Energy Change Q's

Lindsius · Probably a bot. Joined: 23 Sep 2006 Posts: 1

Hello,

I've been pondering over these for a while now and still can't seem to get anywhere, any help would be appreciated.

1) When 1g of ethanol undergoes complete combustion, 29.8kJ of heat energy is given out. How much heat would be given out for one mole of ethanol?

I figured: C2H5OH + 3O2 --> 2CO2 + 3H20
3231kJ + 1488kJ --> 2972kJ + 2778kJ
4719 - 5750 = -1031kJ/mol

One mole C2H5OH = 46g, 1g = 29.8kJ therefore -1031/46 should equal 29.8...it doesn't it equals -22.4kJ.

Where have I gone wrong?

2) When sulphur is burnt in oxygen it forms sulphur dioxide. Calculate the heat given out when 1.6g of sulphur is burnt.
S + O2 --> SO2 Energy Change = -297kJ/mol

I thought perhaps -297 x 20 would work?

Any help is appreciated, thanks.

adrian · Posted: Mon Sep 25, 2006 6:33 pm Post subject:

1. An easier way could be, as I think:
29.8 (kJ/g) x 46 (g/mole) = .... (kJ/mole);
I don't understood wherefrom you introduced all data: 3231kJ + 1488kJ --> 2972kJ + 2778kJ ??

2. Here we have the reverse: 297 kJ/ mole S.
But 1 mole S weight 32 g. 1.6g mean 1/20mole, i.e. the energy shall be 297/20= ...kJ.