Work
Definition and meaning of Work in chemistry.
Work is energy transferred into or out of a system when a force moves something through a distance. In chemistry, work most often means pressure-volume work. That is the energy involved when a gas expands and pushes outward, or when something compresses it.
In more detail
Work and heat are the two ways energy crosses the boundary of a system. Heat moves because of a temperature difference. Work moves because something pushes or pulls.
The first law of thermodynamics ties them together. It says that the change in internal energy equals heat plus work, written as ΔU = q + w. Internal energy is the total energy stored inside the system.
Chemists use a sign rule to keep track of direction. Work is negative when the system does work on its surroundings. A gas expanding against outside pressure is the common case, and energy leaves the system, so the sign is negative.
Work is positive when the surroundings do work on the system, such as when a gas is compressed. Energy enters the system, so the sign is positive. For a process at steady outside pressure, the amount is w = -P ΔV.
Here P is the outside pressure and ΔV is the change in volume. If the volume grows, ΔV is positive, so w comes out negative. A common mix-up is treating work as something a system stores, like a fuel.
It is not. Work is energy while it is moving, and it only exists during the transfer itself. Once the piston stops, there is no work left in the gas, only internal energy.
Key facts
| Symbol | w (sometimes W) |
|---|---|
| SI Unit | Joule (J), often reported in kilojoules |
| Common Form | Pressure-volume work, w = -P ΔV |
| Sign Rule | Negative when the system expands, positive when compressed |
| Partner Quantity | Heat, the other way energy crosses a boundary |
| Field | Physical Chemistry |
Picture a gas sealed in a cylinder under a movable piston. Heat the gas and it expands, pushing the piston up against the air above it. The gas has done work on its surroundings, so w is negative. Now push the piston back down. You are doing work on the gas, energy flows into it, and w is positive. A car engine and an aerosol can both run on this same idea.
Frequently asked questions
What is the difference between work and heat?
Both move energy across the boundary of a system. Heat flows because one side is hotter than the other. Work happens when a force moves something, such as a gas pushing a piston. They are two different routes for the same thing, which is energy.
Why is work negative when a gas expands?
The sign shows which way the energy went. When a gas expands it pushes on its surroundings, so energy leaves the system. Chemists write that as a negative number. When a gas is compressed, energy enters the system and work is positive.
Does a system contain a certain amount of work?
No. Work is not stored inside anything. It only exists while energy is being transferred. Once the transfer stops, the energy that arrived is counted as internal energy, not as work.