Gen Chem -> Concepts + Trends C9

What equation connects pressure, volume, moles, and temperature?

PV = nRT.

What temperature scale must be used in gas laws?

Kelvin; K = °C + 273.15.

What causes gas pressure?

Gas particles colliding with container walls.

What does Boyle’s law describe?

Pressure and volume are inversely proportional at constant temperature and amount of gas.

If volume is halved at constant temperature and moles, what happens to pressure?

Pressure doubles.

What does Charles’ law describe?

Volume and Kelvin temperature are directly proportional at constant pressure and amount of gas.

If Kelvin temperature doubles at constant pressure and moles, what happens to volume?

Volume doubles.

Why does a heated gas expand at constant pressure?

Faster particles would raise pressure, so the gas expands to maintain the original pressure.

How does temperature affect pressure in a rigid container?

Higher Kelvin temperature causes higher pressure when moles stay constant.

What does Avogadro’s law describe?

Volume is directly proportional to moles at constant pressure and temperature.

What happens to pressure when more gas is added at constant volume and temperature?

Pressure increases.

If temperature increases by a factor of 4 and volume doubles, how does pressure change?

Pressure doubles because 4 divided by 2 equals 2.

What does kinetic molecular theory assume about particle motion?

Particles move constantly and randomly.

What does the ideal gas model assume about molecular volume?

It is negligible compared with the container volume.

What does the ideal gas model assume about intermolecular forces?

Particles do not attract or repel each other.

What is an elastic collision?

A collision in which total kinetic energy is conserved.

What determines the average kinetic energy of gas particles?

Kelvin temperature only.

Do different gases at the same temperature have the same average kinetic energy?

Yes.

Do different gases at the same temperature have the same average speed?

No; lighter molecules move faster on average.

How does molar mass affect RMS speed at the same temperature?

RMS speed decreases as molar mass increases.

If Kelvin temperature quadruples, what happens to RMS speed?

It doubles because speed is proportional to the square root of temperature.

What is diffusion?

The spreading and mixing of particles.

What is effusion?

The escape of gas particles through a tiny opening.

Which gases effuse faster under comparable conditions?

Gases with lower molar masses.

What does Graham’s law relate?

Gas diffusion or effusion rates to the inverse square root of molar mass.

What can be said about equal gas volumes at the same pressure and temperature?

They contain equal numbers of molecules.

For equal volumes of gas, what ratio helps compare the number of molecules?

Pressure divided by Kelvin temperature, P/T.

For equal masses of different gases, which has more molecules?

The gas with the lower molar mass.

For equal masses at the same volume and temperature, which gas has higher pressure?

The gas with the lower molar mass because it contains more moles.

What is the gas density equation?

d = PM/RT.

How does pressure affect gas density at constant temperature?

Higher pressure increases density.

How does temperature affect gas density at constant pressure?

Higher temperature decreases density.

How does molar mass affect density at the same pressure and temperature?

Higher molar mass increases density.

What does Dalton’s law state?

Total pressure equals the sum of the gases’ partial pressures.

What is partial pressure?

The pressure a gas in a mixture would exert alone at the same volume and temperature.

How do you calculate mole fraction?

Moles of one gas divided by total moles.

How do you calculate partial pressure in an ideal gas mixture?

Multiply mole fraction by total pressure.

Should mass fraction or mole fraction be used to calculate partial pressure?

Mole fraction.

Under what conditions do real gases behave most ideally?

High temperature and low pressure.

Under what conditions do gases usually deviate most from ideal behavior?

Low temperature and high pressure.

Why does low pressure favor ideal behavior?

Particles are far apart, so attractions and molecular volume have smaller effects.

Why does high temperature favor ideal behavior?

Intermolecular attractions are less important relative to particles’ kinetic energy.

What two real-gas effects does the van der Waals equation correct for?

Intermolecular attractions and finite molecular volume.

What is the van der Waals equation?

[P + a(n/V)^2](V − nb) = nRT.

What does the van der Waals parameter a represent?

The strength of intermolecular attractions.

What does a larger a mean?

Stronger attractions and a larger downward pressure correction, with other variables fixed.

How do intermolecular attractions affect pressure?

They tend to lower pressure by reducing momentum transferred to the walls.

Why is a(n/V)^2 added to P inside the van der Waals equation?

It corrects the measured pressure upward to account for attractions.

What does the van der Waals parameter b represent?

Effective excluded volume per mole associated with molecular size.

What does nb represent?

The total excluded-volume correction for n moles.

What does V − nb represent?

The corrected volume available for molecular motion.

Why is nb subtracted from container volume?

Real molecules occupy space, leaving less available volume.

How does the excluded-volume correction affect predicted pressure?

It raises pressure compared with using the full container volume.

What does a larger b do when other variables stay fixed?

It decreases available volume and increases the pressure contribution.

What is the van der Waals equation solved for pressure?

P = nRT/(V − nb) − a(n/V)^2.

Can real gas pressure be higher than ideal gas pressure?

Yes; excluded-volume effects can outweigh attractions.

Can real gas pressure be lower than ideal gas pressure?

Yes; attractions can outweigh excluded-volume effects.

When does van der Waals behavior approach ideal behavior?

When nb is small compared with V and the attraction correction is small.

What must you do if a van der Waals problem gives gas mass?

Convert mass to moles before substituting.

What quantity is squared in the van der Waals attraction correction?

The entire ratio n/V.

What is a memory aid for a and b?

a means attractions; b means bulk or molecular size.