Fixing the gas laws for real gases

Science · Chemistry · ages 20-21

Name ______________________ Date ____________

At very high pressure, the compressibility factor Z rises above 1 for almost every gas. Why?
- Because attraction always wins at high pressure
- Because the gas cools down and turns into a liquid
- Real size resists squeezing at high P
- Because the ideal gas law becomes more accurate at high pressure
A real gas has a compressibility factor Z less than 1 at a given pressure and temperature. What does that tell you?
- Attraction pulls volume below ideal
- Repulsion between molecules is pushing the gas into a bigger volume than the ideal gas law predicts
- The gas is behaving exactly like an ideal gas
- The gas has condensed into a liquid
A real gas has Z below 1. What does that tell you?
- Attraction wins; gas is smaller
- Repulsion pushes the gas larger than ideal predicts
- The gas matches the ideal law exactly
Fugacity is an effective pressure that keeps ideal equations working for real gases.
Circle one: True False
A student replaces activities with concentrations in a concentrated solution. What goes wrong?
- Nothing, since they are always identical
- Constants and potentials are wrong
- The solution always turns ideal
Two gases have the same a but different b values in the van der Waals equation. What does the gas with the larger b have?
- Stronger attraction between molecules
- Bigger molecules taking up more room
- Molecules that move faster
- A higher temperature
In the van der Waals equation (P + a n^2 / V^2)(V - n b) = n R T, which physical effect does the constant a stand for?
- Attraction between molecules
- The volume the molecules occupy
- Heat flow into the gas
- Collisions with the container walls
A compressibility factor chart usually plots Z against reduced pressure and reduced temperature, so one chart can work for many different gases.
Circle one: True False
Why does a Z plot swing furthest from 1 near the critical point?
- Because the gas becomes perfectly ideal there
- Because the gas constant changes value there
- Attraction plus size together
A student says activity is just another name for concentration. What is missing?
- Effective concentration plus standard state
- Activity applies only to ideal gases
- Activity is always larger than concentration

Answer key

For grown-ups. Fold this page away before handing over the rest.

Fixing the gas laws for real gases W1-mt_HzRqV-KXfN-s1

Real size resists squeezing at high P · At high pressure the molecules are squeezed close together. Their own volume stops being negligible, and the gas resists being squeezed further than the ideal law expects.
Attraction pulls volume below ideal · Z compares the real gas volume to what the ideal gas law expects. A value under 1 means the real gas is smaller than expected, which happens when attraction wins.
Attraction wins; gas is smaller · A smaller than expected volume means attraction is winning.
True · It stands in for pressure wherever the ideal form is kept.
Constants and potentials are wrong · Concentrated solutions deviate far, so plain concentrations corrupt the result.
Bigger molecules taking up more room · b corrects the volume for the space the molecules fill themselves, so a larger b means bigger molecules.
Attraction between molecules · Attractions pull molecules back from the walls, so the measured pressure is lower than ideal; the a term adds that pressure back.
True · Reduced pressure and temperature are each gas's pressure and temperature divided by its own critical values. That scaling lets very different gases land on roughly the same curve.
Attraction plus size together · Both faults act together at high density, so neither can be ignored.
Effective concentration plus standard state · The standard state reference is what makes activities usable in the old equations.