Entropy (and free energy)
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Please explain the difference between enthalpy and entropy?
Ulex writes ...
If you are going to make any progress in chemical thermodynamics you will need to get this distinction straight, so it is appropriate for me to say: “I’m glad you asked me that.” !
Enthalpy
Imagine, if you will, 1 mole of molecules of some familiar substance; let’s choose carbon dioxide but it doesn’t matter all that much what the example is. Let’s further suppose that this mole of carbon dioxide is at room temperature and pressure. These molecules possess a certain amount of energy due to several things which are happening: they are moving about which involves kinetic energy. The atoms within the molecules have electronic energy and there is vibrational and rotational energy due to the ways the molecules and the atoms within them are moving. All this energy added together is called the internal energy of the carbon dioxide and is designated by the symbol U. If the carbon dioxide takes part in some chemical reaction, new substances are produced. The U values for the reactants can be added together and the U values for the products can also be totalled. The difference between the total U values for the products and the reactants , both measured at standard temperature (298K), is the internal energy change of the reaction,DU. A little bit must be added to or subtracted from the value of DU if the reaction takes place at constant pressure because the reaction mixture may expand or contract and do work against the atmosphere or have work done on it by the atmosphere. As most of our reactions take place at constant pressure, i.e. in apparatus which is open to the atmosphere, this small adjustment is nearly always appropriate. The resulting change in energy is called the enthalpy change, DH. We appreciate this enthalpy change as a production of heat (exothermic reaction) or an absorption of heat (endothermic reaction).
The bottom line is: the enthalpy change of a reaction measures the extent to which the reaction is exo- or endo – thermic. It is measured in J or kJ per mole.
Entropy
Entropy measures a different, but related, property of matter. If you can imagine our 1 mole of carbon dioxide at the absolute zero of temperature (difficult, I know, but its important to try) all its molecules and the atoms within them would be completely still. We would know exactly where each one was – there would be perfect order. There would be no internal energy content so we wouldn’t have to think about how much energy was rotational, vibrational etc because there is no energy present of any of those kinds. This is a state of zero entropy. At any higher temperature than the absolute zero, the molecules and atoms within them move and become more random or disordered both in their position and in the possible ways in which their energy can be distributed. The extent of this randomness or disorder is called the entropy of the material and is designated by the symbol S. The value of S obviously depends on how much energy there is in the material concerned and the temperature it is at. S is defined by:
S = H/T
and therefore has units of J (or kJ) per mole per Kelvin. If the material is at standard temperature and 1 atmosphere pressure, its standard entropy can be looked up in the Book of Data; the standard entropy of carbon dioxide is 213.6 J mol-1 K-1, for example.
When a reaction happens, the reactants and products will have different entropies and the difference between the entropies of the reactants added together and the products added together is the entropy change of the chemicals themselves or, as we say, the ‘system’. This is designated DSsystem. You can find a value for this by using the entropies in the Nuffield Book of Data,Tables 5.2, 5.3 and 5.5.
However, when the reaction happens, energy is exchanged with the surroundings so the entropy of the surroundings (air, bench, walls, ceiling etc) changes too, DSsurroundings. The value for DSsurroundings depends on what the temperature is and how much energy is transferred.
DSsurroundings = -DH/T ( the minus sign is there because exothermic reactions have negative enthalpy change values but put energy into the surroundings which increases their entropy).
If you add together the entropy changes in the system and the surroundings you arrive at the total entropy change for the reaction. If this turns out to be positive, the reaction is a feasible one and you can predict whether or not this is the case without having to try the reaction in practice.
You can’t predict how fast it will go, though, but that’s another story!
Risk assessment
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updated: 11 March 2004
