# Homework Help: Chemistry For engineers: thermodynamics

1. Oct 23, 2007

### nboyd1

[SOLVED] Chemistry For engineers: thermodynamics

I was given a question: Reasonable efficiency for converting chemical energy to electrical energy via combustion is 30%. Calculate the how many pounds of coal must be burned to run your reading lamp for two hours. Atypical reading lamp has a 75-W bulb. Assume that coal is pure graphite. ( 1W=1J/s)

Yo be honest this is nothing like the examples given in class of even in the book. I'm not sure what where to even begin, which equation to use. My intial instinct was it should be a heat capacity equation, but I don't know how the other information relates or how to calculate that given this information.

2. Oct 24, 2007

### chemisttree

Start by writing the equation for the combustion of carbon (graphite to be exact). You will need to look up the enthalpy of the reaction.

3. Oct 24, 2007

### nboyd1

i think i got the answer. i found delta h for the reaction then used E=time*power, the divind by 30 percent and setting up a proportion with weight of carbon to get the number of grams and finally converted to pounds