Without constant input of energy, the room would eventually be quite disorganized (this isn’t strictly the same thing but is an interesting parallel).Īll processes have only two ways to transfer or change internal energy Our rooms are rarely in perfect order unless we expend energy to keep it so. We can see an analogy of this in our day to day lives in non scientific ways as well. The second law, thus, is a statement describing the driving force of all spontaneous processes. ( )įor any spontaneous process (closed or not) the entropy of the universe must increase. įor any spontaneous process in a closed system the entropy of the system must increase. Entropy is the scientific name for the macroscopic measure we use to evaluate these microstates of the system and is given the symbol. Clearly, there are more microstates available to the system if the gas expands into both chambers. A given instantaneous combination of position and energy of all the molecules taken together is called a microstate. The driving force seems to be an increase in freedom of motion of the individual molecules. This increase in room allows more freedom of motion for the individual gas molecules. The gas has more room in the increased volume of the double bulb container. The process is obviously spontaneous but energy is not involved. Gas initially in the chamber on the left (shaded) expands adiabatically into the evacuated second chamber (adiabatic means no energy transfer between system and surroundings). To start our thought experiment, we open the valve. The diagram on the left depicts an insulated, closed system consisting of two interconnected chambers separated by a valve. Back to Top 15.2 Entropy and the Second law of Thermodynamics To understand the driving force, we must introduce a new concept. It turns out that the First Law of Thermodynamics completely eliminates enthalpy alone as a driving force Although we will see later that it does have a place in the overall equations to determine spontaneity. Equation (2) is endothermic and so there is some other driving force which pushes the reaction. If we look at these two reactions, we have difficulty deciding anything except that enthalpy change alone is not sufficient to decide if a reaction is spontaneous or not.Įquation (1) is very exothermic and this in itself is a strong tendency towards spontaneity. Obviously, the evolution of enthalpy does not drive a reaction as one might first expect. If we look at some processes which we know to be spontaneous, maybe we can find a common trend, or at least eliminate some possibilities.Ĭonsider The following two spontaneous reactions: What is the driving force that creates spontaneity? How do we measure it? We’ve seen from experience that some chemical (and other) processes are spontaneous while others are not. Michael Mombourquette 15.1 Spontaneous Processes Since S = 0 corresponds to perfect order.15 Thermochemistry II Spontaneity, Entropy and Gibbs Energy The entropy of a pure crystalline substance at absolute zero (i.e. Nonetheless, the combination of these two ideals constitutes the basis for the third law of thermodynamics: the entropy of any perfectly ordered, crystalline substance at absolute zero is zero. In practice, absolute zero is an ideal temperature that is unobtainable, and a perfect single crystal is also an ideal that cannot be achieved. Such a state of perfect order (or, conversely, zero disorder) corresponds to zero entropy. The only system that meets this criterion is a perfect crystal at a temperature of absolute zero (0 K), in which each component atom, molecule, or ion is fixed in place within a crystal lattice and exhibits no motion (ignoring quantum effects). A perfectly ordered system with only a single microstate available to it would have an entropy of zero. The greater the molecular motion of a system, the greater the number of possible microstates and the higher the entropy. These forms of motion are ways in which the molecule can store energy. due to the reduction in the degrees of freedom, the system is more ordered after the reaction). There is a reduction in the disorder of the system (i.e.The reaction has resulted in a loss of freedom of the atoms (O atoms).Since they are now physically bonded to the other molecule (forming a new, larger, single molecule) the O atoms have less freedom to move around.The product of this reaction (\(NO_2\)) involves the formation of a new N-O bond and the O atoms, originally in a separate \(O_2\) molecule, are now connected to the \(NO\) molecule via a new \(N-O\) bond.
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