6.2 Flashcards
What is internal energy?
The sum of the randomly distributed kinetic and potential energies of the particles in a body
What is the first law of thermodynamics?
The change of internal energy of the object is equal to the total energy transfer due to work done and heating
What is the first law of thermodynamics linked to?
Conservation of energy
What happens when a sample is heated? (3)
- heat energy supplied increases internal energy
- Ek increases, so mean molecular speed increases
- also mean separation slightly increased so small increase in molecular Ep
What happens when a samples changes state?
- temperature remains constant
- so mean Ek is constant
When a sample is changing state, what is the energy being used for?
To break bonds as the sample melts or boils
When will two bodies be in thermal equilibrium?
When two objects at different temperatures are placed in thermal contact and heat flows from the higher to the lower temperature until the temperatures equalise
What is the specific heat capacity of a substance?
The energy needed to raise the temperature of unit mass of the substance by 1K without change of state
Properties of solids?
- maintain shape
- constant volume
- cannot be easily compressed
- molecules/atoms close together and vibrate in fixed positions
Properties of liquids?
- flow and take shape of vessel
- constant volume
- cannot be easily compressed
- molecules/atoms are close together and can move around each other
Properties of gases?
- flow and take any shape, fill any space
- take the volume of any vessel or space
- can be easily compressed
- molecules/atoms are far apart and are free to move
What is latent heat?
The energy needed to make a change of state occur
What is the latent heat of fusion?
The energy needed to melt a substance
What is specific latent heat?
How much energy is needed to make a unit mass of a pure substance change state
What are the units for specific latent heat?
J kg⁻¹
On a temperature-time graph, what does the gradient represent?
The rate of change of temperature
What is pressure affected by?
- temperature
- volume
- mass of gas particles
What type of collisions do gas molecules have with the walls of the container?
Elastic
Why do gas molecules move at the same speed after they have collided with the container wall?
The collisions are elastic
What is the magnitude of pressure proportional to?
The rate of collisions with the container wall
How can the pressure of a gas be increased? (3)
- increasing temperature - particles move faster
- reducing volume of container - increases chance of particles colliding with wall
- adding more gas - increasing the number of particles
What is Boyle’s law in words?
The pressure of a fixed mass of gas at constant temperature is inversely proportional to its volume
What is Boyle’s law in equations?
p ∝ 1/V
pV = constant
What is Charles’ law?
Reducing the temperature of a gas but maintaining the same pressure causes the volume to decrease
In Charles’ law, what does volume increase in proportion to?
Absolute temperature
In a graph showing Charles’ law, where will the x-intercept always occur?
At -273.15 °C (absolute zero)
In a Charles’ law experiment, what condition must there be for the x-intercept to be at absolute zero?
The gas must be ideal
What must happen if a volume of gas is compressed but the pressure is maintained?
Heat must be transferred
What happens when you reduce the temperature of a gas at a fixed volume?
Pressure is reduced
What is an ideal gas?
One which obeys the gas law exactly
When can real gas behaviour be classed as ideal?
When gases are considered at low pressures and higher temperatures
What can the gas laws be combined to give?
PV/T = constant
or
P₁V₁ / T₁ = P₂V₂ / T₂
What must be true of a gas to be ideal? (3)
- particles themselves can be thought of as taking up no volume
- no significant forces between particles
- motion of particles is random
What is Brownian motion?
The random movement of particles in a fluid
What is responsible for diffusion?
Brownian motion
What are the 6 assumptions that have to be made to create a kinetic theory equation?
- the particles themselves take up no volume
- there is no force of attraction between the particles
- their motion is random
- their collisions are elastic
- the length of time of the collisions against the walls of the container and each other are negligible
- Newton’s laws of motion can be applied
What are the three factors that affect the pressure of a gas in a given volume?
- mass of molecules
- speed of molecules
- how many molecules there are in the container
How is energy transferred between particles in a system?
Collisions between particles.
What are the three gas laws and their equations?
- Boyle’s Law -> pV = constant
- Charles’ Law -> V/T = constant
- Pressure Law -> p/T = constant
Define internal energy.
The sum of the randomly distributed kinetic and potential energies of all the particles in a body.
Does internal energy change when there when particles collide in a closed system, providing there is no work being done and temperature is constant?
- No it stays constant.
- Average speed of the particles also stays constant
Does a closed system have a constant total internal energy?
What are the conditions?
Yes, as long as:
* It’s not heated or cooled
* No work is done
How can the internal energy of a system be increased
- Heating it
* Doing work on it
Have can you decrease internal energy?
Doing work to to remove energy or cooling the system
During a change of state, what happens to kinetic and potential energies?
- Kinetic energy -> Constant
* Potential energy -> Changes
What happens to temperature when a substance changes state?
Stays the same
What does a graph of temperature against internal energy look like during a change of state?
What is an assumption of the 3 gas laws?
The mass of the gas is constant.
What is the equation for internal energy?
Internal energy = Kinetic energy + Potential energy
Does internal energy change when there is a change of state? Why?
Yes, because the potential energy of the particles is increased.
Define specific heat capacity.
The amount of energy needed to raise the temperature of 1kg of a substance by 1K (or 1 degree C).
Define specific latent heat.
The quantity of thermal energy require to change the state of 1kg of a substance.
Why does Boyles Law happen (particles)?
If you reduce the volume, the particles will be closer together and collide more often = pressure increases
Why aren’t perfect gases perfect? (which law don’t they follow perfectly and why?)
Perfect gases aren’t quite perfect as they don’t follows Boyles law perfectly:
Boyles law assumes that the particles don’t have any size or (more importantly) volume of their own.
Describe the graph for Charles’ Law.
- Volume against temperature plotted
- Straight line with positive gradient
- x-intercept is at -273°C or 0K
Why does Charles law happen (particles)?
Heat gas = particles gain KE = move more quickly = (at a constant pressure) they move further apart = volume of gas increases
Describe the graph for Boyle’s Law. (3)
- Pressure against volume plotted
- Like a 1/x curve, depending on the temperature
- The higher the temperature, the further the curve is from the origin.
What is the Pressure Law? (2)
- p/T = Constant
- At a constant volume, the pressure of a gas is directly proportional to the temperature
Describe the graph for the Pressure Law.
- Pressure against temperature is plotted
- Straight line with positive gradient
- x-intercept is at -273°C or 0K
What happens in the pressure law (particles)?
Heat gas = particles gain KE = Moves faster = (If volume is constant) particles collide with each other more often and at higher speeds = increasing pressure
When does the ideal gas equation work best?
At low pressures and fairly high temperatures.
What is the equation for the work done to expand a gas?
W = p x ΔV
(NOTE: This only applies when pressure is constant.)
How can the work done to expand a gas be found using a p-V graph?
It is the area under the graph.
What are some of the simplifying assumptions used in kinetic theory? (8)
1) Molecules continually move about randomly.
2) Motion of molecules follows Newton’s laws.
3) Collisions between molecules or at the walls of the container are perfectly elastic.
4) Except for during collisions, the molecules always move in a straight line.
5) Any forces that act during collisions last for much less time than the time between collisions.
6) All molecules of the gas are identical.
7) The gas contains a large number of molecules.
8) Molecules have negligible volume compared with the volume of the container (i.e. they act as point masses)
What are the simplified kinetic theory assumptions and how do you remember it?
If temperature is increased and volume is fixed, why does pressure increase (collisions and force)?
More collisions between molecule and the walls in a given amount of time.
On average, a collision will result in a larger change in momentum (higher average speed), and so exert a larger force on the walls on the container.
If temperature is increased and pressure is fixed, why does volume increase? Why does pressure stay constant?
If volume is larger, there will be a longer time between molecule-wall collisions, so rate of change of momentum and therefore the force on the walls of the container will be reduced.
As volume increases, the surface area of the walls increases. Pressure is defined as force per unit area, increasing area stops the pressure from increasing.
How is the average kinetic energy of a gas related to the absolute temperature?
It is proportional.
How can Brownian motion be seen?
When large, heavy particles (e.g. smoke) are moved with Brownian motion by smaller, lighter particles (e.g. air) travelling at high speeds.
Why does Boyles Law happen (particles)?
If you reduce the volume, the particles will be closer together and collide more often = pressure increases.
