Chemistry · Lesson
Systems, surroundings, heat and work
Choose the system boundary before assigning heat and work. Describe what can cross it and use a clear sign convention for energy received by the system.
Back to the Chemistry mapBy the end, you can
- Distinguish open, closed and isolated system descriptions.
- Treat heat and work as energy transfers.
- Assign signs to heat and expansion work using the chemistry convention.
Start by deciding what you are studying
A system is the part of the world selected for a calculation; its surroundings are everything outside the chosen boundary. For a reacting solution, you might call only the reacting material the system and include the container in its surroundings. For another question, you might include both material and container in the system.
The boundary determines the ledger. An open system can exchange matter and energy. A closed system does not exchange matter but can exchange energy. An ideally isolated system exchanges neither. Real apparatus can approximate these descriptions over a useful observation interval.
Energy transfer is different from energy stored
Heat is energy transferred because of a temperature difference. Work is energy transferred through other organized interactions, such as moving a boundary against a force. An object has internal energy; describing it as containing a fixed stock of heat confuses a state property with a transfer.
Thermal insulation can reduce heat transfer. An adiabatic idealization sets heat transfer q to zero for the process, but still permits work. A well-insulated gas can lose energy while pushing a piston, so adiabatic does not automatically mean isolated.
Give each arrow a sign relative to the system
In the chemistry convention used here, heat entering the system has q > 0 and heat leaving has q < 0. Work done on the system has w > 0; work done by the system on its surroundings has w < 0. The signs report energy entering or leaving the selected system.
Keep the boundary and convention explicit. A transfer that is positive for one side is negative for the other side in a simple exchange. These signs are bookkeeping statements, not a judgment that a process is desirable or undesirable.
| Transfer | Direction relative to system | Sign in the ledger | Example interpretation |
|---|---|---|---|
| Heat received | Surroundings → system | q > 0 | System receives energy through a temperature difference |
| Heat released | System → surroundings | q < 0 | Energy leaves the system through heat transfer |
| Work done on system | Surroundings → system | w > 0 | Compression can transfer energy to the gas |
| Work done by system | System → surroundings | w < 0 | Expansion can transfer energy to the surroundings |
Chemistry sign convention. The boundary is fixed for the description; the arrows identify transfer direction rather than stored heat.
Expansion work uses external resistance
For volume expansion against a constant external pressure, boundary work on the system is w = −p_extΔV. Expansion has ΔV > 0 and therefore w < 0; compression has ΔV < 0 and gives positive work on the system. Use the external pressure resisting motion.
For an irreversible expansion, the gas’s internal pressure need not equal the external pressure throughout the process. The constant-external-pressure formula is a stated model. If external pressure changes along the path, the work needs the corresponding pressure–volume integral.
Worked examples
Calculate work against a fixed external pressure
An illustrative gas system expands by 2.00 L against a constant external pressure of 100.0 kPa. Find the boundary work on the system.
- Use w = −p_extΔV.
- 1 kPa L = 1 J, so compatible units can be used directly.
- w = −100.0 × 2.00 = −200 J.
- Report three significant figures as −2.00 × 10² J.
w = −2.00 × 10² J. The negative sign means the system does work on its surroundings.
Try it yourself
Can a closed system exchange energy even though it does not exchange matter?
Show the answer and reasoning
Yes.
Heat and work can cross its boundary.
Does an adiabatic process necessarily have zero work?
Show the answer and reasoning
No.
Adiabatic sets q = 0; work can still transfer energy.
In this convention, what sign has work done by an expanding gas on its surroundings?
Show the answer and reasoning
Negative.
Energy leaves the selected gas system through work.
Which pressure appears in the stated constant-pressure expansion-work formula?
Show the answer and reasoning
External pressure.
It is the pressure resisting the moving boundary.
What to carry forward
- Define the system boundary before its energy ledger.
- Heat and work describe transfers, not stored quantities.
- Adiabatic does not automatically mean isolated.
- Use the stated sign convention and external resistance for boundary work.
References
Teaching resources and reference material supporting this lesson’s explanations and examples.