Question

Which of the following situations would cause the greatest increase in the motion of molecules in a system?
A. The system absorbs 100 J of heat and has 100 J of work done to it.
B. The system absorbs 100 J of heat and does 100 J of work.
C. The system releases 100 J of heat and does 100 J of work.
D. The system releases 100 J of heat and has 100 J of work done to it.

Answer 1

Answer: A. The system absorbs 100 J of heat and has 100 J of work done to it.

Explanation: According to first law of thermodynamics:

$\Delta E=q+w$

$\Delta E$=Change in internal energy

q = heat absorbed or released

w = work done or by the system

w = work done by the system=$-P\Delta V$

{ Work is done by the system as the final volume is greater than initial volume and is negative and work done on the system is positive.}

1. The system absorbs 100 J of heat and has 100 J of work done to it.

$\Delta E=+100+(100)=+200J$

2. The system absorbs 100 J of heat and does 100 J of work.

$\Delta E=+100+(-100)=0J$

3. The system releases 100 J of heat and does 100 J of work.

$\Delta E=-100+(-100)=-200J$

4. The system releases 100 J of heat and has 100 J of work done to it.

$\Delta E=-100+(+100)=0J$

As internal energy is the function of temperature, a positive increase in internal energy means the temperature has increased and thus the motion of particles will be greatest.