Thermal Flashcards
Define Internal Energy
The sum of the randomly distributed kinetic energies and potential energies of the particles in a body
How can you increase the internal energy of a system
By either heating or doing work on the system
When doing work on a system, what is the energy transferred to the system equal to
- The final kinetic energy of the object doing the work
- Which is equal to the GPE
- Work done=mgh
- mgh=mcΔT or Work done= Pressure x Change in Volume
What is the 1st Law of Thermodynamics
The increase in internal energy of a system is equal to the sum of the energies transferred to the system by heating and the energy transferred to the system by work done on it by an external force
Describe how KE and PE change, during a change of state
During a change in state, a substances internal energy changes, but its KE and temperature stay the same, therefore the PE of the particles changes instead
Define Specific Latent Heat
The amount of energy required to change the state of 1kg of a substance without a change in temperature
What is the formula for specific latent heat
Q=mL
Q=Energy Change (J or Nm)
m=mass(kg)
L=Specific Latent Heat (JKg-1)
Define Specific Latent Heat of Vaporisation
The amount of energy required to change 1kg of a liquid to a gas without a change in temperature
Define Specific Latent Heat of Fusion
The amount of energy required to change 1kg of a Solid to a Liquid without a change in temperature
Why is the specific latent heat of vaporisation much greater than the specific latent heat of fusion for water
Because the energy required to completely overcome the intermolecular forces is much larger than the energy required to just increase the separation of the molecules
Define Specific Heat Capacity
the energy needed to raise the temperature of 1kg of the material by 1C without a change in state
Specific Heat Capacity Formula
Q=mcΔT
m=mass
c=specific heat capacity
ΔT= temperature change
What is Absolute zero
0 Kelvin, all particles have the minimum possible kinetic energy
How do you convert from degrees Celcius to Kelvin
K= Degrees C + 273
In the Ideal gas equation pV=nRT, what do each of the letters stand for and what are their units
p=Pressure (Nm^-2 or Pa) V= Volume (m^3) n=Number of Mole (mol) R= Molar Gas Constant (8.31 JK^-1mol^-1) T=Temperature (K)
k= Boltzmann’s constant, what is its value
1.38x10^-23 JK^-1
Define Pressure
Force per unit area
Define Avogadro’s Constant
It is equal to the number of atoms in 12grams of Carbon-12, 6.02x10^23
In what conditions are the gas laws followed closest, and why
Gases at moderate pressures and densities, becasue the average separation of the molecules is much greater than the molecular diameter, and the intermolecular forces are weak so they have negligible effects
What is the formula for work done on a gas and how else can it be worked out
pΔV- where pressure is constant
Area under a pressure Vs Volume graph
What does Boyles Law state
At a constant temperature, the pressure and volume of a gas are inversely proportional. pV=Constant
Why is pressure inversely proportional to volume (Boyles Law)
As you increase the volume, the particles are colliding with the walls less frequently. This results in a smaller force per unit area on the container walls (smaller pressure) because there is an decrease in momentum change
What does Charles Law State
At a constant pressure the volume of a gas is directly proportional to its absolute temperature V/T=Constant
Why is volume directly proportional to temperature (Charles Law)
When the gas is heated the average speed of molecules increases. The faster molecules would result in more momentum change per collision, increasing the pressure, if the volume was kept constant.However if the volume was greater, there would be fewer collisions per second. Therefore to maintain a constant pressure the volume of the gas must increase when the gas is heated.
What does the Presure Law state
At constant volume, the pressure of a gas is directly proportional to its absolute temperature. p/T=Constant
Why is pressure directly proportional to temperature (Pressure Law)
When the gas is heated the average speed of molecules increases. therefore there are more collisions (with the wall) per second and more momentum change per collision. This results in a greater force per unit area on the container walls (greater pressure) because there is an increase in momentum change per second producing a greater force (Ft = p).
How do you work out the number of moles in a substance
Number of Moles= Mass/ Molar Mass
OR
Number of Moles= Number of Atoms/ Avogadro’s Number
What is Brownian Motion
Brownian motion is the random motion of particles suspended in a fluid (a liquid or a gas) resulting from their collision with the fast-moving atoms or molecules in the gas or liquid.
What is Brownian motion evidence for
the existence of atoms
What is the difference between empirical and theory
Empirical laws are based on observations and evidence, meaning they can predict what will happen, but they can’t explain why, e.g. the Gas law
Whereas Kinetic theory is based on theory, because its based on assumptions and derivations from knowledge we already had
What are the 6 assumptions leading to Average KE, being directly proportional to Absolute Temperature
1- The molecules continually move about randomly
2- The motion of the molecules follows Newtons Laws
3- The collisions are perfectly elastic
4- The time of collisions is much less than the time between collisions
5- The volume of particles is much less than the volume of the box
6- Except for during collisions the molecules always move in straight lines
Properties of an ‘Ideal Gas’
- Obeys assumptions for kinetic theory
- Follow the 3 gas laws
- Its internal energy is dependent only on the kinetic energy of the particle
- The potential energy=0J, as there are no forces between the particles except when they are colliding
When do real gases behave most like ideal gases
When they are at moderate pressure, and the temperature is reasonably high compared with their boiling point
Describe changes that occur in the motion of a typical molecule of gas during expansion due to an increase in temperature
- Speed/Kinetic Energy Increased
- Collision rate decreases
- The distance between collisions increases
If you only knew the temperature of gas molecules how could you work out the average KE
Average KE= 3/2kT
Define thermal equilibrium
no net flow of (thermal) energy (between two or more bodies) bodies at same temperature
Explain what is meant by random motion
molecules have a range of speeds (1) they have no preferred direction of movement (1)
Describe how the motion of gas molecules can be used to explain the pressure exerted by a gas on the walls of its container.
molecules collide (with the walls) (1) walls exert a force on the molecules (1) molecules exert an (equal) force (on the walls) (1) creating pressure (1) molecule momentum changes
