M2_5s - Unit 5 How do we predict chemical change The...

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Chemistry XXI Unit 5 How do we predict chemical change? M3. 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 M4. 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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Chemistry XXI Unit 5 How do we predict chemical change? Module 2: Comparing Free Energies Central goal: To quantitatively determine the directionality and extent of chemical reactions.
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Chemistry XXI The Challenge Imagine that you were interested in QUANTITATIVELY determining which of two or more chemical processes could have occurred on the primitive Earth. What measurable properties of the system can be used to make the prediction? How could we actually quantify the directionality and extent of a chemical reaction? Transformation How do I change it?
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Chemistry XXI Relevant Factors The directionality and extent of a chemical reaction depend on three main factors: ENERGETIC FACTORS H rxn ENTROPIC FACTORS S rxn TEMPERATURE T But how?
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Chemistry XXI What Changes? Consider this chemical process: CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) Is this a favored process? Why?
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Chemistry XXI The Facts It may be favored if the process: a) leads to more energetically stable compounds. energy is released (Exothermic: H rxn < 0). CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) q H rxn < 0 ( - )
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Chemistry XXI It may be favored if the process: b) Leads to the formation of substances with more distinguishable configurations: S rxn > 0 ( + ) The Facts CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) Certainly favored: H rxn < 0 ( - ) (exothermic) S rxn > 0 ( + ) S rxn > 0 ( + )
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Chemistry XXI Let’s Think Consider the possible decomposition of CO 2 in the atmosphere: CO 2 (g) C (s) + O 2 (g) Analyze the sign of H rxn and S rxn and decide whether the process is favored or not.
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Chemistry XXI The Facts If, as in the previous case, But what happens if: H rxn > 0 ( + ) (exothermic) S rxn < 0 ( - ) ? or H rxn < 0 ( - ) (endothermic) S rxn > 0 ( + ) ? The outcome depends on the TEMPERATURE .
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Chemistry XXI In general, energetic effects become less relevant at high temperature, where entropic effects dominate. 2 nd Law of Thermodynamics At constant T and P: G rxn < 0 for thermodynamically favored processes. The Law The sign of this quantity ( Gibbs Free Energy , G ): G rxn = H rxn T S rxn can be used to determine process directionality.
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Chemistry XXI Experimental results indicate that processes at constant T and P occur in the direction in which the Gibbs free energy G of the system decreases. A
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This note was uploaded on 03/29/2010 for the course CHEM 152 taught by Professor Grall during the Spring '08 term at Arizona.

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M2_5s - Unit 5 How do we predict chemical change The...

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