per hour. Calculate the per second (Ans. 42.5 ms') Problem 4. Ethyl alcohol boils at 78.5°C and freezes at -117°C, at one atmosphere pressure. What is boiling point and freezing point of ethyl alcohol in degrees kelvin ? [Ans. B.P. = 351.65 K (ii) F.P. = 156 K)
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per hour. Calculate the per second (Ans. 42.5 ms') Problem 4. Ethyl alcohol boils at 78.5°C and freezes at -117°C, at one atmosphere pressure. What is boiling point and freezing point of ethyl alcohol in degrees kelvin ? [Ans. B.P. = 351.65 K (ii) F.P. = 156 K)
Answer:
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Contents Home Bookshelves General Chemistry Map: Principles of Modern Chemistry (Oxtoby et al.) Unit 4: Equilibrium in Chemical Reactions 12: Thermodynamic Processes and Thermochemistry Expand/collapse global location
12.3: Heat Capacity, Enthalpy, and Calorimetry
Last updatedFeb 4, 2021
12.2: The First Law of Thermodynamics - Internal Energy, Work, and Heat
12.4: Illustrations of the First Law of Thermodynamics in Ideal Gas Processes
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Learning Objectives
Explain the technique of calorimetry
Calculate and interpret heat and related properties using typical calorimetry data
To use calorimetric data to calculate enthalpy changes.
Heat Capacity
We now introduce two concepts useful in describing heat flow and temperature change. The heat capacity ( C ) of a body of matter is the quantity of heat ( q ) it absorbs or releases when it experiences a temperature change ( ΔT ) of 1 degree Celsius (or equivalently, 1 kelvin)
C=qΔT(12.3.1)
Heat capacity is determined by both the type and amount of substance that absorbs or releases heat. It is therefore an extensive property—its value is proportional to the amount of the substance.
For example, consider the heat capacities of two cast iron frying pans. The heat capacity of the large pan is five times greater than that of the small pan because, although both are made of the same material, the mass of the large pan is five times greater than the mass of the small pan. More mass means more atoms are present in the larger pan, so it takes more energy to make all of those atoms vibrate faster. The heat capacity of the small cast iron frying pan is found by observing that it takes 18,140 J of energy to raise the temperature of the pan by 50.0 °C
Csmall pan=18,140J50.0°C=363J/°C
The larger cast iron frying pan, while made of the same substance, requires 90,700 J of energy to raise its temperature by 50.0 °C. The larger pan has a (proportionally) larger heat capacity because the larger amount of material requires a (proportionally) larger amount of energy to yield the same temperature change:
Clarge pan=90,700J50.0°C=1814J/°C
The specific heat capacity ( c ) of a substance, commonly called its specific heat, is the quantity of heat required to raise the temperature of 1 gram of a substance by 1 degree Celsius (or 1 kelvin):
c=qmΔT(12.3.2)
Specific heat capacity depends only on the kind of substance absorbing or releasing heat. It is an intensive property—the type, but not the amount, of the substance is all that matters. For example, the small cast iron frying pan has a mass of 808 g. The specific heat of iron (the material used to make the pan) is therefore:
ciron=18,140J(808g)(50.0°C)=0.449J/g°C
The large frying pan has a mass of 4040 g. Using the data for this pan, we can also calculate the specific heat of iron:
ciron=90,700J(4,040g)(50.0°C)=0.449J/g°C
Although the large pan is more massive than the small pan, since both are made of the same material, they both yield the same value for specific heat (for the material of construction, iron). Note that specific heat is measured in units of energy per temperature per mass and is an intensive property, being derived from a ratio of two extensive properties (heat and mass). The molar heat capacity, also an intensive property, is the heat capacity per mole of a particular substance and has units of J/mol °C (Figure 12.3.1 ).
The picture shows two black metal frying pans sitting on a flat surface. The left pan is about half the size of the right pan.