Matter Flashcards
What is absolute zero?
0K - lowest possible temp. where particles don’t move
Why does everything theoretically stop at absolute zero?
All particles have the minimum possible kinetic energy as energy is proportional to temp.
What does a change in temperature mean?
Thermal energy has been transferred
What is thermal equilibrium?
When everything is the same temp
What is specific thermal capacity?
The amount of energy needed to raise the temperature of 1Kg of a substance by 1K
What is the unit for specific thermal capacity?
J Kg-1 K-1
How can you measure the specific thermal capacity in a lab?
Heat the substance with an electric heater so its temp. increases
Attach an ammeter and voltmeter to the electric heater so that work done can be calculated (W=VIt)
And then use q = mcΔT
What heater do you use when finding specific thermal capacity of a liquid?
Heating coil (or and electric heater)
Why is the experimental value of c higher than the actual value?
Energy from the heater is also transferred to the air and the container
Resistance in the circuit
How can you minimise the effects of heat dissipation when finding the specific thermal capacity experimentally?
Start below and finish above room temperature to cancel out gains and losses in energy
When is an ideal gas a good approximation?
Low pressure
High temp (compared to bp of substance)
What is Boyle’s Law?
At a constant temperature, the pressure and volume of a gas are inversely proportional
pV = constant
What does the graph of pressure - volume look like?
y=k/x graph
How does temperature affect a pressure - volume graph?
The higher the temperature the further the curve is from the origin
What is Charles’ Law?
At constant pressure, the volume of a gas is directly proportional to absolute temperature
V/T = constant
What does the graph of volume - temp look like?
Straight line graph that crosses the x-axis at 0K or -273°C
How do you test Boyle’s Law?
Use a pump and pressure gauge and change the pressure in a sealed tube with oil and air
Record pressure against volume (volume will change as pressure changes) and then times them together to get a constant
How do you test Charles’ Law?
Use a sealed capillary tube containing a drop of conc. sulfuric acid halfway up the tube, so that a column of air is trapped between the bottom of the tube and the acid drop
Place capillary tube in large beaker of water and record temperature of water against length of the column. Take repeats and averages.
Plot a graph of length against temperature (length is proportional to volume) and graph should be a straight line
What is the Pressure Law?
At constant volume, the pressure of a gas is directly proportional to temperature.
p/T = constant
What is the graph for pressure - temp?
Straight line that crosses x-axis at 0K or -273°C
How do you test the Pressure Law?
Submerge a flask that is connected to a Bourdon gauge in water. The volume of the tubing must be much less than the volume of the flask
Record the pressure of the air in the flask and the temperature of the water. Do repeats and take averages
Plot a graph of pressure against and temperature and should get a straight line
What is the ideal gas equation?
pV=nRT
How do you derive the ideal gas equation?
Combine the three gas laws to get the equation:
pV / T = constant
the constant depends on the amount of gas used and is equal to nR, where R is the gas constant
What is the value of the gas constant?
8.31 J mol-1 K-1
What is Avogadro’s constant?
Na 6.02 x 10 23 mol -1
What is the unit for Avogadro’s constant?
mol -1
What is the Boltzmann constant?
k = R / Na
What is the difference between the gas constant and the Boltzmann constant?
gas constant is for one mole of gaseous
Boltzmann constant is for one gas particle
What is the alternative form of the ideal gas equation?
pV = NkT
What are the assumptions used in kinetic theory?
- Gas contains a large no. of identical molecules
- All collisions between molecules and walls are perfectly elastic
- Time taken for collisions is negligible compared to the time between collisions
- No intermolecular forces so molecules don’t attract each other
- Particles occupy a negligible volume compared to the volume of the container
- Gas particles obey Newton’s laws of motion
- Molecules have Brownian motion
How can you model the movement of particles?
Random walk
What does the random walk assume?
Each particle starts in one place, moves N steps in random directions, and ends up somewhere else
When does a particle change direction?
When it collides with another particle
What is the average distance moved by a particle proportional to?
√N
N = no. steps
(distance = √N x length of one step)
What is the distance usually travelled between collisions?
≈ 10 -7
How do gas particles exert pressure on their containers?
They continuously collide with each other and the walls of their container, causing a change in momentum which produces a force (F = Δp / Δt)
This force results in a pressure (P= F / A)
What does the area under a force - time graph represent?
Change in momentum (impulse)
How does speed of particle affect the force it exerts on the wall?
The greater the speed, the greater the force (F = Δp / Δt)
as Δt is smaller
What is the shape of a force - time graph of a collision?
bell shaped
What is the root mean square speed?
Measure of average speed
What is the average velocity of particles in a gas?
zero as all moving in different directions
How do you calculate the root mean square speed?
Find the root of the average of all the velocities squared
Why are the velocities squared when finding the root mean square speed?
Makes all negative velocities positive so won’t cancel out when finding the average
What is internal energy?
Sum of the kinetic and potential energy of the particles within a system
What is the internal energy of an ideal gas the same as?
Kinetic energy (there’s no potential energy)
What is the equation for average kinetic energy of a particle?
1/2 m √c2 (KE) = 3/2 kT
What two equations do you use to derive the equation for average kinetic energy of a particle?
pV = NkT
pV = 1/3Nm √c2
What is the average energy per particle approximately equal to?
≈ kT
What does a rise in absolute temperature mean for the internal energy of particles?
Increase KE of each particle which causes the internal energy to rise
How do we get the fraction in the equation pV = 1/3Nm √c2?
Particle can move in three dimensions, so the pressure exerted in any one dimension is 1/3 of the total pressure.
Estimate a third of the particles are travelling in one dimension at any time
How do you calculate the force exerted by one particle on a wall of the box?
change in momentum = 2mc
(c is speed of the particle and 2 as completely changes direction)
Force = change in momentum / time
time between two consecutive collisions = 2x /c (x is length of one side of cube)
F = 2mc / (2x/c)
F = mc2 / x
How do you find the pressure of one particle given the force of one particle is equal to mc2 / x?
Pressure = force / area
(mc2 / x) / x2 (as area is product of two lengths and as cube all lengths are x)
p = mc2 / x3
x3 = volume
pV = mc2
How do you find the pressure exerted by all the particles on one wall given pV = mc2?
As this equation is for a single particle multiply by 1/3 N to find the total pressure as we assume a third of all particles will be travelling in that direction
pV = 1/3 N m c2
What is activation energy?
Energy needed to break the bonds that hold together particles
What happens when the ratio Ea / kT is too big?
nothing
What happens when ratio of Ea/kT is around 15-30 and what does it mean?
Process happens at an appreciable rate
Some particles must have energies 15-30 times greater than the average energy
How do particles gain energy above the average energy?
Every time particles collide there’s a chance one of them will gain extra energy and if that happens several times in a row particles can gain energies much higher than the avergae
What is the Boltzmann factor? and official definition
e - E / kT
The ratio of the numbers of particles in energy states E joules apart, at temperature T kelvins
What does the Boltzmann factor tell you?
The ratio of particles in two energy states
When Ea /kT is 15 the Boltzmann factor is 10-7, what does that mean?
Only one in 10^7 particles have enough energy to overcome the activation energy
Why if only one in 10^7 particles have enough energy to overcome the activation energy do reactions occur?
Gas particles collide ≈10^9 times a second so every time there’s a collision there’s an attempt at a a reaction, so only a few particles with enough energy mean the reaction can happen in a matter of seconds
Why are reactions more likely at high temperatures?
As temp increases, Boltzmann factor approaches 1 so a greater proportion of particles have energy greater than the activation energy
What is the rate of reaction with activation energy Ea proportional to and why?
Boltzmann factor, e -Ea/kt
As both are proportional to temperature
