Question

Calculate the entropy change of the universe (in J/K) when 43.4 g of ethanol vapor is condensed in a laboratory at 20.4 oC. Report your answer in scientific notation to three significant figures.

If something is condensed, does that mean the system gaines heat, or the surroundings gain heat?

Also, would I use enthalpy of fusion or enthapy of vaporization?

Answer 1

You would use the heat of vaporization. And if something is condensed, that means it gives off heat so the universe will gain it.

Answer 2

You can remember the heat absorbed or heat given off easily if you remember this:
I must ADD heat to water at 100 degrees C in order to turn it into steam. Everyone know that. <b>Therefore, if steam is condensing, it must give off heat.</b>

I must ADD heat to ice to melt it. Everyone knows that. <b> Therefore, if liquid water is freezing to ice, it must give off heat.</b>

You may also turn these into equations.
H2O(liquid) + heat ==> H2O(gas)
This shows we add heat to go from liquid water to steam (endothermic). Reverse the equation and it shows heat being given off (exothermic) for condensation.

H2O(ice) + heat ==> H2O(liquid)
This shows we add heat to ice to melt it (endothermic). Reverse the equation and it shows heat is given off when we freeze liquid water (exothermic).
It may be a little tough to think of freezing water as exothermic but the equation helps get it correct.