Heat capacity or thermal capacity is an extensive property of matter, that defines its physical property. Heat Capacity is the amount of heat that must be applied to an object in order to cause a unit change in temperature. Heat capacity is measured in Joules per Kelvin (J/K), which is its SI unit. When heat capacity is divided by the mass of the substance, gives the corresponding intensive property called Specific Heat Capacity. Moreover, heat capacity divided by the amount of substance in volumes gives Molar Heat Capacity. Hence, in this article, we'll understand the important concepts of Heat Capacity like its definition, explanation, formula, unit, specific heat capacity, molar heat capacity
What is Heat Capacity?
The amount of heat required to increase the temperature of a given mass of a substance by one unit without causing a phase change is known as the heat capacity (C) or thermal capacity. It describes a substance's ability to absorb thermal energy.
Being an extensive property, heat capacity is based on the sample's mass and size. This suggests that, for a sample containing twice as much material as another sample, twice as much heat energy (Q) would be needed to produce the same change in temperature.
The heat capacity of various substances will determine how much their temperatures will rise. On a hot day, a metal chair left in the direct sun may get rather warm to the touch whereas equal amounts of water won't heat up nearly as much when exposed to the same amount of sunlight. This indicates that water has a high heat capacity. Metals typically do not react well to temperature fluctuations while water does.
Heat Capacity Formula
Mathematically, Heat Capacity for the unit mass of the substance is defined as,
C = ΔQ / ΔT
where
ΔQ is the amount of heat supplied to the substance
ΔT is the change in temperature
SI Unit of Heat Capacity
The SI unit for the heat capacity of a substance is Joule per Kelvin (J/K or J⋅K−1). When the temperature is increased by 1°C is the same as an increase of 1 K, which is the same unit as J/°C.
Dimension of Heat capacity is [L2 M T−2 Θ−1]
Specific Heat Capacity
Specific heat capacity is defined as the number of heat changes i.e. heat absorbed or rejected by a substance per unit mass in order to change its temperature by one unit.
When the same quantity of heat is applied to the same mass of different substances, the resulting temperature changes are not the same. It indicates that each substance has a distinct value for the quantity of heat absorbed or rejected to change the temperature of its unit mass by one unit. The specific heat capacity of a substance is the measurement of this quantity. It is symbolized by the letter s.
If C is the amount of heat absorbed or rejected i.e. ΔQ by a substance of mass m when it experiences a temperature change ΔT, then the specific heat capacity, of that substance is given by
S = C/m = (1/m) (ΔQ / ΔT)
Hence, the specific heat capacity is the property of the substance which determines the change in temperature of the given quantity of a substance which is the amount of heat absorbed or rejected. The given substance is undergoing no phase change during temperature change. It is dependent on the substance's nature and temperature. J kg–1 K–1 is the SI unit for specific heat capacity.
Molar Heat Capacity
The amount of heat required to raise the temperature of one mole of any substance by one degree Kelvin is called the Molar Heat Capacity. Molar heat capacity and specific heat capacity are almost the same and the basic difference between them is that specific heat capacity is measured per gram whereas molar heat capacity is measured per mole.
Unit: Molar Heat Capacity is measured in Joule/mole.
The formula for finding molar heat capacity is,
Q = nC∆T
where,
q is the heat supplied
n is the moles of the substance
C is the molar heat capacity of the body
Molar Heat Capacity at Constant Pressure Cp
The amount of heat required to raise the temperature of one mole of any substance by one degree Kelvin at constant pressure is called the Molar Heat Capacity at Constant Pressure and it is denoted by Cp.
The formula for calculating Molar Heat Capacity at Constant Pressure Cp is,
Cp = dH/dt (at constant pressure)
where,
Cp represents the specific heat at Constant Pressure
dH is the Change in Enthalpy
dt is the Change in Temperature
Molar Heat Capacity at Constant Volume Cv
The amount of heat required to raise the temperature of one mole of any substance by one degree Kelvin at constant volume is called the Molar Heat Capacity at Constant Volume and it is denoted by Cv.
The formula for calculating Molar Heat Capacity at Constant Pressure Cv is,
Cv = dH/dt (at constant volume)
where,
Cv represents the specific heat at Constant Pressure
dH is the Change in Enthalpy
dt is the Change in Temperature
Heat Capacity Ratio
The ratio between heat capacity at constant pressure (Cp) and heat capacity at constant volume (Cv) is called the Heat Capacity Ratio.
It is denoted by the Greek symbol γ.
γ = Cp / Cv
Relation between Cp and Cv
Cp and Cv are the specific heats of an ideal gas at constant pressure and at constant volume respectively. They indicate the amount of heat required to raise the temperature of unit mass by one degree Kelvin at constant pressure and at constant volume. The relation between Cp and Cv is given as,
Cp - Cv = R
where R is universal Gas Constant
Also, Check
Solved Examples on Heat Capacity
Example 1: A 88.3 g sample of metal at 95.24 C is added to 35.10 g of water that is initially at 17.27 °C. The final temperature of both the water and the metal is 29.20 °C. The specific heat of water is 4.184 J/(g°C). Calculate the specific heat of the metal.
Solution:
Given,
Mass of metal is 88.3 g
The initial temperature of the metal is 95.24 °C.
Mass of water: 35.10 g.
The initial temperature of the water is 17.27 °C.
The final temperature of the water and the metal is 29.20 °C.
The specific heat of water is 4.184 J/(g°C).
Therefore, the expression where the energy from the hotter metal transfers to the cooler water is
−moCoΔTo = mwCwΔTw
where
mo = mass of a metal object
ΔTo = temperature change of metal object
Co = specific heat capacity of metal object
mw = mass of water
ΔTw = temperature change of water
Cw = specific heat capacity of water
Rearrange the above expression,
Co = (mwCwΔTw)/(moΔTo)
Substitute the values in the above expression,
Co = [35.10 4.184(29.20−17.27)]/[88.3(29.20-95.24)]
= 0.301 J/g°C
Example 2: A 30.5 g sample of an alloy at 93.0 °C is placed into 50.0 g of water at 22.0 °C in an insulated coffee cup with a heat capacity of 9.2 J/K. If the final temperature of the system is 31.1 °C, what is the specific heat capacity of the alloy?
Solution:
Heat absorbed = heat lost
then the specific heat capacity, of that substance is given by
s = (1/m)(ΔQ/ ΔT)
Rearrange the above expression,
ΔQ=smΔT
ΔQalloy = ΔQwater+ΔQcup
Temperature of the water is equal to the temperature of the cup = 22.0 °C.
Temperature of the alloy is 93.0 °C.
Final Temperature is 31.1 °C.
30.5×(93.0 - 31.1)s = 9.2×(31.1-22.0) + 50.0×4.2×(31.1-22.0)
1887.95×s = 1994.72
s = 1.057 J/gK
Example 3: The specific heat of water is 4.18 J/(g°C). Calculate the molar heat capacity of water. Express your answer to three significant figures and include the appropriate units.
Solution:
Specific Heat of water is 4.18 J/(g°C).
Expression to convert gram into mole is
4.18 J/gC x (18.0 g / mole) = 75.24 J/mole C
Hence, the molar heat capacity of water is 75.24 J/mole C
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