For the processes described below, state whether it is appropriate to use delta
ID: 490230 • Letter: F
Question
For the processes described below, state whether it is appropriate to use delta H = Q. Briefly explain why in each case. Two gases mix in a container of fixed size and react chemically. Two gases mix in a container with a freely-moving piston and react chemically. A solid is added to a liquid in a beaker open to the atmosphere. Remember that the Q in the first law of thermodynamics is the heat transfer to the system of interest. In A and B. consider processes that may include physical or chemical changes of a system, perhaps the mixture of substances in a beaker. As a process takes place, the system of interest feels hot to the touch. Is this process exothermic or endothermic? In this process, is Q positive, negative, or zero? If the process takes place a constant pressure, is delta H positive, negative, or zero? As a process takes place, the system of interest feels cool to the touch. Is this process exothermic or endothermic? In this process, is Q positive, negative, or zero? If the process takes place a constant pressure, is delta H positive, negative, or zero?Explanation / Answer
1]
Fixed size - volume constant
Work = pdV = 0
H = E + PV and
q = E + W [ W = - pdV ]
delta H = delta E + pdV + vdP
constant pressure and volume is fixed
So
Q = E
delta H = delta E = Q
freely moving pistion
delta H = delta E + pdV
Q = E + pdV
delta H not equal to Q
Solid added to a liquid
The reaction is at constant pressure ...
At constant pressure ....delta H = Q
So delta H = Q
3]
A] Hot totouch
Exothermic
Q - negative - heat is released
delta H is also - negative
B]
cool to touch
Endothermic reaction
Q - positive [ heat is absorbed]
delta H = positive ]