Gases and Intermolecular Forces

Question 1

overview of gases

Answer 1

• atoms and molecules are generally not in contact

- intermolecular bonding can be observed

Question 2

pressure

Answer 2

• gas molecules exert pressure on their container through collisions
• bigger than 0 pascals, the molecules are in motion
• collective result of collisions is pressure (p) which is a macroscopic property

Question 3

equation for pressure

Answer 3

p = F/A

force in newtons (N)
unit area in square meters (m2)

Question 4

atmospheric pressure

Answer 4

weight of a column of air molecules in the atmosphere above an object

Question 5

Boyle’s law and equations

Answer 5

if you decrease volume, you increase pressure
if you increase volume, you decrease pressure
PV = k (constant)

P1V1 = P2V2

Question 6

what do you need to do to correct for pressure gauges?

Answer 6

pressure gauges measure above atmospheric pressure so add 1 atm to internal pressure of a tire etc.

Question 7

Charles’ law

Answer 7

as gas heats up, it expands

V is proportional to T

V1/T1 = V2/T2

Question 8

avagadro’s law

Answer 8

as the number of moles of gas increases, the volume increases (constant P and T).

limitation: only works with ideal gas laws (can’t keep things constant)

Question 9

intramolecular force

Answer 9

attractions and interactions between atoms within a molecule

Question 10

intermolecular forces

Answer 10

forces between molecules (relatively weak)

Question 11

dispersion forces

Answer 11

fluctuating, temporary dipole that results in relatively weak electrostatic attraction. applies to all atoms and molecules

Question 12

instantaneous dipole

Answer 12

electrons (in motion) are distributed asymmetrically for an instant

Question 13

induced dipole

Answer 13

instantaneous dipole in one atom distorts the electrons in another. they form dispersion forces

Question 14

trends in dispersion forces

Answer 14

• larger/heavier atoms and molecules have stronger dispersion forces (I2 is solid but F2 is a gas)
• as atoms/molecules increase in size and mass, melting and boiling points increase (more heat needed)
• forces strengthen as surface area increases (long)

Question 15

polarizability

Answer 15

how easily an electrostatic charge can distort a molecule’s charge distribution

Question 16

dipole-dipole attraction

Answer 16

electrostatic force between the partially positive end of one polar molecule and the partially negative end of another

• high boiling point than non polar molecules
• can be liquids at room temp

Question 17

hydrogen bonding

Answer 17

intermediate forces between hydrogen of one molecule and a more electronegative atom on another (FON)

• group 15, 16 and 17 hydrides increase boiling points down the groups (dipole-dipole become weaker instead)
• lightest of the hydrides should have lower boiling point but don’t (hydrogen bonding!)

Question 18

intermolecular forces of halogens

Answer 18

• form diatomic molecules such as H2
• single covalent bonds
• end on overlap of p orbitals

Question 19

compare Cl2, F2, B2 and I2

Answer 19

Cl2 and F2 expand and contract in response to pressure, move freely, collide - intermolecular forces are small (gases)

Br2 move freely but not as freely (less room), cannot be compressed, doesn’t expand a lot, intermolecular forces are stronger so is liquid

I2 has more electrons than Br2, little space between molecules, doesn’t expand or contract much, ordered in arrays and forces are strong enough to prevent molecules from sliding past each other. can vibrate (solid).

reasons:

• large KE molecules remain seperate as a gas
• intermolecular forces are large and force molecules close as a liquid or gas

Question 20

ideal gas law with Pa or kPa

Answer 20

PV = nRT

P = Pa
V = m3
n = mol
R = 8.314J/mol/k
T = k 
1000L in m3 so can use kPa and L instead

Question 21

ideal gas law with atm

Answer 21

PV = nRT

P = atm
V = L
n = mol
R = 0.08206L/atm/mol/k

Question 22

rearrangement of ideal gas law

Answer 22

M = mRT/PV

Question 23

density

Answer 23

varies with conditions

p (ro) = m/V

p = m/V = PM/RT

Question 24

rules of density

Answer 24

• density increases as pressure increases
• density decreases with increasing temperature
• density increases with M

Question 25

why can CO2 extinguish fire?

Answer 25

• higher molecular weight than air - denser

- displaces oxygen around the fire

Question 26

what does the ideal gas law assume?

Answer 26

• gas has negligible forces between its constituent atoms and molecules
• negligible volumes
  this is not true for real gases!!

Question 27

van der Waals equation for real gases

Answer 27

(p + n^2a/V)(V-nb) = nRT

corrects volume and negligible forces
- gases can be liquified!!

Question 28

why is PV/nRT less than 1 at low pressure?

Answer 28

intermolecular attractions - lower pressure than expected

Question 29

why is PV/nRT greater than 1 at high pressure?

Answer 29

gases do have a volume - finite volume of gas atoms/molecules