculate the energy needed for the lamp

(2 hour * 60 min / 1hr)*(60 s / 1 min)= 7200 s

75 W * 7200 s = 540000 W

540000 W * 1 J /s = 540000 J

540000 J * 1 kJ / 1000 J = 540 kJ

now lets calculate the mass of the coal

540 kJ * 12.01 g C / 393.5 kJ = 16.5 g C

now lets find the mass of coal using the percent efficiency

16.5 C * 100 % / 30.0 % = 55 g C

now lets convert gram to lb

55 g C * 1 lb / 453.5 g = 0.121 Lb

therefore we need 0.121 lb coal for the lamp to work for 2 hour