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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: happychappie1700 on January 02, 2019, 05:34:11 PM

Title: Entropy change of Universe - System and Surroundings
Post by: happychappie1700 on January 02, 2019, 05:34:11 PM

Hi Guys,

I wanted to ask you about an example calculation about the entropy change of the universe in regards to whether water will freeze to ice at 5°c ( 278 K ):

ΔSuniv=ΔSsurr+ΔSsys

If we already know that the entropy change of the system for water converting to ice is -22.0 J K-1mol-1 and that the entropy change of surroundings for water converting to ice at 5°c ( 278K ) is 21.6 J K-1 mol-1

Then ΔSuniv=ΔSsurr+ΔSsys:

                  = 21.6 - 22.0
                  = -0.4 J K-1mol-1(meaning entropy is reduced in the
                                                                               universe )

My question is :
where did the - negative sign come from in deducting the 21.6 - 22.0 ?
(when the equation ΔSuniv=ΔSsurr+ΔSsys involves addition )

Title: Re: Entropy change of Universe - System and Surroundings
Post by: mjc123 on January 03, 2019, 04:53:19 AM

Because as you said

Quote

we already know that the entropy change of the system for water converting to ice is -22.0 J K-1mol-1

Adding -22 is the same as subtracting 22.