Entropy Rules - Chemistry LibreTexts
The central concepts of thermodynamics are: entropy S, temperature T, and .. the relation between thermodynamic temperature T and Celsius temperature ϑ, it is . In tables, the product Cm⋅T is usually given instead of entropy capacities. If the process occurs at constant temperature, for example if an ice cube at 0°C is melting on a table top Absolute entropy is not inversely proportional to absolute temperature. Why does entropy have an inverse relation with temperature?. A temperature vs. specific entropy diagram, or T–s diagram, is used in thermodynamics to The thermal efficiency of the cycle is the ratio of the white area (work) divided by the sum of the white and red areas (total heat). If the cycle moves in a.
Observations and Explanations in terms of Entropy changes system and process source of entropy increase of system A deck of cards is shuffled, or coins, initially heads up, are randomly tossed. The cooler block contains more unoccupied microstates, so heat flows from the warmer block until equal numbers of microstates are populated in the two blocks. A gas expands isothermally to twice its initial volume. The increased thermal energy makes additional microstates accessible. The increase is by a factor of about ,,, , Equal volumes of two gases are allowed to mix.
The effect is the same as allowing each gas to expand to twice its volume; the thermal energy in each is now spread over a larger volume. One mole of dihydrogen, H2, is placed in a container and heated to K.
The above reaction mixture is cooled to K. The composition shifts back to virtually all H2because this molecule contains more thermally accessible microstates at low temperatures. Entropy is an extensive quantity; that is, it is proportional to the quantity of matter in a system; thus g of metallic copper has twice the entropy of 50 g at the same temperature.
This makes sense because the larger piece of copper contains twice as many quantized energy levels able to contain the thermal energy. Entropy and "disorder" Entropy is still described, particularly in older textbooks, as a measure of disorder. In a narrow technical sense this is correct, since the spreading and sharing of thermal energy does have the effect of randomizing the disposition of thermal energy within a system.
Carrying these concepts over to macro systems may yield compelling analogies, but it is no longer science. Entropy and Probability The distribution of thermal energy in a system is characterized by the number of quantized microstates that are accessible i.
The more such microstates, the greater is the probability of the system being in the corresponding macrostate. But even though it is beyond human comprehension to compare numbers that seem to verge on infinity, the thermal energy contained in actual physical systems manages to discover the largest of these quantities with no difficulty at all, quickly settling in to the most probable macrostate for a given set of conditions.
If someone could make a movie showing the motions of individual atoms of a gas or of a chemical reaction system in its equilibrium state, there is no way you could determine, on watching it, whether the film is playing in the forward or reverse direction. At this level, time clearly has a direction, and it is often noted that because the entropy of the world as a whole always increases and never decreases, it is entropy that gives time its direction.
It is for this reason that entropy is sometimes referred to as "time's arrow". But there is a problem here: So we are faced with the apparent paradox that thermodynamics, which deals only with differences between states and not the journeys between them, is unable to describe the very process of change by which we are aware of the flow of time.
The direction of time is revealed to the chemist by the progress of a reaction toward its state of equilibrium; once equilibrium is reached, the net change that leads to it ceases, and from the standpoint of that particular system, the flow of time stops.
If we extend the same idea to the much larger system of the world as a whole, this leads to the concept of the "heat death of the universe" that was mentioned briefly in the previous lesson.
Absolute Entropies Energy values, as you know, are all relative, and must be defined on a scale that is completely arbitrary; there is no such thing as the absolute energy of a substance, so we can arbitrarily define the enthalpy or internal energy of an element in its most stable form at K and 1 atm pressure as zero.
In other words, as the absolute temperature of a substance approaches zero, so does its entropy.
This principle is the basis of the Third law of thermodynamicswhich states that the entropy of a perfectly-ordered solid at 0 K is zero. Third law of thermodynamics The entropy of a perfectly-ordered solid at 0 K is zero.
Think about some everyday experiences you have with chemical reactions. Your ability to melt and refreeze ice shows you that H2O has two phases and that the reaction transforming one to the other is reversible--apparently the crystallization of ice requires removing some heat.
Frying an egg is an example of an irreversible reaction. If you dissolve halite in water you can tell that the NaCl is still present in some form by tasting the water.
Why does the NaCl dissolve? Does it give off heat? Does it require energy? How is it that diamond, a high-pressure form of C, can coexist with the low pressure form, graphite, at Earth's surface?
Do diamond and graphite both have the same energy? If you burn graphite and diamond, which gives you more energy? When dynamite explodes, why does it change into a rapidly expanding gas, which provides the energy release, plus a few solids? Chemical thermodynamics provides us with a means of answering these questions and more.
A Few Definitions A system is any part of the universe we choose to consider.
Matter and energy can flow in or out of an open system but only energy can be added to or subtracted from a closed system. An isolated system is one in which matter and energy are conserved. A phase is a homogeneous body of matter. The components of a system are defined by a set of chemical formula used to describe the system.