write and simplfy the closed system energy balance ( Equation : Q-W= delta U+ de

Question

write and simplfy the closed system energy balance ( Equation : Q-W= delta U+ delta Ek+ delta Ep) for each of the follwing processes and state whether nonzero heat and work terms are positive or negative

a. the contects of a closed flask are heated from 25 celsius to 80 celsius

b. a tray filled with water at 25 celsius is pur into a freezer. the water turns into ice at -4 celsius. note: when a substance expands it does work on its surroundings and when it contracts with surroundings do work on it .

Explanation / Answer

a) Since there is no change in position or motion of the system EK = 0 & Ep = 0
   Therefore the equation will be reduced to,
Q - (-W) = U
since the system is heated is Q will be positive; work is done on the system so W will be negative.

b) The temperature is brought down from 25oC to 0oC , q1
when temperature is brought down from 0oC to -4oC , q2
   By the temperature difference we can say that q2 > q1
  q1 + q2 = negative = Q
Contraction is taking place so that means surroundings do work on the system this implies
   that work is done on the system by surroundings hence negative
(-Q) - (-W) = U

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