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The concepts of thermodynamics and equilibrium in chemistry. It delves into the factors that influence the directionality of chemical reactions, including enthalpy, entropy, and temperature. Gibbs free energy as a key concept for predicting reaction spontaneity and explores the relationship between gibbs free energy, equilibrium constant, and temperature. It also includes examples and exercises to illustrate the application of these concepts.
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M4. Measuring Rates Analyzing the factors that affect reaction rate. M2. Comparing Free Energies Determining the directionality and extent of a chemical reaction. M1. Analyzing Structure Comparing the relative stability of different substances M3. Understanding Mechanism Identifying the steps that determine reaction rates. The central goal of this unit is to help you identify and apply the different factors that help predict the likelihood of chemical reactions.
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We have learned the directionality of all processes is determined by energetic and entropic factors: Product favored by ionic forces Reactant favored by more moles of gas Relevant Factors
Temperature is also Relevant The directionality and extent of a chemical reaction depend on three main factors: ENERGETIC FACTORS DH rxn
rxn TEMPERATURE T But how?
Chemical Thinking Consider a chemical reaction with these conditions: DS rxn
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Connecting DS and DH to PEC diagrams E p S
What does this mean for the reactants & products? Products have higher S / more configurations (S going up) Products have lower energy H (H going down) What would the PEC diagram look like for this type of reaction?
Chemical Thinking How can we associate each of these conditions with the corresponding E p -S diagram? DS rxn
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p S
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Remember: larger S means more configurations In other words: DS rxn
0 = increasing configs A B C D
The Idea Can we define a single measurable quantity that takes into account both energetics (DH rxn ) and entropy (DS rxn ) to determine if a reaction favors the reactants or products? Measure of Thermodynamic Stability Resistance to Change This quantity is called Gibbs Energy (or Gibbs Free Energy) and is defined as:
In general, energetic effects become less relevant at high temperature, where entropic effects dominate. The Law WHY? Gibbs Energy is G = H – TS
Gibbs Free Energy G = H – TS G more negative Negative H and Positive S (Very stable) G more positive Positive H and Low (or negative) S (Not very stable) Product-Favored Processes G final < Ginitial DG = G final
initial < 0 DG is negative Reactant-Favored Processes G final
Ginitial DG = G final
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0 DG is positive DG = DH – TDS
2 (g) + O 2 (g) 2 H 2 O(g) Under what conditions will this reaction favor the products? A. At no temperatures (never) B. At high temperatures C. At low temperatures D. At all temperatures E. More information is needed Hint: First, you should predict if DH and DS are positive or negative
DHrxn < 0 DSrxn > 0 DHrxn > 0 DSrxn > 0 DHrxn < 0 DSrxn < 0 DHrxn > 0 DSrxn < 0 DGrxn < 0 always DGrxn > 0 never DGrxn < 0 Low T DGrxn < 0 High T Under what conditions will a reaction favor the products? E p S
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Summary
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4 (g) 2 NO 2 Colorless (g) Red- Brown What does the gas color tell us about the reaction? T = -78 oC T ~-10 oC T = 20 oC DH rxn
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Product-favored only at High T (20 vs - o C) We can use thermodynamics to make inferences about reactions.
Gibbs Free Energy Change DG rxn
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rxn DG = DH – DS T y = b + m x RF PF DH > 0, DS > 0 DG T For example: DH (+) m = - DS (negative slope) b = DH m = - DS x = T Rearrange the equation to make it like the equation for a line
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