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<0) means that products are favored at equilibrium (K>1)

• 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