Some Trends In Entropy Values
A close inspection of the entropyA thermodynamic state function, symbol S, that equals the reversible heat energy transfer divided by temperature; higher entropy corresponds to greater dispersal of energy on the molecular scale. See also standard entropy. values in the Table of Molar Entropies reveals several trends which can be explained in terms of the factors of molecular mass and restriction of movement.
|Compound||Smo /J K-1mol-1||Compound||Smo /J K-1mol-1|
|Liquids||Polyatomic Gases( > 3)|
Solids, Liquids, and Gases Perhaps the most obvious feature of the table of molecular entropies is a general increase in the molar entropy as we move from solids to liquids to gases. In a solid, the molecules are only capable of restricted vibrations around a fixed point, but when a solid melts, the molecules, though still hampered by their mutual attraction, are much freer to move around. Thus when a solid melts, the molar entropy of the substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. increases. When a liquid vaporizes, the restrictions on the molecules’ ability to move around are relaxed almost completely and a further and larger increase in the entropy occurs. When 1 mol of ice melts, for example, its entropy increases by 22 J K–1, while on boilingThe process of a liquid becoming vapor in which bubbles of vapor form beneath the surface of the liquid; at the boiling temperature the vapor pressure of the liquid equals the pressure of the gas in contact with the liquid. the entropy increase is 110 J K–1.
Molecular Complexity A second clear trend in the table is the higher molar entropy of substances with more complex molecules. To some extent this is due to the mass since on the whole more complex molecules are heavier than simpler ones. However, we still find an increase of entropy with complexity when we compare molecules of very similar masses:
|Substance|| || || || |
|Sm°/J K–1 mol–1|| || || || |
|Molar mass/g mol–1|| || || || |
|Number of atoms|| || || || |
The more atoms there are in a molecule, the more ways the molecule can change its shape by vibrating. In consequence there are more ways in which the energyA system's capacity to do work. can be distributed among the molecules.
Strength of Bonding Another trend in entropy, most noticeable in the case of solids, is the decrease in the entropy as the forces between the atoms, molecules, or ions increases. A good example is the three solid compounds
|Substance|| || || |
|Sm°(298 K)/J K–1 mol–1|| || || |
|Molar mass/g mol–1|| || || |
which are isoelectronicHaving the same number of electrons. Sometimes used to describe species that have the same number of valence electrons. with sodium fluoride. Since there is very little difference in the molar masses, the entropy decrease can only be attributed to the increase in the coulombic attraction between the ions as we move from the singly charged ions Na+ and F– through the doubly charged ions Mg2+ and O2–, to the triply charged ions A13+ and N3–. (While it is true that there is a fair degree of covalent character to the bonding in AIN, the effect of this will be to increase the strength of the bonding.)
EXAMPLE 1 From each of the following pairs of compounds choose the one with the higher standard molar entropy at 25°C. Give brief reasons for your choice.
a) HBr(g), HCl(g) c) ND3(g), Ne(g) e) C2H6(g), C2H4(g)
b) Cs(s), Cs(l) d) KCl(s), CaS(s)
SolutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture.
a) HBr and HCl are very similar except for their mass. HBr will have a higher entropy because of its greater mass.
b) At the same temperatureA physical property that indicates whether one object can transfer thermal energy to another object., the liquid form of a substance always has a higher entropy than the solid.
c) ND3 (D = deuteriumThe isotope of hydrogen having one neutron in its nucleus.) and Ne have almost identical molar masses (20 g mol–1) However, since ND3 is more complex, it can vibrate and rotate while Ne cannot. ND3 will have the higher entropy.
d) KCl and CaS are isoelectronic. Because both anionA negatively charged ion. An ion that is attracted toward the anode in an electrolytic cell. and cationA positively charged ion, attracted toward the cathode in an electrolytic cell. are doubly charged in CaS, the ions are more tightly held to each other and can vibrate less readily. Thus KCl must have the higher entropy.
e) On all counts C2H6 must have a higher entropy than C2H4. C2H6 is heavier and more complex than C2H4. In addition there is free rotation about the C—C bond in C2H6 but hindered rotation about the bond in C2H4.