MNSR 21 - Heat Flashcards
(25 cards)
States/Phases of Matter
Intermolecular forces and thermal energy of molecules determines whether object is in solid, liquid, or gas state
Changing State
As temperature increases, molecules get more energy, can vibrate at greater amplitudes, material undergoes thermal expansion, can overcome some of the intermolecular forces and object can change state
Solid - liquid = fusion
Liquid - gas = vaporization
Heat & temperature
- Temperature and heat are closely related but they are not the same thing!
- Temperature is a property which indicates the relative “hotness” of an object.
Heat
- Heat is a measure of thermal energy which is transferred between systems due to a temperature difference.
- Heat energy is always transferred from a higher temperature to a lower temperature
Thermometer
A thermometer is used to measure temperature.
It is calibrated to a reference material and its THERMOMETRIC PROPERTY. i.e a mercury thermometer and the thermal expansion of the mercury
An increase in the height of a column of mercury can be related to a change in temperature
Mercury/Alcohol expands/contracts linearly within that temp range (if temp goes up by certain amount, expands/contracts by same amount)
Calorimeter
Device used to measure heat
Thermometric Property
As the mercury/alcohol heats up in the thermometer, it will expand in a linear fashion for this temperature region and after calibration, a thermometer can be made.
Heat Capacity
The amount of energy required to increase the temperature of a substance by 1°C
Specific Heat Capacity, C
Energy required to increase the temperature of 1kg of a substance by 1°C.
𝑄=𝑚𝑐∆𝑇
Q = amount of energy (J)
m = mass of the substance (kg)
c = specific heat capacity (J/kg.K)
∆𝑇 = change in temperature °C.
How much heat energy is required to raise the temperature of 3L of water from 20 to 80°C? SHC of water is 4200 J/kg.K
Q = 𝑚𝑐∆𝑇
m = 3 kg (1L water = 1kg)
∆𝑇 = 80-20 °C = 353-293 K = 60 K
C = 4200 J/kg.K
Q = (3)(4200)(60) = 7.56𝑥10^5 J
Evaporation
Some liquid molecules have enough KE to escape from the bulk liquid.
The rate of evaporation increases with temperature.
Vapor Pressure
Molecules evaporating from the liquid surface generate a VP
At equilibrium, number of molecules evaporating off is equal to the number of vapor molecules re-entering the liquid
Volatile compound turns from liquid to gas very easily, has a high VP (e.g. perfume turns into gas)
VP increases as temperature increases
Boiling Point
Temperature at which VP = Patm
If Patm decreases -> BP decreases -> lower temperature required (BP at Everest is 90-95 degrees)
At BP, visible evaporation occurs throughout the bulk of the liquid
BP is the only temp at which a liquid and vapor can coexist in equilibrium for a given pressure
Latent Heat
- To change from solid to liquid (at same temp) = Latent Heat of Fusion must be supplied to complete the phase change
- To change from liquid to gas (at same temp) = Latent Heat of Vaporization must be supplied to complete the phase change
Takes a lot more energy to turn from liquid to gas than solid to liquid = latent heat of vaporization generally much larger than latent heat of fusion.
Specific Latent Heat
- Specific Latent Heat of Fusion, Lf = energy required to convert 1kg of solid to 1kg of liquid (at the same T)
- Specific Latent Heat of Vaporisation, Lv = energy required to convert 1kg of liquid to 1kg of gas (at the same T)
Evaporation of Sweat
Takes more energy to evaporate than melt
Each water molecule that evaporates from skin removes a significant amount of heat energy
Same thing, birds require twice as much heat to melt snow from solid to liquid at 0 C than is needed to heat it from 0 C to their body temp of 40 C - takes them so much energy
Phase Diagram
Temp on x, Pressure on y
Temperature at which phase change occurs varies with absolute pressure
- Triple Point characterizes the only conditions under which all 3 phases can exist simultaneously
Positive Slope of Liquid-Gas Boundary
Can boil water at less than T<100 C is the gas pressure above the liquid surface is lowered
In-flight cabin pressure
2/3 atmospheric pressure, water boils <100 C
At 1500m deep in the sea, hydrothermal vents heat water up to 340 C, but water does not boil
H2O is unusual, negative phase boundary between Solid & Liquid
Freezing/melting point temp increases as pressure is reduced
Freeze Drying
By reducing air pressure, water will freeze at temps greater than 0 C
Water is removed from food/sample by sublimation
Sublimation
Substance goes directly from solid to gas phase
Plasma
Formed when molecules are given so much energy they become electrically charged ionized gas
e.g. neon lights, aurora, disinfection
Bose-Einstein Condensate
At micro-Kelvin Temperatures, the Pauli-Exclusion Principle is relaxed.
Atoms occupy identical ground states and get as close to each other as Heisenberg Uncertainty Principle allows.
