6.Predict whether the values of q, w , U and H are positive, zero, or negative f
ID: 885966 • Letter: 6
Question
6.Predict whether the values of q, w , U and H are positive, zero, or negative for each of the following processes:
Metling of ice at 1 atm and 273K
Melting of solid cylcohexane at 1 atm and the normal melting point
Reversible isothermal expansion of an ideal gas
Reversible adiabatic expansion of an ideal gas
For most substances, the volume increases upon melting, but for the ice melting to water under the given conditions, the volume decreases. In all cases the change in volume between solid and liquid phases is very small and work associated will be small also. For the ideal gas U and H depend on the temperature only
Explanation / Answer
Metling of ice at 1 atm and 273K
heat is giving to the system. work done on the system negative heat giving to the system positive
temperature is constant at melting point . so dU = Cv dT = 0 , dH = CpdT = 0
q = +ve
w = -ve
U = 0
H = 0
Melting of solid cylcohexane at 1 atm and the normal melting point
q = +ve
w = -ve
U = 0
H = 0
Reversible isothermal expansion of an ideal gas
isothermal temperature constant so dU = dH =0
U = 0
H= 0
W = +ve
q = +ve
because dQ = dU + dW
dU = 0
dQ = dW
Reversible adiabatic expansion of an ideal gas
in adiabatic heat constant
q = 0
H = 0
U = -ve (dU = Cv dT , dU = -ve)
w = +ve
in adiabatic expanision work done by system is equal to decreasing in its internal energy
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