Latent Heat, Types of Latent Heat, Vapour Pressure

1. STATES OF MATTER & PROPERTIES OF
MATTER
Ms. Punam Dilip Bagad
Assistant Professor, Dept. of Pharmaceutics
GES’s Sir Dr. M. S. Gosavi College of Pharm. Edu. & Research,
Nashik-422005, INDIA
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2. Latent Heat
• The term was introduced around 1762 by British chemist Joseph
Black.
• It is derived from the Latin latere (to lie hidden).
• Heat is an important component of phase changes.
• As you apply heat to the water, the temperature of the water
increases until it reaches 100 °C, the boiling point of water.
• The water will stay at this temperature until all of the water
changes from liquid to a gas.
• During this process, you are continuing to add heat energy to the
water, but the water temperature does not increase.
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3. Latent Heat
 Normally when heat energy is added to or removed from an object,
the temperature of the object changes; however, during phase
changes, the temperature of an object stays constant.
 The temperature remains the same because energy is required for an
object to change phases.
 Latent heat is the heat energy per mass unit required for a phase
change to occur.
OR
 Latent heat (hidden heat) is defined as the total energy
absorbed or released when a substance change its physical state
completely at a constant temperature.
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4. Latent Heat
Specific latent heat:
• A specific latent heat expresses the amount of energy in the form of
heat required to completely effect a phase change of a unit of mass,
usually 1 kg of a substance.
• The formula for latent heat is:
• Q = 𝒎 × 𝑳
• This equation relates the heat Q that must be added or removed for
an object of mass m to change phases.
• The object's individual latent heat is noted by L.
• The unit of latent heat is J/kg
• The values of latent heat are variable depending on the nature of the
phase change taking place.
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5. TYPES OF Latent Heat
• The change of one phase to another phase of matter happens at a
specific temperature and pressure.
• The change happens with the absorption or release of heat followed
by a change in the volume and density of the substance.
1. Latent heat of fusion (Lf)
2. Latent heat of Vaporization (Lv)
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6. TYPES OF Latent Heat
Sr. No. Term used for phase change Description of phase change
1 Fusion/Melting Solid to liquid
2 Freezing Liquid to solid
3 Vaporization/Boiling Liquid to gas
4 Condensation Gas to liquid
5 Sublimation Solid to gas
6 Deposition Gas to solid
7 Evaporation Liquid to gas
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7. TYPES OF Latent Heat
Latent heat of fusion (Lf):
 The energy needed to break the bond in a solid and change it into
a liquid. It is the heat absorbed by melting solid.
 For Ice at 0 °C you would need 80 calories to melt each gram of ice.
 Solid water (ice) melts at 0 °C. If an ice cube is heated, it will begin
to melt.
 The interesting thing is that its temperature is kept constant in
this process: Even though heat is being supplied!
 That is because heat energy is being used to overcome the
intermolecular forces which keep the water molecules together in
a lattice.
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8. TYPES OF Latent Heat
Latent heat of fusion (Lf):
• Different materials will take different amounts of energy to melt. This
is called the latent heat of fusion.
• It is the energy required to transform a solid substance (already
heated to its melting point) into a liquid.
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9. TYPES OF Latent Heat
Latent heat of Vaporization (Lv):
• It is the energy needed to break the bond in a liquid and change it to a
gas.
• It is the heat absorbed during boiling and for ice it is 540 calories per
gram. When water boils, it does so at a constant temperature.
• The heat in this case is also used to overcome intermolecular forces
that are present
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10. Numericals
Q. 1: Calculate the amount of heat added to 1 gram gold to change
phase from solid to liquid. The heat of fusion for gold is
64.5*103J/Kg.
Q. 2: Determine the amount of heat absorbed by 1 kg water to
change phase from liquid to vapor. Heat of vaporization for water
= 2256*103J/Kg.
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11. Vapour pressure
 Vapour pressure of liquid solutions is defined as the pressure exerted
by the vapors on of a liquid in its equilibrium state with the pure
liquid at a given temperature.
 Liquid Solution is formed when we dissolve a solid, liquid or gas in a
particular liquid solvent.
 It varies with the nature of liquid and temperature of the
surroundings.
 Some characteristics of the vapour pressure of liquid solutions are as
follows:
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12. Vapour pressure
• The pure liquid has more vapour pressure as compared to liquids
solution.
• Example: Take two beakers, one filled with water and other with
lemon juice and water, you will find that the beaker filled with water
has more vapour pressure
• Vapour pressure is inversely proportional to forces of attraction
between molecules of a liquid.
• Vapour pressure increases with increase in temperature, as
molecules gain kinetic energy and vaporise quickly.
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13. Vapour pressure
 Liquid solution can contain volatile solute and solvent as well. In
most cases, the solvent is volatile in nature and the solute left out to
be non-volatile. Vapour pressure can be evaluated in two cases which
are:
 Vapour pressure of liquid-liquid solutions that is, solute and solvent
both are in liquid phase
 Vapour pressure of solid-liquid solutions that is, solute is in solid
state and solvent is in liquid phase.
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14. Vapour pressure
Vapour Pressure of Solutions of Solids in Liquids:
 Another type of solutions is solids in liquid solution, in which we take
solid as the solute and liquid as the solvent.
 For example, when we dissolve glucose, sugar or salt in water, we get
solids in liquids type of solution.
 Generally, the solute is non-volatile in nature and the vapour pressure
is less than the pure vapour pressure of the solution.
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15. Vapour pressure
The decrease in vapour pressure is due to:
 As evaporation is a surface phenomenon, the more the surface, the
greater the evaporation and hence more the vapour pressure. In a
pure liquid, there is more surface area available for the molecules to
vapourise, thereby have more vapour pressure. On the other hand,
when we add a non-volatile solute, the solvent molecules get less
surface to escape and hence experience low vapour pressure
 The number of molecules evaporating or leaving the surface is much
greater in pure liquid solutions to that of non-volatile solute in
solvent.
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16. Vapour pressure
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17. Vapour pressure
Factors affecting Vapor pressure
a) Temperature
Temperature has the most significant impact on the vapor pressure as
with higher temperature, the energy of molecules rises and there is an
increase in their ability to escape. A lower temperature, on the other
hand, makes it difficult for the molecules to escape from the liquid.
b) Molecule Type
The intermolecular force and the vapor pressure are inversely related to
each other. When the intermolecular force between the molecules is
strong, the vapor pressure will be low. However, if the intermolecular
force is weak, the vapor pressure is likely to be higher.
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18. Vapour pressure
Factors affecting Vapor pressure
c) Surface Area
As against the other factors, the surface area of the solid or liquid will
have no impact on the vapor pressure.
In other words, it will hardly matter whether you use a larger surface
area or a smaller surface area, the vapor pressure will still be the same
with constant molecule type and temperature.
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