ideal gas law and IMF

What 2 things does the ideal gas law assume?

• particles are point masses with no volume

• there are no forces between molecules

How does the ideal gas law not account for volume of gas particles?

• behaviour deviates at small volumes, high pressures

• real gases have finite volume, reduces the space available for compression

Problems caused by IMF for ideal gas law

• at short intermolecular separation there will be repulsion between particles (Pauli exclusion)

• IMF forces slow down molecules which reduces the frequency of collisions with the container = reduces pressure

What is Van der Waals equation?

What 3 things affect the values of a in Van der Waals equation?

• dipole moment

• polarisability

• ionisation energy

What does dipole moment mean?

how big the dipole is, the difference in electronegativity

What does polarisability mean?

how easy for induced dipole to form

What is the Lennard Jones potential graph?

potential energy of interaction vs separation

Why does Lennard Jones potential graph never cross the y axis?

can’t put two atoms directly touching

• limit to how close atoms can be

What does ε represent in Lennard Jones Potential graph?

What does minimum represent?

ε represents how strong interactions are between molecules (how deep graph is)

most attraction, most potential energy

What does σ represent in Lennard Jones Potential graph?

as close as they can be, above that they are being forced together

• x intercept

How to calculate force from Lennard Jones Potential graph?

gradient using F = - dE / dr

• steeper gradient = stronger force

What is r₀ in Lennard Jones Potential graph?

equilibrium point where attraction = repulsion

At what temperatures does the ideal gas equation break down?

low temperatures, particularly at high pressures

What happens at lower temperatures (energy)?

less translational energy (energy used to move) = slower molecules = stronger attractive forces (have more time to move past each other)

What happens at higher temperatures (speed)?

a gas has a higher average speed and broader distribution of speeds

How do phases change at

• low temp

• high pressure

• low temp = strong interactions between molecules form condense phase

• high pressure = gas turns into liquid as molecules are held so closely together

What does enthalpy change mean?

The amount of heat transferred at constant pressure

What is an isotherm?

A graph plotted at constant temperature

Why is ice less dense than water?

Hydrogen bonds hold water molecules apart in a lattice when solid

Why are there stronger interactions at the surface of a liquid?

Why weaker interactions in the bulk of liquid?

imbalance of forces = pulled more strongly into middle

weaker interactions in the bulk as each particle experiences attractive forces in all directions

Why does water form larger meniscus than other liquids?

larger surface tension = hydrogen bonding means larger force of attraction at surface

What is capillary action?

some liquids rise up thin tubes against gravity

Why does the meniscus appear concave?

H bonds between water and SiO₂ (in glass) are stronger than surface tension

How does surface tension contribute to droplet size?

Equation?

• liquids with stronger IMF form larger droplets

• 𝛾 = surface tension