4.2 Thermal Physics

Question 1

Define Boyle’s Law

Answer 1

For a fixed mass of gas at constant temperature, the volume (V) is inversely proportional to the applied pressure (p)

Question 2

What is the Boyle’s law equation

Answer 2

pV = a constant where p is in Pascals

Question 3

Define Charles’ Law

Answer 3

For a fixed mass of gas at constant pressure, the volume (V) is directly proportional to the kelvin temperature (T)

Question 4

What is the Charles’ Law equation

Answer 4

V/T = a constant

Question 5

Define the Pressure Law

Answer 5

For a fixed mass of gas at constant volume, the pressure (p) is directly proportional to the kelvin (or absolute) temperature (T)

Question 6

What is the Pressure Law equation

Answer 6

P/T = a constant

Question 7

What is the ideal gas equation

Answer 7

pV/T = a constant or pV=nRT (n = no. moles in gas) or. pV = NkT (N = no. molecules of gas)

Question 8

what does the ideal gas equation depend on

Answer 8

a constant no. molecules in the sample of gas considered

Question 9

Define a mole

Answer 9

The mole is the amount of substance which contains as many particles as there are atoms in 0.012kg of carbon 12

Question 10

Define avogadros number

Answer 10

the number of particles per mole

Question 11

what is avogadros number

Answer 11

6.02 x 10^23

Question 12

equation for the number of moles in a substacne

Answer 12

no. particles/avogadros no.

Question 13

what is the relative molecular mass

Answer 13

mass of 1 mol in grams

Question 14

equation for the relative molecular mass

Answer 14

RMM = mass of gas sample in grams/no.moles

Question 15

Define kinetic theory

Answer 15

it is the collision of gas molecules with the walls of a container the produce an outward force or pressure

Question 16

What are the 5 assumptions of kinetic theory

Answer 16

1. There are no intermolecular forces
2. the molecules themselves have a negligible volume compared to the volume of the gas
3. the collisions between molecules and the walls of the container are elastic (i.e. energy is conserved)
4. the duration of a collision is negligible when compared w the time between collision
5. between collisions molecules move with constant velocity

Question 17

the total force on the container walls is the

Answer 17

sum of the forces of all colliding molecules

Question 18

Explain Boyle’s Law in terms of kinetic theory

Answer 18

a reduced volume means that the molecules have a shorter distance to travel to the walls of the container. Thus more collisions occur per second. Pressure increases

Question 19

explain Charles’ Law in terms of kinetic theory

Answer 19

Increase in temp means ke, momentum and velocity of the molecules all increase
to maintain same pressure, no. collisions per second decreases
expansion (increased volume) leads to a greater distance between collisions
longer time interval thus leads to a decreased no. collisions per second and pressure remains constant

Question 20

explain the pressure law in terms of kinetic theory

Answer 20

Increase in temp means ke, momentum and velocity increase
momentum change per collision and no.collisions per s increases
pressure increases

Question 21

what is R

Answer 21

the universal gas constant (applies to all gases)
8.31 Jmol^-1K^-1

Question 22

What is the Boltzmann constant, k

Answer 22

1.88 x 10^-23

Question 23

what is the equation for the Boltzmann constant, k

Answer 23

k = R/Avogadro’s no

Question 24

define translational kinetic energy

Answer 24

the kinetic energy of straight line motion is the only energy possessed by the atoms

Question 25

Define the internal energy of a gas

Answer 25

the internal energy of a real gas is the sum of the potential and kinetic energy of its molecules

Question 26

what is a monatomic gas

Answer 26

one in which atoms are not bound to each other eg. helium

Question 27

what is a polyatomic molecule

Answer 27

one which consists of 2 or more atoms

Question 28

what kind of gases are monatomic

Answer 28

ideal gases

Question 29

Why do ideal gases have entirely kinetic energy as their internal energy

Answer 29

there are no forces of attraction between the atoms. Ideal gases possess no potential energy.

Question 30

Use kinetic theory to explain the pressure exerted by a gas on container walls

Answer 30

Molecules collide elastically with the walls of the container. Each collision results in a momentum change of -2mv for the molecule.
A momentum change (by Newton’s second law) implies that a force was exerted on the molecules by the wall and on the wall by the molecules. (according to N’s third law)
The total force on the wall is the sum of the forces exerted by all the colliding molecules.
The pressure on the wall is force per unit area.

Question 31

Why can we be confident that the volume of a gas is not directly proportional to the celsius temperature?

Answer 31

If it was, then 0 degrees celsius air would have no volume. This clearly does not happen.