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AP Chemistry 9.1-9.6 Basic Overview

9.1- Introduction to Entropy

^^Entropy^^: dispersal of matter/energy in sample of matter

  • changes of entropy can be seen as how dispersed the matter/energy is

  • entropy increases when matter is more dispersed

    • (ex. a phase change from solid to liquid, liquid to gas)

  • individual particles at increased entropies are more free to move and occupy more space

  • With gases:

    • entropy of gas increases when volume, increase because gas molecules are able to move in a larger space with same speed
    • if total number of moles of gaseous products>total number of gaseous reactants, entropy increases

  • entropy also increases when energy is more dispersed

  • according to KMT, K.E among particles broadens when temperature increases

  • entropy increases when temperature increases

9.2- Absolute Entropy and Entropy Change

  • entropy change can be calculated from absolute entropies in individual species

    • unit: J/K

  • most substances have a nonzero value for absolute entropy unlike enthalpy

  • when calculating, number of moles of each substance have to be considered

  • ^^Find delta S with: ΔS=∑S(products)−∑S(reactants)^^

  • entropy is positive if : phase changes occur as, solid to liquid to gas or if number of moles increase from reactants to products

  • entropy is negative if: phase changers occur as, gas to liquid to solid

9.3- Gibbs Free Energy and Thermodynamic Favorability

^^Gibbs free energy:^^ △G describes if a reaction is thermodynamically favorable or unfavorable

^^Thermodynamically favorable:^^ equation proceeds to equilibrium with no outside factors

  • reminder! just because reaction is favorable does not mean it happens quickly

  • in Gibbs free energy all reactants and products are in standard states (pure substance, 1.0M, 1 atm)

^^Find delta G with: ΔG=∑G(products)−∑G(reactants)^^

  • thermodynamically favorable, G=negative

  • thermodynamically unfavorable, G=positive

  • G can be calculated from enthalpy and entropy with: ^^ΔG=ΔH-TΔS^^ * t=temperature

    • if both enthalpy and entropy are favorable or both unfavorable, there is no need to find G to see if its favorable

9.4-Thermodynamic and Kinetic Control

  • processes that are favorable but do not make products at measurable rate, are under kinetic control
    • things under kinetic control usually have large activation energy (Ea), making the rate slow down
  • a catalyst (ex. enzyme) can decrease Ea and increase reaction rate, but has no effect on favorability
  • even if the process doesn't happen at a noticeable rate, it does not mean it's at not equilibrium

9.5- Free Energy and Equilibrium

  • thermodynamically favored (ΔG
  • at equilibrium, no net change in concentration of reactants and products occurs

^^Find K with: K=e-GT/RT^^

Find Delta G with: ΔG° = -RTlnK *R = 8.314 J mol-1 K-1

  • -when ΔG is neg, K>1, reaction favors products
  • when ΔG is pos, K<1, reactions favors reactants
  • when ΔG=0, reaction is at equilibrium

9.6-Coupled Reactions

  • a process with positive ΔG is unfavorable
  • different paths occurs to make process happen when the reaction is unfavorable
    • paths can be external sources of energy (ex. sunlight, power source)
    • or it can be coupled to another favorable reaction

^^Coupling:^^ when two reactions share an intermediate, they can be coupled, Hess’s law can be applied and the sum of the reactants’ ΔG values makes overall process favorable when added