KEY2CHEM

Energy Conservation     

As stated by the First Law of Thermodynamics, energy is neither created or destroyed. Energy can be transferred between system and surroundings; the magnitude of energy is unchanged even as it is transferred. Energy can be transferred as heat (\(q\)) or work (\(w\)), with the total change in internal energy described as \(\Delta E\) and where \(\Delta E = q + w\).

Transferring energy from the surroundings into the system increases the energy of the system, so \(\Delta E > 0\). Transferring energy from the system to the surroundings decreases the energy of the system, so \(\Delta E < 0\).


Example 1.

 

When the energy of the system decreases, what is the sign of \( \Delta E _{system}\)?

 

A. positive

B. zero

C. negative

 

 

 

 

Solution

 

C. negative

When the system of the energy decreases, energy is transferred out of the system, so \(\Delta E_{system} < 0\).

 


Example 2.

A system gains \(25\text{ J}\) of heat from its surroundings and does  \(15\text{ J}\) of work on its surroundings. What is \( \Delta E\) of the system?

 

A. \(-40\text{ J}\)

B. \(10\text{ J}\)

C. \(-10\text{ J}\)

 

 

 

 

 

Solution

 

B. \(10\text{ J}\)

Energy transfer into the system is positive, and energy transfer out of the system is negative. \(\Delta E = q + w = 25 \text{ J} + (-15 \text{ J}) = 10 \text{ J} \)


Example 3.

 

Which would result in an increase in energy of the system?

 

A. The surroundings absorb \(5\text{ J}\) of heat from the system.

B. The surroundings do \(8\text{ J}\) of work on the system.

C. The system does \(3\text{ J}\) of work on the surroundings. 

 

 

 

Solution

B. The surroundings do \(8\text{ J}\) of work on the system.

When the surroundings do work on the system, the system of the energy is increased (\(\Delta E > 0\)).