Spontaneous and Nonspontaneous Processes - Entropy

Spontaneous vs. Nonspontaneous Processes The AP Board now prefers the term thermodynamically favored to substitute for spontaneous although saying spontaneous will never lose points. In thermodynamics, we must predict spontaneity. A spontaneous process is one that does not require outside intervention. The spontaneity of a reaction is the direction in which and extent to which a chemical reaction proceeds. (NOT HOW FAST IT OCCURS) Predicting if a chemical process is thermodynamically favored can be challenging. In order to do so we must: – 1.) Develop a chemical potential that predicts the direction of a chemical system. – 2.) We must not confuse spontaneity with speed.

Chemical Reactions that are Not Thermodynamically Favored A nonspontaneous reaction can not be made spontaneous by use of a catalyst…however, this doesn’t mean nonspontaneous reactions are impossible. Nonspontaneous reactions can be made possible by coupling them with highly spontaneous reactions. – Example: Iron can be separated from its ore if external energy, usually by means of another chemical reaction that is spontaneous, is supplied.

Entropy and the Second Law of Thermodynamics Most thermodynamically favored processes are exothermic, but some are endothermic. – Examples: Ice melting at 0 o C The evaporation of liquid water to gaseous water The dissolution of sodium chloride in water – All three examples are endothermic processes and still they are spontaneous. The reason that this is true is because the criterion for spontaneity is the entropy of the universe. Entropy (S) – the disorder or randomness at the molecular level.

Entropy Entropy (S) is the thermodynamically function that increases with the number of energetically equivalent ways to arrange the components of a system to achieve a particular state. S = k ln W (k is known as the Boltzmann constant (the gas constant divided by Avogadro’s number: 1.38x J/K) W is the number of energetically equivalent ways to arrange the components of a system). As W increases, so does entropy! You will not be expected to know the equation above for the AP exam. You are however expected to understand that as the randomness of a system increases, so does entropy (S) and the value of entropy would be positive. This is because S final > S initial, So… ΔS is positive. For any spontaneous process, the entropy of the universe increases (ΔS univ > 0) Entropy, like enthalpy, is a state function…it’s value depends only on the state of the system, not on how the system arrived at that state.

Let’s Try a Practice Problem! Consider these three changes in the possible distribution of 6 gaseous particles within three interconnected boxes. Which change has a positive ΔS? a.) Less to more random

Entropy Changes Associated with a Change in State Entropy increases when matter changes from a solid to a liquid, and again from a liquid to a gas. Gases are more disordered than both solids and liquids, and a gas has more energetically equivalent configurations because it has more ways to distribute its energy. Entropy also increases as the number of moles of gas increases during a chemical reaction.

Let’s Try Another!!! Predict the sign of ΔS for each process: a.) the boiling of water b.) I 2 (g)  I 2 (s) c.) CaCO 3 (s)  CaO(s) + CO 2 (g) a.) ΔS is positive b.) ΔS is negative c.) ΔS is positive

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