ch14

Question 1

describe the motion of gas molecules

Answer 1

gas molecules move with constant brownian motion which is random motion of molecules caused by collisions with larger particles

Question 2

how do gases exert a force (pressure) on it a container

Answer 2

the molecules continuously collide with each other and the walls of the container.

The collisions cause a change in momentum (impulse) which produces a force equal to the rate of change of momentum

this force gives pressure (P = F/A)

Question 3

how do you get impulse from a force-time graph

Answer 3

impulse is equal to the area under a force-time graph

Question 4

describe the pathing of particles

Answer 4

particles take a random path
they don’t travel in a straight line, but are constantly changing direction due to collisions, which is why diffusion is slow

Question 5

equation for average distance moved (displacement)

Answer 5

distance moved = √N * step length (mean free step)
N = number of steps moved
step length = length of one step

Question 6

boyle’s law

Answer 6

when gas is at a constant temperature, pressure P and volume V are inversely proportional
pV = constant

Question 7

why does boyle’s law still work

Answer 7

as volume decreases, particles are closer together, increasing density and colliding more frequently
this exerts a greater force and therefore greater pressure on the container

Question 8

1 bar = normal atmospheric pressure
the pressure in a car tyre is 2.6 bar
the tyre volume is 6 litres and the temp is constant

calculate the volume of gas that escapes if the tyre is punctured, assuming the tyre keeps its shape

Answer 8

pV = constant
first calculate constant
2.6 * 6 = 15.6 bar Litre

air will escape until the tyre pressure matches atmospheric pressure, 1 bar so work out volume after it’s expanded to that pressure
pV = 15.6 bar litre

V = 15.6 / P = 15.6 / 1
volume = 15.6 litres
but 6 litres remains constant in deflated tyre

volume escaping = 15.6 litre - 6 litre = 9.6 litre

Question 9

how to calculate number of particles

Answer 9

number or particles = number of moles * Avogadro constant Na

Question 10

how is pressure affected when the volume of a gas increases

Answer 10

when the volume of a gas increases, the space between molecules increases and so the time between collisions is larger

this cause the rate of collisions and so rate of change of momentum to decrease

this means the force exerted is lower, causing a decrease in pressure

Question 11

what does Charles’ Law

Answer 11

at a constant pressure, volume V is directly proportional to absolute temperature T

Question 12

number of moles =

Answer 12

number of moles =

mass of sample / mass of 1 mole

Question 13

explain charles’ law

Answer 13

as temp increases, the average kinetic energy of the molecules increases

pressure is constant so the force and rate of change of momentum is constant

to keep it like this, the volume increases so the faster speed of molecules is compensated by the larger gaps between them

Question 14

state pressure law

Answer 14

when a gas has a fixed volume, pressure is directly proportional to the absolute temperature

Question 15

explain pressure law

Answer 15

as temperature increases, the average kinetic energy increases so speed of molecules increases

this increases the rate of collisions so producing a larger rate of change of momentum

this leads to a greater force exerted and so an increase in pressure

Question 16

what does kinetic theory assume gases to be

Answer 16

ideal gases

Question 17

assumptions of ideal gases

Answer 17

the gas contains a large number of molecules

the gas molecules are identical to each other

all collisions between molecules and container are perfectly elastic (energy is conserved)

time taken for collisions is negligible compared to time between collisions

there are no intermolecular forces so molecules do not attract each other

molecules are in constant random Brownian motion

gas particles obey Newton’s laws of motion

Question 18

ideal gas equations:

Answer 18

pV = nRT

p = pressure (Pa)
V = volume (m³)
n = number of moles
R = gas constant 8.314 J mol^-1 K^-1
T = temp in K

pV = NKT

N = number of molecules
K = boltzmann constant
1.38 * 10^-23 J K^-1

Question 19

Boyle’s law relationship

Answer 19

pV = constant

Question 20

charles’ Law relationship

Answer 20

V / T = constant

Question 21

pressure law relationship

Answer 21

p / T = constant

Question 22

how can you work out nR using Nk

Answer 22

Nk = nR
N = number of molecules
k = boltzmann constant
n = number of moles
R = gas constant

Question 23

equation for force F caused by a single molecule on a wall

Answer 23

F = (mc²) / x
F = force in newtons
m = mass of molecule
c = velocity
x = length of box

Question 24

final theoretical ideal gas equation

Answer 24

pV = 1/3 Nmc²

Question 25

how do you get R.M.S, root mean square speed,

the average speed of molecules

Answer 25

you find the mean of the speeds squared

c² (small line over c)

Question 26

derivation of 1/2mc(bar)² = 3/2kT

what does the equation tell us

Answer 26

we have 2 equations:
pV = 1/3 Nmc² and pV = NKT

mc² is very similar Ek = 1/2mv²
so pV = 1/3 Nmc² =
1/3N(1/2mc²) = but 1/3 * 1/2 doesn’t equal to 1/3, so we must have 2/3 and 1/2 to get the original 1/3

pV = 2/3N(1/2mc²)
NKT = 2/3N(1/2mc²)
KT = 2/3(1/2mc²) so:

3/2KT = 1/2mc²
for a mixture of gases like air the mean kinetic energy is the same at a given temperature

Question 27

approximation of kinetic energy

Answer 27

3/2KT

3RT / 2Na

Question 28

what is internal energy and how do you calculate it

Answer 28

internal energy is the amount of energy contained in a system

internal energy is the sum of kinetic and potential energies of its particles

Question 29

internal energy in an ideal gas

Answer 29

there are assumed to be no intermolecular forces, so no potential energy
so assumption is internal energy = kinetic energy

Question 30

equation of internal energy.

the first law of thermodynamics

Answer 30

ΔU = W + Q
ΔU = change in internal energy
Q = energy transferred thermally 
W = work done (energy)
you calculate work done from W = force (N) * distance (m)
W and Q can be negative

Question 31

define specific heat (thermal) capacity

Answer 31

the energy needed to raise the temperature of a 1kg object by 1 degrees

Question 32

equation for specific heat capacity

Answer 32

E (energy transferred) = ΔU (change in internal energy)

E = m c Δθ

E = energy (J)
m = mass (KG)
c = specific heat capacity
θ = temperature (t)

Question 33

equation for power, work done and time

Answer 33

power is the rate at which work is done

P = ΔU * t

P = power (W)
ΔU = energy transferred
t = time