Need Chemistry explanation about heat

Chemistry
Tutor: None Selected Time limit: 1 Day

(a)If 375J of heat are absorbed by the system, and it has 425 cal of work done by it, compute the change in the internal energy, and interpret the sign of your result. 1 cal= 4.184J

(b)When 3.50 moles of N2(g) is heated in a piston having a constant pressure of 3.00 atm from 0.00 degrees Celsius to 225 degrees celcius, calculate the work, expressed in J, associated with this process. Interpret the sign of your answer.

Nov 11th, 2015

Thank you for the opportunity to help you with your question!

a)Heat is absorbed by the system so q=375J

Work is done by the system ,so w=-425cal=-425*4.184 J=-1778.2 J

According to first law of thermodynamics,deltaU=q+w=-1403.2J

Interpretation of sign:Therefore, heat is taken away from the system making it exothermic and negative.

b)As the presure is constant therefore the work doe by the gas is given by the formula ,W=nR(dT)

Here dT=change in temperature=225-0=225

R=.8.314J/K

n=3.5

Wrok done=3.5*225*8.314 =6547.275J

As the pressure is constant therefore the volume of the system will increase and therefore the work is done by the system.

But work done by the system is negative therefore,work done=-6547.275 J

Please rate my answer so that I can help you further in the future.
Nov 11th, 2015

When it says work is done "by" the system, it means it's positive not negative

Nov 11th, 2015

When it says work is done "by" the system, it means it's positive not negative

Nov 11th, 2015

When it says work is done "by" the system, it means it's positive not negative

Nov 11th, 2015

Yes you are correct if you are a physicists because physicists use the sign convention said by you but in chemistry the sign convention is just the opposite as explained by me.There may arouse a lot of confusion therefore please remember this sign convention.

I have given the sign convention considering the Chemistry point of view:

Work done by the system=negative

Work done on the system=Positive

But in Physics

Work done by the system=positive

Work done on the system=negative.

Please consider which subject this question is from and accordingly put the sign.

Thanks

Nov 11th, 2015

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