# Homework Help: Energy problem - ice melted by burning ethanol

1. Apr 18, 2010

### philtered

1. The problem statement, all variables and given/known data
How many grams of ice at 0°C could be melted by the heat produced when 10.0g of ethanol is burned?

2. Relevant equations
So I wrote out the combustion reaction:
C2H5OH +3O2 ---> 2CO2 + 3H2O

3. The attempt at a solution
First I though I had to find heat of formation for that reaction, which I found to be -1235.6kJ (So this would be the heat given off when ethanol is burned correct?)
Hf=products-reactants
Hf=[-393.6(2)+-242(3)]-[-277.6+0]
Hf=-1235.6kJ

I wasn't sure if I had to use that number or if I had to just pull the Hf number off my chart for ethanol (-277.6kJ)

I have the answer given to me, but I'm stuck at converting my kJ to g of water. (Answer is 802g)

Tried: 1235.6kJ[1mol/6.03kJ][18.02g/1mol]

2. Apr 18, 2010

### Dickfore

You need to constants. The latent heat of fusion of ice (let us denote it by $$L_{f}$$) and the energy released by burning a unit mass of ethanol (let us denote it by $$q$$). Then, you simply have:

Heat needed to melt mass $$m(\texrm{ice})$$ = $$L_{f} \cdot m(\textrm{ice})$$

Heat released by burning mass $$m(\textrm{ethanol})$$ = $$q \cdot m(\textrm{ethanol})$$

These are equal, so, you have an equation for $$m(\textrm{ice})$$:

$$m(\textrm{ice}) = \frac{q}{L_{f}} m(\textrm{ethanol})$$

3. Apr 18, 2010

### philtered

Thanks for the help. It worked out with that. Also the q value I was using was wrong, had forgot to take into account the 10g of ethanol being burned.

4. Apr 18, 2010

### Dickfore

cool. am i right to assume you're a chem student?

5. Apr 19, 2010

### philtered

Yeah I am. Right now just the high school course, next year I'm doing Physics with a minor in Chem