scotty 3 - Ozaer Faroqui 1 17.2: Spontaneous and...

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Ozaer Faroqui – 1 17.2: Spontaneous and Non-spontaneous Processes A fundamental goal of thermodynamics is the prediction of spontaneity. A spontaneous process is one that occurs without ongoing outside intervention. The spontaneity of a chemical reaction should not be confused with the speed of a chemical reaction. o In thermodynamics, we study the spontaneity of a reaction – the direction in which and extent to which a chemical reaction proceeds. o In kinetics we study the speed of the reaction – how fast a reaction takes place. o Thus, a reaction may be thermodynamically spontaneous but kinetically slow. o A catalyst may increase the rate of a spontaneous reaction, but won’t increase the rate of a non-spontaneous reaction. A non-spontaneous process is not impossible, it can be made spontaneous by coupling it to another process that is spontaneous, or by supplying energy from an external source. 17.3: Entropy The criteria for spontaneity in a chemical system depends on the entropy and not on the enthalpy which is a quantity related to disorder or randomness at the molecular level. Entropy (S) is a thermodynamic function that increases with the number of energetically equivalent ways to arrange the components of a system to achieve a particular state. o S = k ln W k = Boltzmann constant or the gas constant divided by Avogadro’s number (R/N A = 1.38 * 10 -23 J/K). W = the number of energetically equivalent ways to arrange the components of the system (As W increases, entropy increases). Since W is unitless, the units of entropy are joules per Kelvin (J/K). Entropy is a state function – its value only depends on the state of the system, not on how the system got to that state. o Therefore, for any process, the change in entropy is just the entropy of the final state minus the entropy of the initial state. ΔS = S final - S initial
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Ozaer Faroqui – 2 Entropy determines the direction of chemical and physical change. o A chemical system proceeds in a direction that increases the entropy of the universe – it proceeds in a direction that has the largest number of energetically equivalent ways to arrange its components. Entropy of a state increases with the number of energetically equivalent ways to arrange the components of the system to achieve a particular state. o The state with the highest entropy also has the greatest energy dispersal or randomization. A state in which a given amount of energy is more dispersed or randomized has more entropy than a state in which the same energy is more concentrated. 17.3: Thermodynamics Zeroth Law – If two systems are the same time in thermal equilibrium with a third system, they are in thermal equilibrium with each other. First Law – Energy can be transformed (changed from one form to another), but it
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scotty 3 - Ozaer Faroqui 1 17.2: Spontaneous and...

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