Latent Heat:
When a substance is heated, the temperature of the substance increases gradually until it reaches its melting point where the substance start to melt. As the substance start to melt, the temperature of the substance remains constant until the melting is completed. During this time that the object start to melt to the time that the melting is completed, The thermometer thermometer will not indicate any increase in temperature. The heat that is supplied from the time the object stated to melt to the time the melting is completed, is used to overcome the intermolecular force that binds the molecules of the substance together and make the substance change from solid state to liquid state.
Also, when a liquid is heated, the temperature of the liquid gradually increases until it reaches the boiling point of the liquid, at which the liquid starts to boil. This temperature at which the liquid start to boil is called saturation temperature. During this time that the liquid is boiling, the temperature of the boiling water remain constant at 100°C, though more heat is added. The water changes to vapour and escape to the atmosphere. The thermometer did not read any increase in temperature because the additional heat is hidden. The additional heat that is applied to the boiling water is used to break the intermolecular force of attraction between the water molecules and make the water changed to vapour / gas.
Definition Of Latent Heat:
Latent heat is the quantity of heat energy that is required to change a substance from one state to another at constant temperature and pressure. The unit of latent heat is Joule (J), and the symbol used for latent heat is Q.
Formula Of Latent Heat Capacity:
The quantity of heat that is required to change a substance from one state to another is directly proportional to the mass of the substance.
Mathematically:
Quantity of heat energy ∞ mass of substance
Q ∞ m
Introducing the constant of proportionality: Q = L*m
L is the constant of proportionality. It is called the specific latent heat of the substance.
M is the mass of the substance measured in kilogram.
Q is the quantity of heat measured in Joule.
Factors That Determine The Quantity Of Heat Required To Change The State Of A Substance:
The following factors determine the quantity of heat energy that is required to change a substance from one state to another:
I. The nature of the substance.
II. The mass of the substance.
III. The quantity of energy supplied.
IV. The volume of the substance
V. The area of the substance
VI. The state of the substance as the time the heat energy is supplied.
Specific Latent Heat Of A Substance:
Specific latent heat of a substance is the quantity of heat energy that is required to change 1kg mass of the substance from one state to another at constant temperature.
Specific Latent Heat Of Fusion of Ice Lice :
Specific latent heat of fusion of ice is the quantity of heat energy that is required to change 1kg mass of ice block into liquid at constant temperature Or melting point of the ice block. It is a scalar quantity.
Specific Latent Heat Of Fusion of Ice L ice = 336 J/g
Formula Of Specific Latent Heat Of Fusion Of Ice:
Quantity of Heat energy Q = mass of ice block * specific latent heat of ice block
Q = m*Lice
Specific latent beat of ice Lice = Q/m
Worked Examples:
I. Calculate the quantity of heat energy that is required to change 25.6 kg mass of ice to liquid at constant temperature.
Solution:
Data given in the question:
Mass of ice block m = 25.6 kg = 25.6 * 1000 = 25600g, Lice = 336J/g, Q = ?
Formula: Q = m*Lice
Substitution: Q = 25600*336 ➡ Q = 8601600 Joules
II. The quantity of heat that is required to change X g of ice to liquid is 235 J. If the Lice is 336J/g, calculate the mass of the object.
Solution:
Data given in the question:
Mass = ?, Q = 235 J, Lice = 336J/g
Formula: Q = m*Lice
Substitution: 235 = m* 336
Make m the subject: m = 235/336 ➡. M = 0.699 g
Experiment To Determine Latent Heat Of Fusion Of Ice:
Aim:
To determine the latent heat of fusion of ice.
Apparatus:
Beaker, funnel, heater, ice block,
Setup Diagram:
Procedures:
Insert the heater into ice block contained in a funnel. Switch on the heater so that current flow for some times t. Adjust the rheostat so that steady current flow.
Collect the melted ice in a beaker and measure its mass.
Data:
Current that flow in the heater = I
Voltage supplied = V
Time for which current flow = t
Resistance of the heater = R
Mass of melted ice = Mice
Specific latent heat of fusion of ice = L
Theory Of Calculation:
Quantity of Heat Supplied by heater = Quantity of Heat Gained by melted ice
Formula of Calculation:
I²Rt = M*Lice
Precautions:
1. Use dry ice only
2. Add small quantity of ice at the a time
3. Stir the mixture gently and continuously for equal temperature.
4. Lag the calorimeter to prevent heat loss.
5. The amount of temperature below and above room temperature for the final and initial temperature should be the same.
Effect Of Expansion And Contraction On Fusion:
Glass bottle of water crack on freezing because water expands on freezing and because of unequal expansion of the bottle.
When water freezes into ice, it expands, become less dense and float on water with nine - tenth (9/10 ) of its volume submerged in water. Ice contracts on melting.
Effect Of Pressure on Freezing Point of a Substance:
I. Increase in pressure lower the melting point of ice or the freezing point of water. This is called regelation of ice.
II. An increase in pressure lowers the freezing point of any liquid which expands on solidifying. E.g water.
III. Increase in pressure increases the freezing point of any substance which contract on solidifying. Eg paraffin-wax.
Regelation of Ice:
When a wire to whose ends two weights are tied is hung on ice block, the pressure of the wire on the ice block, due to the weights, reduces the melting point of the ice block and the ice block starts to melt as the wire cuts through the ice. When the pressure is removed, the freezing point of the melted ice block increases and the water refreezes again. On refreezing, it gives out latent heat which is conducted by the wire to melt more ice underneath it. The process is repeated till the wire passes through the ice block without cutting it into two.
When two ice blocks are pressed together, they stick together. The pressure between the contact of the ice blocks caused the ice block to melt. When the pressure is removed, the melt ice refreeze and the two ice blocks join together.
Definition of Regelation Of Ice block:
Regelation of ice is the process whereby when ice block is subjected to high pressure, its melting point increases and the ice block melts, but when the pressure is removed, its freezing point increase and the ice refreezes.
Effect Of Impurities On Freezing Point Of Substance
The presence of impurities in a substance lower the melting point of pure substance
Worked Examples:
I. An electric heater rated 30W, 220V is used to
Specific Latent Heat Of Vaporization Of
Formula Of Specific Latent Heat Of Vapourization Of Steam (Lsteam):
Specific latent heat of vaporization of steam is the quantity of heat energy that is required to change 1kg mass of a liquid into steam / gas at constant temperature ( boiling point) and pressure. It is a scalar quantity. The unit of specific latent heat of vaporization of steam is J/Kg or J/g.
Relationship Between Specific Latent Heat Of Fusion Of Ice Lice and Specific Latent Heat Of Vaporization Of Steam Lsteam:
Specific latent heat of fusion of ice Lice = 1/7 of specific latent heat of vaporization of steam
L ice = 1/7 * Lsteam
Worked Examples:
I. If the latent heat of fusion of ice is 336J/g, what is e specific latent heat vaporization of steam?
Solution:
Data given in the question:
Specific latent heat of fusion of ice Lice = 336j/g. Lsteam = ?
Formula:
specific latent heavy of fusion of ice Lice = 1 /7 Specific latent heat of steam
Lice = 1/7 of Lsteam
Substitution: 336 = 1/7 of Lsteam
Make Lsteam the subject: Lsteam = 7*336 ➡ Lsteam = 2352J/g
II. What is the specific latent heat of fusion of ice if that of steam is 2352J/g?
Solution:
Data given in the question:
Specific latent heat of vaporization of steam Lsteam = 2352J/g, Lice = ?
Formula : Lice = 1/7 of L steam
Substitution: L ice = 1/7 * 2352. ➡ L ice = 336 J/g
Graph Of Temperature Against Time Of Change Of State Of Ice At 0°C To Steam At 100°C:
Worked Examples:
Exercises:
1. Explain in brief two reasons, two ways by which a quantity of liquid can be made to evaporate quickly.
2. What is meant by the specific latent heat of fusion?
3. Explain specific heat capacity andbe a method of finding the latent heat of steam.
4. Define specific latent heat of ice. Explain the effect n terms of kinetic energy of the molecules.
5. A copper calorimeter of mass 120g contains 70g of water and 10g ice at 0°C. What mass of steam at 100°C must be passed into the calorimeter to raise the temperature by 40°C? ( Cw = 4.2j/kg, Lice = 336j/g, Lsteam = 2260j/g, Cw = 0.4j/kg )
6. A piece of copper of mass 1kg at a temperature of 90°C is quickly transferred into a vessel of negligible thermal capacity containing 40g of water at 20°C. If the final temperature is 60°C, what mass of water will boil away? ( Cc = 0.4j/kg, Lsteam = 226j/g )
7. Differentiate between specific heat capacity and specific latent heat of vaporization. What is specific latent heat of fusion?
8. Explain what is meant by the statement “ the specific latent heat of fusion of ice is 3.4*105 J/kgK?” Describe an experiment to determine the specific latent heat of fusion of ice. State two precautions necessary to obtain accurate result. Using the kinetic theory of matter, explain why ice can change to water at 0°C without any change in temperature.
9. An electric heater is used to raised the temperature of water from 50°C to 100°C in 5minutes. After 20minutes, it was noticed that some water has boiled away. Assuming there was no heat loss, calculate the specific latent heat of vaporization of the water.
10. 654g of water at a temperature of 15°C is completely converted into ice at 0°C y removing 1800J of heat from it. If Cw = 4200J/kgk, calculate the specific latent heat of fusion of the ice.
11. What is the heat required to vaporize 67g of water initially at 96°C. If Cw = 4.2 J/gk, ( specific latent heat of vaporization of water is 2260J ).
12. 2A of current which passes through 8 ohms resistor for 25 seconds generate heat which is used to evaporate 7g of water at its boiling point. What is the specific latent heat of the liquid?
13. Heat is supplied at a steady rate of 800J per minute to 100g of solid substance in an insulated container. The temperature of he substance first rise steadily from -5°C to 30°C in 4 minutes. It again rises steadily to 115°C at a rate of 6 degrees per minute, during which the mass decrease by 2g. Use the information to sketch the temperature variation of the substance, its (ii) specific heat capacity, (iii) its specific latent heat of fusion (iv) specific latent heat of vaporization.
14. A heating coil of resistance 20ohms connected to a 220V source is used to boil a certain quantity of water in a container of heat capacity 100J/kg for 2 minutes. If the initial temperature of the water is 40°C, calculate the mass of water in the calorimeter. ( Cw = 4200J/kgk, assume b.p = 100°C )
15. A 90W immersion heater is used to supply energy for 5 minutes. The energy supplied is used to completely melt 160g of solid at it melting point. Calculate the specific latent heat of fusion of the solid.
16. How much heat is required to convert 20g of ice at 0°C to water at the same temperature? ( L ice = 336j/g )
17. What is the difference of the amount of heat given out of 4kg steam and 4kg of water when both are cooled from 100°C to 80°C? ( L steam = 2.26*106 j/kg, C w = 4200j/kgk )
18. What amount of current would pass through a 100W coil if it takes 21 seconds for the coil to just melt a lumps of ice of mass 10g at 0°C if there is no heat loss? ( L ice = 336j/g )
19. Calculate the heat required to convert 20g of ice at 0°C to water at 16°C ( C w = 4200j/kgk, L ice = 336j/g )
20. If an electric kettle 1500W boils away 300 g of a liquid at its boiling point in 300sec, calculate the specific latent heat of vaporization of the liquid.
21. 85% of the heat supplied to 40g block of ice at 0°C completely s into water at 0°C. Calculate the total heat energy supplied. ( L ice = 336j/g )
22. What is the quantity of heat released when 100g of steam at 100°C condenses to water ( L vaporization = 2.3*106 j/kg )
23. A cup containing 120 g of pure water at 30°C is placed in a refrigerator. If the refrigerator extract heat at the rate of 840J per minute, calculate the time taken for the water to freeze. ( neglect heat capacity of the material of he cup, C w = 4200j/kgk, L ice = 336j/g )
24. Calculate the heat energy required to change 100g of ice at 0°C to water boiling at 100°C. ( L ice = 336j/g, Cw = 4200 j/gk )
25. How much heat is required to convert 60g of ice at 0°C to water at its boiling point? ( Lice = 336j/g, Cw = 4200j/kgk )
When a substance is heated, the temperature of the substance increases gradually until it reaches its melting point where the substance start to melt. As the substance start to melt, the temperature of the substance remains constant until the melting is completed. During this time that the object start to melt to the time that the melting is completed, The thermometer thermometer will not indicate any increase in temperature. The heat that is supplied from the time the object stated to melt to the time the melting is completed, is used to overcome the intermolecular force that binds the molecules of the substance together and make the substance change from solid state to liquid state.
Also, when a liquid is heated, the temperature of the liquid gradually increases until it reaches the boiling point of the liquid, at which the liquid starts to boil. This temperature at which the liquid start to boil is called saturation temperature. During this time that the liquid is boiling, the temperature of the boiling water remain constant at 100°C, though more heat is added. The water changes to vapour and escape to the atmosphere. The thermometer did not read any increase in temperature because the additional heat is hidden. The additional heat that is applied to the boiling water is used to break the intermolecular force of attraction between the water molecules and make the water changed to vapour / gas.
Definition Of Latent Heat:
Latent heat is the quantity of heat energy that is required to change a substance from one state to another at constant temperature and pressure. The unit of latent heat is Joule (J), and the symbol used for latent heat is Q.
Formula Of Latent Heat Capacity:
The quantity of heat that is required to change a substance from one state to another is directly proportional to the mass of the substance.
Mathematically:
Quantity of heat energy ∞ mass of substance
Q ∞ m
Introducing the constant of proportionality: Q = L*m
L is the constant of proportionality. It is called the specific latent heat of the substance.
M is the mass of the substance measured in kilogram.
Q is the quantity of heat measured in Joule.
Factors That Determine The Quantity Of Heat Required To Change The State Of A Substance:
The following factors determine the quantity of heat energy that is required to change a substance from one state to another:
I. The nature of the substance.
II. The mass of the substance.
III. The quantity of energy supplied.
IV. The volume of the substance
V. The area of the substance
VI. The state of the substance as the time the heat energy is supplied.
Specific Latent Heat Of A Substance:
Specific latent heat of a substance is the quantity of heat energy that is required to change 1kg mass of the substance from one state to another at constant temperature.
Specific Latent Heat Of Fusion of Ice Lice :
Specific latent heat of fusion of ice is the quantity of heat energy that is required to change 1kg mass of ice block into liquid at constant temperature Or melting point of the ice block. It is a scalar quantity.
Specific Latent Heat Of Fusion of Ice L ice = 336 J/g
Formula Of Specific Latent Heat Of Fusion Of Ice:
Quantity of Heat energy Q = mass of ice block * specific latent heat of ice block
Q = m*Lice
Specific latent beat of ice Lice = Q/m
Worked Examples:
I. Calculate the quantity of heat energy that is required to change 25.6 kg mass of ice to liquid at constant temperature.
Solution:
Data given in the question:
Mass of ice block m = 25.6 kg = 25.6 * 1000 = 25600g, Lice = 336J/g, Q = ?
Formula: Q = m*Lice
Substitution: Q = 25600*336 ➡ Q = 8601600 Joules
II. The quantity of heat that is required to change X g of ice to liquid is 235 J. If the Lice is 336J/g, calculate the mass of the object.
Solution:
Data given in the question:
Mass = ?, Q = 235 J, Lice = 336J/g
Formula: Q = m*Lice
Substitution: 235 = m* 336
Make m the subject: m = 235/336 ➡. M = 0.699 g
Experiment To Determine Latent Heat Of Fusion Of Ice:
Aim:
To determine the latent heat of fusion of ice.
Apparatus:
Beaker, funnel, heater, ice block,
Setup Diagram:
Procedures:
Insert the heater into ice block contained in a funnel. Switch on the heater so that current flow for some times t. Adjust the rheostat so that steady current flow.
Collect the melted ice in a beaker and measure its mass.
Data:
Current that flow in the heater = I
Voltage supplied = V
Time for which current flow = t
Resistance of the heater = R
Mass of melted ice = Mice
Specific latent heat of fusion of ice = L
Theory Of Calculation:
Quantity of Heat Supplied by heater = Quantity of Heat Gained by melted ice
Formula of Calculation:
I²Rt = M*Lice
Precautions:
1. Use dry ice only
2. Add small quantity of ice at the a time
3. Stir the mixture gently and continuously for equal temperature.
4. Lag the calorimeter to prevent heat loss.
5. The amount of temperature below and above room temperature for the final and initial temperature should be the same.
Effect Of Expansion And Contraction On Fusion:
Glass bottle of water crack on freezing because water expands on freezing and because of unequal expansion of the bottle.
When water freezes into ice, it expands, become less dense and float on water with nine - tenth (9/10 ) of its volume submerged in water. Ice contracts on melting.
Effect Of Pressure on Freezing Point of a Substance:
I. Increase in pressure lower the melting point of ice or the freezing point of water. This is called regelation of ice.
II. An increase in pressure lowers the freezing point of any liquid which expands on solidifying. E.g water.
III. Increase in pressure increases the freezing point of any substance which contract on solidifying. Eg paraffin-wax.
Regelation of Ice:
When a wire to whose ends two weights are tied is hung on ice block, the pressure of the wire on the ice block, due to the weights, reduces the melting point of the ice block and the ice block starts to melt as the wire cuts through the ice. When the pressure is removed, the freezing point of the melted ice block increases and the water refreezes again. On refreezing, it gives out latent heat which is conducted by the wire to melt more ice underneath it. The process is repeated till the wire passes through the ice block without cutting it into two.
When two ice blocks are pressed together, they stick together. The pressure between the contact of the ice blocks caused the ice block to melt. When the pressure is removed, the melt ice refreeze and the two ice blocks join together.
Definition of Regelation Of Ice block:
Regelation of ice is the process whereby when ice block is subjected to high pressure, its melting point increases and the ice block melts, but when the pressure is removed, its freezing point increase and the ice refreezes.
Effect Of Impurities On Freezing Point Of Substance
The presence of impurities in a substance lower the melting point of pure substance
Worked Examples:
I. An electric heater rated 30W, 220V is used to
Specific Latent Heat Of Vaporization Of
Formula Of Specific Latent Heat Of Vapourization Of Steam (Lsteam):
Specific latent heat of vaporization of steam is the quantity of heat energy that is required to change 1kg mass of a liquid into steam / gas at constant temperature ( boiling point) and pressure. It is a scalar quantity. The unit of specific latent heat of vaporization of steam is J/Kg or J/g.
Relationship Between Specific Latent Heat Of Fusion Of Ice Lice and Specific Latent Heat Of Vaporization Of Steam Lsteam:
Specific latent heat of fusion of ice Lice = 1/7 of specific latent heat of vaporization of steam
L ice = 1/7 * Lsteam
Worked Examples:
I. If the latent heat of fusion of ice is 336J/g, what is e specific latent heat vaporization of steam?
Solution:
Data given in the question:
Specific latent heat of fusion of ice Lice = 336j/g. Lsteam = ?
Formula:
specific latent heavy of fusion of ice Lice = 1 /7 Specific latent heat of steam
Lice = 1/7 of Lsteam
Substitution: 336 = 1/7 of Lsteam
Make Lsteam the subject: Lsteam = 7*336 ➡ Lsteam = 2352J/g
II. What is the specific latent heat of fusion of ice if that of steam is 2352J/g?
Solution:
Data given in the question:
Specific latent heat of vaporization of steam Lsteam = 2352J/g, Lice = ?
Formula : Lice = 1/7 of L steam
Substitution: L ice = 1/7 * 2352. ➡ L ice = 336 J/g
Graph Of Temperature Against Time Of Change Of State Of Ice At 0°C To Steam At 100°C:
Worked Examples:
Exercises:
1. Explain in brief two reasons, two ways by which a quantity of liquid can be made to evaporate quickly.
2. What is meant by the specific latent heat of fusion?
3. Explain specific heat capacity andbe a method of finding the latent heat of steam.
4. Define specific latent heat of ice. Explain the effect n terms of kinetic energy of the molecules.
5. A copper calorimeter of mass 120g contains 70g of water and 10g ice at 0°C. What mass of steam at 100°C must be passed into the calorimeter to raise the temperature by 40°C? ( Cw = 4.2j/kg, Lice = 336j/g, Lsteam = 2260j/g, Cw = 0.4j/kg )
6. A piece of copper of mass 1kg at a temperature of 90°C is quickly transferred into a vessel of negligible thermal capacity containing 40g of water at 20°C. If the final temperature is 60°C, what mass of water will boil away? ( Cc = 0.4j/kg, Lsteam = 226j/g )
7. Differentiate between specific heat capacity and specific latent heat of vaporization. What is specific latent heat of fusion?
8. Explain what is meant by the statement “ the specific latent heat of fusion of ice is 3.4*105 J/kgK?” Describe an experiment to determine the specific latent heat of fusion of ice. State two precautions necessary to obtain accurate result. Using the kinetic theory of matter, explain why ice can change to water at 0°C without any change in temperature.
9. An electric heater is used to raised the temperature of water from 50°C to 100°C in 5minutes. After 20minutes, it was noticed that some water has boiled away. Assuming there was no heat loss, calculate the specific latent heat of vaporization of the water.
10. 654g of water at a temperature of 15°C is completely converted into ice at 0°C y removing 1800J of heat from it. If Cw = 4200J/kgk, calculate the specific latent heat of fusion of the ice.
11. What is the heat required to vaporize 67g of water initially at 96°C. If Cw = 4.2 J/gk, ( specific latent heat of vaporization of water is 2260J ).
12. 2A of current which passes through 8 ohms resistor for 25 seconds generate heat which is used to evaporate 7g of water at its boiling point. What is the specific latent heat of the liquid?
13. Heat is supplied at a steady rate of 800J per minute to 100g of solid substance in an insulated container. The temperature of he substance first rise steadily from -5°C to 30°C in 4 minutes. It again rises steadily to 115°C at a rate of 6 degrees per minute, during which the mass decrease by 2g. Use the information to sketch the temperature variation of the substance, its (ii) specific heat capacity, (iii) its specific latent heat of fusion (iv) specific latent heat of vaporization.
14. A heating coil of resistance 20ohms connected to a 220V source is used to boil a certain quantity of water in a container of heat capacity 100J/kg for 2 minutes. If the initial temperature of the water is 40°C, calculate the mass of water in the calorimeter. ( Cw = 4200J/kgk, assume b.p = 100°C )
15. A 90W immersion heater is used to supply energy for 5 minutes. The energy supplied is used to completely melt 160g of solid at it melting point. Calculate the specific latent heat of fusion of the solid.
16. How much heat is required to convert 20g of ice at 0°C to water at the same temperature? ( L ice = 336j/g )
17. What is the difference of the amount of heat given out of 4kg steam and 4kg of water when both are cooled from 100°C to 80°C? ( L steam = 2.26*106 j/kg, C w = 4200j/kgk )
18. What amount of current would pass through a 100W coil if it takes 21 seconds for the coil to just melt a lumps of ice of mass 10g at 0°C if there is no heat loss? ( L ice = 336j/g )
19. Calculate the heat required to convert 20g of ice at 0°C to water at 16°C ( C w = 4200j/kgk, L ice = 336j/g )
20. If an electric kettle 1500W boils away 300 g of a liquid at its boiling point in 300sec, calculate the specific latent heat of vaporization of the liquid.
21. 85% of the heat supplied to 40g block of ice at 0°C completely s into water at 0°C. Calculate the total heat energy supplied. ( L ice = 336j/g )
22. What is the quantity of heat released when 100g of steam at 100°C condenses to water ( L vaporization = 2.3*106 j/kg )
23. A cup containing 120 g of pure water at 30°C is placed in a refrigerator. If the refrigerator extract heat at the rate of 840J per minute, calculate the time taken for the water to freeze. ( neglect heat capacity of the material of he cup, C w = 4200j/kgk, L ice = 336j/g )
24. Calculate the heat energy required to change 100g of ice at 0°C to water boiling at 100°C. ( L ice = 336j/g, Cw = 4200 j/gk )
25. How much heat is required to convert 60g of ice at 0°C to water at its boiling point? ( Lice = 336j/g, Cw = 4200j/kgk )
Keep up the good work.
ReplyDeleteKindly post the solution to the 25 questions some are not clear pls
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