Chapter 14- Thermal Flashcards
define the triple point of a substance
a specific temperature and pressure where the three phases of matter can exist in thermal equilibrium
what is the net transfer of thermal energy if one object is hotter than another
energy moves from the hotter object to the colder object
what is meant by two objects being in thermal equilibrium
there is no net transfer of energy/at the same temperature
what is the 0th law of thermodynamics
if A is in thermal equilibrium with B
and B is in thermal equilibrium with C
then A is in thermal equilibrium with C
what are the 2 fixed points on the Celsius scale
- 0 is the freezing point of water
- 100 is the boiling point of water
what are the important things about the Kelvin scale
0 is absolute 0
273 is the triple point of water
the increments are the same as in celcius
e.g. 0 celcius = 273 kelvin etc.
what is the kinetic model
it is a model which describes how all substances are made of atoms or molecules arranged differently depending on phase
summarise the particle layout for a solid
- particles are regularly arranged/packed closely
- strong electrostatic forces
- fixed positions
- particles are able to vibrate around their fixed positions so they have KE
summarise the particle layout for a liquid
- particles are very close together but not in a regular arrangement
- particles can flow over each other
- more KE than solids
- weaker electrostatic forces than solids
- random motion of particles
summarise the particle layout for gases
- negligible electrostatic forces
- fast and random motion
- more KE
- collisions between different particles
- collisions between particles and container walls leads to pressure
what is the idea of Brownian motion and how did Einstein explain it
- Brownian motion is the idea that substances were formed of particles and the particles’ KE and collisions determines its motion
- its the idea of random motion
Einstein said:
- collisions between water molecules and pollen grains are elastic
- there is a transfer of momentum from the water molecules to the pollen grains
how can Brownian motion be observed
- set up a light source going through a convex lens
- the light rays should pass through a smoke cell
- a microscope can be placed above the cell, at a right angle
- the smoke particles can be viewed as they scatter the light
- both the smoke particles and the air particles have random motion but only the smoke particles move slow enough to be viewed (they do have the same KE)
what generally happens to density as you change phase
- density decreases from solid to liquid to gas
- this is because the particles have more KE so collide more frequently and take up more room
define the internal energy of a substance
“the sum of the randomly distributed kinetic and potential energies of atoms or molecules within a substance”
what are the two ways that internal energy is increased
increasing temperature, changing phase
why does increasing the temperature of a substance increase the internal energy
the kinetic energies of the particles increases thus the internal energy increases
why does changing phase increase the internal energy of a substance
the potential energies of the molecules increases,
because work is done on the molecules
they are rearranged and electrostatic attraction between them decreases
why do solids and liquids have a negative potential energy
it requires energy to overcome the electrostatic forces between molecules, liquids are less negative than solids
define specific heat capacity
“the specific heat capacity of a substance is the energy required per unit mass (kg) to change the temperature by a unit temperature (1K)”
what is the equation for specific heat capacity
E = mc(delta(theta)) Energy = mass x SHC x change in temp
what tend to be the units for SHC
J kg^-1 K^-1
what is water’s SHC
4.2KJ kg^-1 K^-1
what is the circuit required for the heater when determining SHC and SLH
- a heater in series with a cell, variable resistor and ammeter
- in parallel with a voltmeter
outline the experiment to determine the SHC of a substance
- record the mass of a substance
- fully immerse a heater and place a thermometer into an insulated substance
- record initial temperature
- switch on the heater and start a timer simultaneously
- record the p.d. and current of the heater
- after a set time, stop the timer and record the change in temperature
- given E = VIt,
- C = VIt/m(delta(theta))
how can this SHC experiment be improved
- plot a graph of temperature (on Y) against time (on x)
- given VIt = MC(delta(theta))
- this means delta(theta) = p/mc (t)
- thus SHC = power/(m)(gradient)
what to do where you mix two different substances and they reach thermal equilibrium
- where 2 substances of different initial temps are mixed
- final temps are =
- known SHC of 1
- known masses’
we assume the energy transferred by both is equal
this implies m1c1theta1 = m2c2theta2
this means if 5 are known a 6th can be calculated
define specific latent heat
“the specific latent heat of a substance (L) is the energy required to change the phase per unit mass (usually kg) while at a constant temperature”
what is the equation for specific latent heat
E = Lm
what is the symbol and term for the transition of
solid –> liquid
specific latent heat of fusion Lf
what is the symbol and term for the transition of
liquid –> gas
specific latent heat of vaporisation Lv
why does changing state require energy
although the temp doesn’t change, the potential energy increases
how to experimentally find specific latent heat of fusion
- weigh the beaker first
- place ice in a funnel with a heater as in SHC experiment a thermometer
- make sure the temperature is 0 degrees C
- just as it starts to melt, switch on the heater and start a timer
- record p.d. and current
- once all ice has melted, switch off timer and heater
- measure mass of water in beaker
- given E = VIt = Lf x mass
- use Lf = VIt/mass
how to experimentally find specific latent heat of vaporisation
- do the same thing as before but use a different setup
- inner flask with heater-> outer flask-> condenser-> beaker
why is SLH (vaporisation) usually higher than SLH (fusion)
- there is a bigger difference in internal energies between liquid and gas than solid and liquid
combining SLH and SHC
SLH and SHC might need to be combined where a substance undergoes a very large temperature change and the substance melts or boils in between
e.g. ice from -40 to 100
(SHC for -40 to 0) + (Lf) + (SHC for 0 to 100) + (Lv)
what is absolute zero
a temperature of 0K
the minimum internal energy a substance can have and the lowest possible temperature
is there any increase of potential energy when a substance’s temperature increases
only a small increase
when heating a sample of a substance to calculate specific latent heats, why may the substance not start changing state immediately
- it may need to heat up (increase temperature) first before it can change phase
why may the temperature of an object not start heating up immediately when the power to a heater is switched on/ why is there sometimes a flat line on a temp-time graph even if its not changing state
the heater is being heated/work is done on the heater
what is the difference between internal energy of a solid/liquid and internal energy of an ideal gas
- internal energy of an ideal gas is purely kinetic as the molecules have no potential energies
- internal energy of a solid/liquid is both kinetic and potential energies
what is an important detail of wording in the SHC experiment
heater is FULLY IMMERSED in water/oil/substance
what are some sources of uncertainty in the SHC experiment and how can these be reduced
- non-uniform temperature distribution, to solve stir as the substance heats (if liquid)
- heat loss to surroundings - insulate substance or start the experiment the same amount below room temperature as you will finish it
- take into account the energy required to heat container/beaker
if you are describing the motion/energy of particles in objects that are in different phases but at the same temperature what should we note
- the particles still have the same KE because they are at the same temperature
what should we do if there’s two experiments for SHC or SLH where one is heated only by the surroundings and one has a heater
- find the difference in mass heated/melted by the two experiments
- THIS is the mass heated by the heater
