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Gas molecules and their spread of speeds

Science · Thermal & Statistical Physics · ages 18-19

Name ______________________ Date ____________

What does the peak of the speed curve mark?
- The average bond length
- The most probable speed
- The total gas pressure
At one temperature, how do molecular speeds compare?
- They spread over a wide range
- They are all exactly equal
- They split into two fixed values
Heating a gas shifts its speed curve rigidly right with the same shape.
Circle one: True False
Why does evaporation draw only on the tail of the curve?
- Only the fastest molecules can escape the surface
- Slow molecules boil more easily
- The peak molecules carry no energy
Order the three speeds from slowest to fastest.
- Mean, most probable, root mean square
- Root mean square, mean, most probable
- Most probable, mean, root mean square
Two gases share a temperature, but one has lighter molecules. What differs?
- Both curves match exactly
- The heavier gas averages faster
- The lighter gas averages faster
A student sketches the heated curve taller and narrower. What is the error?
- Heating flattens and widens the curve
- Heating removes the peak
- Heating leaves the peak fixed
A student says the mean speed explains why slight heating doubles a reaction rate. What is wrong?
- Reaction rates ignore temperature
- Most molecules never react, so the tail count matters, not the mean
- Heating shrinks the fast tail

Answer key

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

The most probable speed · The tallest point of the curve is the speed most molecules have.
They spread over a wide range · One temperature fixes the average energy, not each single speed.
False · Heating flattens and widens the curve while the peak moves right.
Only the fastest molecules can escape the surface · Escaping the liquid takes rare high energy that only tail molecules hold.
Most probable, mean, root mean square · The peak comes first, then the mean, then the root mean square.
The lighter gas averages faster · Equal temperature means equal average energy, so lighter mass means higher speed.
Heating flattens and widens the curve · Higher temperature spreads molecules wider, so the peak drops and broadens.
Most molecules never react, so the tail count matters, not the mean · Only tail collisions clear the bar, and the tail grows far faster than the mean.