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# Work when volume or pressure is constant

There are a few common scenarios where we might want to calculate work in chemistry class, and it helps to be able to recognize them when they come up. We will discuss how work is calculated for these cases.

### Constant volume processes

Sometimes reactions or processes occur in a rigid, sealed container such as a bomb calorimeter. When there is no change in volume possible, it is also not possible for gases to do work because \Delta\text V =0ΔV=0delta, start text, V, end text, equals, 0. In these cases, \text {work}= 0work=0start text, w, o, r, k, end text, equals, 0 and change in energy for the system must occur in other ways such as heat.

### Bench (or stove) top reactions: Constant pressure processes

In chemistry, we will often be interested in changes in energy that occur during a chemical reaction at constant pressure. For example, you may run a reaction in an open beaker on the benchtop. These systems are at constant pressure because the pressure in the system can equilibrate with the atmospheric pressure of the surroundings.

Photograph of soup containing tomatoes, onions, and meat in a clear orange-ish broth. The metal pot is on a white stovetop, and the soup is being stirred with a black plastic spoon.Cooking soup in an open pot is another example of a chemical reaction at constant pressure!In this situation, the volume of the system can change during the reaction, so 0ΔV​= 0, does not equal, 0 and work is also non-zero. Heat can also be transferred between the system (our reaction) and the surroundings, so both work and heat must be considered when thinking about the energy change for the reaction.

The energy contribution from work becomes more significant when the reaction makes or consumes gases, especially if the number of moles of gas changes substantially between the product and the reactants.Other chemical processes result in only a small volume change, such as in the phase change from a liquid to a solid. In these cases, the energy change due to work will also be quite small, and may even be ignored when calculating the energy change. The relationship between work, heat, and other forms of energy transfer is further discussed in the context of the first law of thermodynamics.