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Chapter 19, Part III

Chapter 19, Part III. Spontaneous vs. Non-spontaneous Entropy vs. enthalpy. How do we know a reaction occurs?. Free energy —energy available to do work Spontaneous reaction —a reaction that will occur naturally and favors the formation of the product (K eq >1)

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Chapter 19, Part III

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  1. Chapter 19, Part III Spontaneous vs. Non-spontaneous Entropy vs. enthalpy

  2. How do we know a reaction occurs? • Free energy—energy available to do work • Spontaneous reaction—a reaction that will occur naturally and favors the formation of the product (Keq>1) • All spontaneous reactions release free energy!

  3. How do we know a reaction occurs? • Nonspontaneous reaction—a reaction that does not favor formation of the product and does not occur naturally. • Reactions can be spontaneous at one set of conditions and not at another. • Sometimes nonspontaneous reactions can occur when coupled with spontaneous reactions.

  4. NOTE: This does NOT refer to speed of reaction, only whether or not it naturally occurs.

  5. LOOKING BACK: • Remember that enthalpy change (DH) is the measure of change in heat content for reaction. • Since all spontaneous reactions release free energy, you could expect that they would always be exothermic (release heat = - DH).

  6. This is NOT the case, for example, the melting of ice to water requires energy, but it happens spontaneously. • Thus, enthalpy change is not the only factor that determines whether or not a reaction will be spontaneous.

  7. ENTROPY • The law of disorder states that all processes occur in such a way that they move towards maximum disorder (randomness). • Entropy is the measure of the amount of disorder in a system. • A reaction that absorbs energy can be spontaneous if entropy increases.

  8. How does entropy increase? • Changing to a less organized phase • Solid changing to liquid or gas • Liquid changing to gas • Substance is divided into parts • NaCl is separated to Na+ and Cl- in solution • The product has more molecules than the reactants • 2H2O a 2H2 + O2

  9. Entropy also increases as heat is applied to a system.

  10. Spontaneous or nonspontaneous • So, to decide whether a reaction is spontaneous or not, you have to look at both the change in enthalpy (heat) and the change in entropy (disorder)

  11. Exothermic + entropy increase = Spontaneous reaction • Endothermic + entropy increase = Spontaneous reaction ONLY if entropy change is more than heat absorbed

  12. Exothermic + entropy decrease = spontaneous reaction ONLY if heat change is more than increased order • Endothermic + entropy decrease = nonspontaneous reaction

  13. CALCULATING ENTROPY CHANGE • Entropy is symbolized by S. Standard entropy (entropy @ 25oC & 101.3kPa) is So • DSo = Soprod - Soreact

  14. Practice Problem #1 • Calculate the standard entropy change that occurs when 1 mol H2O(g) condenses to 1 mol H2O(l).

  15. Practice Problem #2 • NO(g) reacts with O2(g) to form NO2(g). What is the standard entropy change for the reaction at 25oC and 101.3kPa.

  16. CALCULATING FREE ENERGY • Gibbs free energy is DG. Because free energy is released in a reaction, when DG is negative (free energy is released) then the reaction is spontaneous. • DG = DH – TDS (T in Kelvin) • At 25oC & 101.3kPa, DGo = DHo – TDSo • Also, DGo = DGoprod – DGoreact

  17. Practice Problem #3 • For the reaction C(s) + O2(g) a CO2(g), find DGo at 25oC, and tell whether or not the reaction is spontaneous.

  18. Practice Problem #4 • Tell whether or not the following reaction is spontaneous at 25oC using the equation DG = DH - TDS: 2Cl2(g) + 2H2O(l) a 4HCl(g) + O2(g)

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