3.2-2 Thermal Properties of Matter
Name ______
Read Tsokos pp 165 - 169
Read Cutnell pp373-380
Assignment A323 Past IB exam
Microscopic description of temperature change of a substance
Any substance is composed of particles, and these are in random motion whether the substance is a solid, liquid, or gas (or plasma). Heating an object causes, on avergae, these particles to move faster. The total thermal energy that is transferredto an object when it warms is shared among all its particles as random kinetic energy (KE).
Note that these is a difference between the organized KE that a moving object’s particles must possess (which equals the KE of the object as a whole) and the random thermal kinetic energy that particles must possess (which is part of the internal energy of the object and is related to its temperature)
Increasing temperature of a subsatnce means that the average random KE of the particles has been increases. The temperature as measured on the thermodynamics scale is proportional to the average KE per particle.
There is no net flow of thermal energy in or out of an object at constant tenperature, but individual molecules are interchannging energy all the time. When energy I s”lost” to th esurroundings this degraded energy is ahred among all th emolecules in the surroundings as disorganized KE. This makes it difficult pratical task to utlize.
(For ideal gases, t he avergae KE is directly proportional to temperature:
KE = 2/3 kT, where k is Boltzmann’s consatnt and T is temperature in K)
Molecular structure and particle motion______
Phase / Solid / Liquid / GasMacroscopic / Both volume and shape are approximately constant / Have variable shape but volume is approx. constant / Has variable volume and shape
Microscopic / Molecules are held in position by bonds and vibrate about a fixed point in the lattice / Molecules vibrate within ordered clusters but there are no permanent bonds and can move / The intermolecular forces are only significant during collision, molecules are in random motion.
Mean Molecular separation / = atom radius / = atom radius / = 10 x atom radius
Number per m3 / 1028 / 1028 / 1025
Phase Change
When a substance changes phase, the potential energy of particles in the substance change as, on average, they move further apart.
Solids, liquids, and gases have different macroscopic properties corresponding to the different structures of substances in molecules terms. The table in the last page summarizes some aspects of these differences, and gives some typical values. A separate fourth phase/state, plasma, exists when a gas is heated to extremely high temperatures (e.g. 104 K or above), sufficient to ionize the atoms and molecules. A plasma consists of a mixture of ions and some neutral particles in random motion. The hydrogen and helium in the Sun are in the plasma state.
Macroscopic description of phase change
Whenever a substance changes state it does so at a fixed temperature, and there is energy associated with the change: this energy is called the latent heat. The amount of thermal energy involved depends on the type of substance, on its mass, and on the phase change involved. It can also be affected by other external air pressure.
The change of state from solid to liquid is called fusion, and the change from liquid to gas is called vaporization. Each of these changes involves energy being given to the substance. The reverse processes (liquefying and solidifying) must involved energy being released from the substance.
The specific latent Heat for a phase transformation (at constant temperature) is defined as the latent heat per unit mass:
Q = mL
Where
Q is the thermal energy transferred to the substance, measures=d in J
m is the mass of the substance, measured in kg
L is the latent heat, measured in J×kg–1
Macroscopic description of phase change
The making or breaking of bonds involves energy. When bonds are broken, the potential energy of the molecules is increased, and this requires energy input. In some changes (for example vaporization) the change is also associated with work being done: when a liquid changes into gas the volume greatly increases, so that duirng the expansion work must be done (pushing the atomosphere away), which adds to the increase in potential energy. When bonds are formed (soldification or liquefaction) then energy must be released.
It is imporatnt to note that when intermolecluar bonds are made or broken, this happens independently of the KE of the individul molecules. When water boils, the temperature of the liquid and the temperature of the vapour is the same: 100oC. The bonds between water molecules are being broken, but the average kinetic energy per molecule remains the same, and thus the molecules do not, on average, move faster.
Evaporation and Boiling
There are two common processes by which liquids can turn into the vapour state: evaporation and boiling. Boiling takes place at one fixed temperature, and happens throughout the liquid concerned (bubbles appear throughout the body of liquid). Evaporation, however, is the process by which the faster-moving molecules can escape from the surface of a liquid (see figure below)
As the faster-moving molecules are the only ones that can escape, evaporation causes the average KE per molecule to decrease. In other words, the temperature reduces. Evaporation causes cooling, and is an additional method of transferring thermal energy.
Specific Latent Heat of Fusion
· When a solid reaches melting point, it absorbs heat Q
Q = mLf
Where
Q is the thermal energy transferred to the substance, measures=d in J
m is the mass of the substance, measured in kg
L is the (specific) latent heat of fusion, measured in J×kg–1
· When a liquid reaches freezing point and starts solidication, it releases heat Q
Q = mLf
Where
Q is the thermal energy transferred to the substance, measures=d in J
m is the mass of the substance, measured in kg
L is the (specific) latent heat of fusion, measured in J×kg–1
Specific Latent Heat of vaporization
· When a liquid reaches boiling point and starts boiling, it absorbs heat Q
Q = mLv
Where
Q is the thermal energy transferred to the substance, measures=d in J
m is the mass of the substance, measured in kg
L is the (specific) latent heat of vaporization, measured in J×kg–1
· When a liquid is frozen, it releases heat Q
Q = mLv
Where
Q is the thermal energy transferred to the substance, measures=d in J
m is the mass of the substance, measured in kg
L is the (specific) latent heat of vaporization, measured in J×kg–1
Latent Heat under 1 atm
<practice problems>
1. Find the energy needed to heat a 0.4-kg ice cube from –5oC and turn it into 120oC vapor.
Specific heat capacity of ice = 2200 J×kg–1×K–1
Specific heat capacity of water = 4186 J×kg–1×K–1
Specific heat capacity of vapor = 2090 J×kg–1×K–1
Latent heat of fusion = 3.33x105 J×K–1
Latent heat of vaporization = 2.26x106 J×K–1
2. The process of heating water
Based on the graph, find
(a) Specific heat capacity of ice
(b) Specific heat capacity of water
(c) Latent heat of fusion
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