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UA CHEM 152 - Exam 1 Study Guide

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CHEM 152 1st Edition Exam # 1 Study Guide Lectures: 1 – 9Chem 152Exam 1 Study GuideHow do we predict chemical change?Unit 5 Module 1Comparing the relative stability of different substances-using relevant chemical features of reactants and products (thermodynamic stability) to make qualitative predictions about the directionality of reactions-energetic and entropic factors- The extent and directionality of the reaction is determined by the thermodynamic stability of the reactants and products- Thermodynamic stability consists of energetic and entropic factors- 2nd Law of Thermodynamics= systems usually “end up “in the state that posses maximumthermodynamic stability- To predict directionality we need to consider:o Energetic factors (Internal Potential Energy)o Entropic factors (Configurations)Energetic Factors- Generally, thermodynamic stability increases with decreasing potential energy of a substance - Components interact strongly= lower potential energy- Bond Strength: o Strong bonds= more energetically stable (decomposition requires energy input)o Homoatomic (A-A) bonds are weaker than heteroatomic (A-B) - Bond Length:o Longer bonds are weaker than shorter bonds- Polarity:o More polar=harder to separate= more favorable- Intermolecular Forces:o Solids have stronger IMFs=stronger bonds=more stableo Gases have weaker IMFs=weaker bonds=less stable - Ionic Compounds: o Strength of ion-ion interactions (electrostatic forces)o Coulomb’s Law: F=q1q2/r2 Larger charge of the ions=stronger forces Smaller size of ions= stronger forceso Stronger forces= lower potential energy= more energetically stable the ionic compoundo Lattice energy- energy released during formation of an ionic compound starting from its ions in the gas phase is a good measure of the potential energy of the compound A+(g) = B (g)  AB(s) o ΔH: lattice energy More negative lattice energy= more energy required to separate ions= more energetically stable- Heat of Reaction ΔH:o Processes will have a higher probability to occur in the direction in which less netenergy needs to invested for a reaction to happen Exothermic (-ΔH) =more energetically favored Endothermic (ΔH) = less energetically favoredo ΔH= Σ (Bond energy bonds broken)- Σ (BE bonds formed)- Standard Enthalpy of Formation ΔHf- change in enthalpy when one mole of substance is formed from the most stable form of its constituent elements in their standard stateo Example: Using as a model: 12Hs(g)+12I2(s)→ HI (g)Create a reaction for the standard state formation of water H2O (l) H2(g)+12O2(g)→ H2O(l)Entropic Factors (S)- Configurational entropy of a system reflects the number of possible unique configurations that the components of a system can take- Generally, thermodynamic stability increases with an increase in configurational entropy- Boltzmann’s Entropy Equation (W=different configurations)o S=kbln (W) where kb=1.38x10−23 J/K- More distinguishable “configurations” are obtained with:o More different types of particleso Moving and interacting in different ways- Standard Molar Entropy of Formation (Sf¿J/(mol K) - value S experimentally determined and measures the different configurations that matter and energy can take in one mole of substance at 25 C and 1 atmo Entropy of a substance at standard conditions oSf is affected by: Single Particle Level= Molar Mass/Molecular complexity Many Particle Level= State of Matter- Ionic compounds= Ion Charge/Size- Phase/State of Matter:o Solids= particles more constrained=lower entropyo Gases=particles less constrained=larger entropy- Molar Mass:o Higher molar mass= entropy increaseo Small molar mass= lower entropy Going from more moles in reactants to less moles in products= not favorable (-ΔS) Going from less moles in reactants to more moles in products= favorable (ΔS)- Molecular Complexity:o More complex= higher entropyo Less complex= lower entropy- Ionic Compounds:o Larger/higher charge=stronger attractions=lower entropyo Small ion radius=more constrained ions=low entropyo Large ion radius=less constrained ions=higher entropy- Change in Entropy of System ΔS:o – ΔS= NOT entropically favored (backward process favored)o ΔS= entropically favored (forward process favored)Directionality: - Directionality will always favor the formation of products if:o The internal potential energy of the products is lower than that of the reactantso The entropy of the products is higher than that of the reactants- If: +ΔH and –ΔS, the directionality definitely favors reactantso Endothermic reactions & low entropy backward favorableo Endothermic= favored at high temps- If: - ΔH and + ΔS, the directionality will always favor productso Exothermic reactions & high entropy forward favorableo Exothermic= favored at low temps- When +ΔS and +ΔH or –ΔS and – ΔH:o The outcome is TEMPERATURE dependentUnit 5 Module 2Comparing free energies-calculating change in Gibb’s Free Energy during a chemical process (ΔG)- depends on ΔH and ΔS-quantitatively determine the directionality and extent of chemical reactions- Thermodynamic stability and extent of a chemical reactions depends on energetic factors, entropic factors, and now TEMPERATURE-ΔSrxn = Σ ΔSproducts−Σ ΔSreactants-ΔHrxn = Σ ΔHproducts−Σ ΔHreactants- Generally, energetic effects become less relevant at high temperatures and entropic effects dominateGibb’s Free Energy, GΔGrxn enables accurate prediction of directionality for any chemical process or reaction at a given TemperatureWe must know: ΔH, ΔS, or Gproducts∧Greactants-ΔGrxn= ΔHrxn−T ΔSrxn-ΔGrxn= Gproducts−Greactants- Where the sign (+) or (-) can be used to determine directionality - 2nd Law of Thermodynamics: At constant T and P ΔG<0 for thermodynamically favored processeso –ΔG favors products- EXERGONIC more (-)= larger extent of reaction/ products more stable than reactants More (-)= more thermodynamically stable productso ΔG favors reactants- ENDERGONIC More (+)= smaller extent of reaction/ reactants more stable than products- Temperature is the “glue” that links ΔH and ΔS that enables quantification of thermodynamic stability of reactants and products- Standard Gibb’s Free Energy of Reaction ΔG °rxn - change in Gibb’s Free Energy for theformation of 1 mol of the substance in it’s standard state (25 C/1


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