Latent Heat and Specific Latent Heat: Definition, Types and Formula
Latent Heat and Specific Latent Heat: Let us do an experiment to understand Latent Heat and Specific Latent Heat. Start heating ice at \( – {4^ \circ }{\text{C}},\) the temperature of the ice will rise up to \({0^ \circ }{\text{C}}{\text{.}}\) At \({0^ \circ }{\text{C}}{\text{,}}\) the temperature of the ice will not rise even if we supply the heat energy. After some time, the ice will melt and convert into water.
When all the ice is converted into water, the temperature will again start increasing. When the water reaches \({100^ \circ }{\text{C,}}\,\) the temperature of the water will again stop rising. When we further heat the water, the steam will start forming. When all the water is converted into steam, the temperature of the steam will again start rising with the heat supply.
From the above experiment, we can say that heating has two effects. The first effect is the change in temperature, and another effect is the change in the state of the material. Supply of heat due to which temperature changes is called sensible heating. The rate of change in temperature depends on the specific heat of the material.
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What is Latent Heat?
In the above experiment, at \({0^ \circ }{\text{C}}\) and \({100^ \circ }{\text{C,}}\,\) the temperature was not changing even after the further supply of heat. At \({0^ \circ }{\text{C}}{\text{,}}\) the ice starts melting, and at \({100^ \circ }{\text{C,}}\,\) the water starts vapourizing. In both, the case, change of state is happening. During the change in state, the intermolecular bonds will break and consumes energy. So, we can say that all the heat supplied at \({0^ \circ }{\text{C}}\) and \({100^ \circ }{\text{C}}\) is utilized in breaking intermolecular bonds. The opposite happens in the case of the reverse process (cooling). The energy is released duringzing at \({0^ \circ }{\text{C}}\) and condensing at \({100^ \circ }{\text{C}}\) due to the formation of new intermolecular bonds. The amount of heat transferred during the change in the state of the given mass \(\left({\text{m}} \right)\) of substance at any specific pressure is called latent heat \(\left( {Q_ {\mathbf{L}}}\right).\) Its SI unit is \({\text{J}}.\)
The amount of heat transferred per unit mass during the change in the state of the substance or latent heat per unit mass is called Specific Latent Heat. Its SI unit is \({\text{J}}\,{\text{k}}{{\text{g}}^{ – 1}}.\)
Specific Latent Heat \(\left( L \right) = \frac{Q_{\mathbf{L}}}{m}\)
Types of Specific Latent Heat
There are three types of specific latent heat. These are as follows,
1. Specific Latent Heat of Fusion:- The amount of heat transfer during melting of \(1\,{\text{kg}}\) solid object to into liquid orzing of liquid into solid without any further increase or decrease in the temperature is known as latent heat of fusion. For example, the specific latent heat of fusion of water is \(333.5\,{\text{kJ}}\,{\text{k}}{{\text{g}}^{ – 1}}.\) This means, when \(1\,{\text{kg}}\) of water willze at \({0^ \circ }{\text{C}},\) it will release \(333.5\,{\text{kJ}}\,.\) Similarly, when \(1\,{\text{kg}}\) of ice melts at \({0^ \circ }{\text{C}},\) it will observe \(333.5\,{\text{kJ}}\,\) heat.
2. Specific Latent Heat of Vapourization:- It is defined as the heat required to change one kilogram of liquid into vapour at its boiling point under standard atmospheric pressure. It is expressed as \({\text{kJ}}/{\text{kg}}.\) The same amount of heat energy is released during the condensation of \({\text{1}}\,{\text{kg}}\) vapour. For example, the specific latent heat of vapourization of water at \({\text{1}}\) atm and \({\text{10}}{{\text{0}}^ \circ }{\text{C}}\) is approximately \(2257\,{\text{kJ}}\,{\text{k}}{{\text{g}}^{ – 1}}.\) This means, when \({\text{1}}\,{\text{kg}}\) of water will vapourize at \({\text{1}}\) atm and \({\text{10}}{{\text{0}}^ \circ }{\text{C,}}\) it will observe \(2257\,{\text{kJ}}{\text{.}}\) Similarly, when \({\text{1}}\,{\text{kg}}\) vapour melts at \({\text{1}}\) atm and \({100^ \circ }{\text{C}},\) it will release \(333.5\,{\text{kJ}}\) heat.
Let us plot the temperature versus supplied heat for a given mass of ice at \( – {10^ \circ }{\text{C}}.\) When the ice reached \({0^ \circ }{\text{C, 334}}\,{\text{J}}\) heat is added per gram without changing the temperature. It is equal to the specific latent heat of the fusion of ice. Again when the water is reached at \({100^ \circ }{\text{C,}}\,{\text{2257}}\,{\text{J}}\) heat is added per gram without changing the temperature. It is equal to the specific heat of vapourization of water.
3.Specificlatent Heat of Sublimation:- It is defined as the quantity of heat observed by the material to convert from solid to vapour directly without melting and boiling.
Importance of Latent Heat in our Daily Life
Ice is used to store the food items such as fish, meat, etc.
Due to the high latent heat of vapourization, the hot vapour is used in many processing industries to carry heat.
The latent heat of fusion of ice cream should be large enough to avoid rapid melting.
Our body maintains the temperature by sweating. Heat is lost from our skin during the evapouration of sweat.
An earthen pot is used to keep cool water during summer. Heat is lost from the outer surface of the pot during evapouration of water leaking from pores.
The air cooler cools the air by losing heat during the evapouration of water.
Summary
The heating has two effects. The first effect is the change in temperature, and another effect is the change in state. During the change of state, the temperature of the material does not change. All the supplied heat is used to break the intermolecular bonds. In case of cooling, heat is released. The amount of heat released or observed during the change in the state of the given material is called Latent heat. Its SI unit is \({\text{J}}{\text{.}}\)
Latent heat per unit mass is defined as specific Latent heat. It is expressed as \({\text{kJ/kg}}{\text{.}}\)
Specific Latent Heat \(\left( L \right) = \frac{{{Q_{\text{L}}}}}{m}.\) Here \({Q_L}\) latent heat of total mass \({m}.\)
There are two types of latent heat, these are,
Specific Latent heat of fusion:- It is the amount of heat transfer during melting of \(1\,{\text{kg}}\) solid object to into liquid orzing of liquid into solid.
Specific Latent heat of vapourization:- It is defined as the heat required to change one kilogram of liquid into vapour at its boiling point under standard atmospheric pressure.
Specific Latent heat of vaporization:- It is defined as the quantity of heat observed by the material to convert from solid to vapour directly without melting and boiling.
Sample Problems on Latent Heat and Specific Latent Heat
Q 1. Calculate the heat required to convert the \(3\,{\text{kg}}\) of ice at \( – {12^ \circ }{\text{C}}\) to steam at \({100^ \circ }{\text{C}}\) at atmospheric pressure. Given the Specific heat capacity of ice \( = 2100\,{\text{J}}\,{\text{k}}{{\text{g}}^{ – 1}}\,{{\text{K}}^{ – 1}},\) the Specific heat capacity of water \( = 4186~{\text{J}}~{\text{k}}{{\text{g}}^{ – 1}}~{{\text{K}}^{ – 1}},\) the Latent heat of fusion of Ice \( = 3.35 \times{10^5}~{\text{J}}~{\text{k}}{{\text{g}}^{ – 1}},\) the Latent heat of vapourization of water \( = 22.56 \times {10^5}~{\text{J}}~{\text{k}}{{\text{g}}^{ – 1}}.\)
Solution:-
We have,
Mass of ice, \(m = 3\,{\text{kg}},\)
Specific heat capacity of ice, \({S_{{\text{ice }}}} = 2100~{\text{J}}~{\text{k}}{{\text{g}}^{ – 1}}~{{\text{K}}^{ – 1}},\)
The Specific heat capacity of water, \({S_{{\text{water }}}} = 4186~{\text{J}}~{\text{k}}{{\text{g}}^{ – 1}}~{{\text{K}}^{ – 1}},\)
The Latent heat of fusion of Ice, \({L_{{\text{ice}}}} = 3.35 \times {10^5}~{\text{J}}~{\text{k}}{{\text{g}}^{ – 1}},\)
The Latent heat of vapourization of water, \({L_{{\text{water }}}} = 22.56 \times {10^5}~{\text{J}}~{\text{k}}{{\text{g}}^{ – 1}},\)
Now, the heat required to bring the ice at \({0^ \circ }{\text{C}},{Q_1} = m{S_{{\text{ice}}}}\Delta T.\)
\( \Rightarrow {Q_1} = 3 \times 2100 \times \left({0 – \left({ – 12} \right)} \right)\)
\(\Rightarrow {Q_1} = 75600~{\text{J}}\)
Heat requires to melt the Ice at \({0^ \circ }{\text{C}}\) to water at \({0^ \circ }{\text{C}},{Q_2} = m{L_{{\text{ice}}}}.\)
\( \Rightarrow {Q_2} = 3~{\text{kg}} \times 3.35 \times {10^5}~{\text{J}}~{\text{k}}{{\text{g}}^{ – 1}}\)
\( \Rightarrow {Q_2} = 1005000~{\text{J}}\)
Heat required to bring the water from \({0^ \circ }{\text{C}}\) to \({100^ \circ }{\text{C}},{Q_3} = m{S_{{\text{Water}}}}\Delta T.\)
\( \Rightarrow {Q_3} = 3 \times 4186 \times \left({100 – 0} \right)\)
\( \Rightarrow {Q_3} = 1255800~{\text{J}}\)
Heat requires to convert the water at \({100^ \circ }{\text{C}}\) to steam at \({100^ \circ }{\text{C}},{Q_4} = m{L_{{\text{Water}}}}.\)
\( \Rightarrow {Q_4} = 3~{\text{kg}} \times 22.56 \times {10^5}~{\text{J}}~{\text{k}}{{\text{g}}^{ – 1}}\)
\( \Rightarrow {Q_4} = 6768000~{\text{J}}\)
So, the total heat required will be equal to, \(Q ={Q_1} + {Q_2} + {Q_3} + {Q_4}.\)
\( \Rightarrow Q = 75600 + 1005000 + 1255800 + 6768000 = 9.1 \times {10^6}~{\text{J}}\)
FAQs on Latent Heat and Specific Latent Heat
Let’s look at some of the commonly asked questions about Latent Heat and Specific Latent Heat:
Q.1. Which is more dangerous, water at \({100^ \circ }{\text{C}}\) or steam at \({100^ \circ }{\text{C}}\)? Ans: When the water is converted into steam at \({100^ \circ }{\text{C,}}\) it absorbs heat called the latent heat of vapourization. So, the steam at \({100^ \circ }{\text{C}}\) will have more heat energy than water at \({100^ \circ }{\text{C.}}\) Thus, steam at is \({100^ \circ }{\text{C}}\) more dangerous than water at \({100^ \circ }{\text{C.}}\)
Q.2. What is the difference between Latent heat and Specific heat? Ans: Latent heat is the quantity of heat transfer during the change in state, whereas Specific heat is the quantity of heat required to change the temperature of unit mass by one degree. Specific heat governs the rate of change in temperature during sensible heating or cooling.
Q.3. Does the latent heat of vapourization depend on pressure? Ans: Yes, the latent heat of vapourization depends on pressure. As the pressure increases, the boiling point increases, and the heat of vapourization. As the pressure tends to critical pressure, the latent heat of vapourization will tend to zero.
Q.4. What is critical pressure? Ans: It is the pressure at which the liquid will change to vapour without boiling. At critical pressure and critical temperature, the latent heat of vapourization of liquid will be equal to zero. For water, the value of critical pressure is 218 atm, and the value of critical temperature is \(647\,{\text{K}}.\)
Q.5. What will be the value of the slope of temperature versus heat supply graph during boiling? Ans: During boiling, the state material changes from liquid to vapour. All the heat supplied during this is utilized in breaking the intermolecular bonds. There will be no sensible heating, so there will be no rise in temperature. So, the slope will be zero.
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