M4_5 - 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 4: Understanding Mechanism Central goal: To use reaction mechanisms to make predictions about reaction rate and vice versa.
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Chemistry XXI The Challenge Imagine that you were interested in understanding why certain types of substances and processes appeared in our planet. How can we use reaction mechanisms to make predictions? How can we deduce reaction mechanisms based on reaction outcomes? Transformation How do I change it?
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Chemistry XXI Reaction Pathways Our ability to predict the most likely outcomes of a chemical reaction improves considerably when we understand the mechanism that leads from reactants to possible products. Most reaction mechanisms involve several steps. However, some steps play a more central role than others in determining the overall rate of reaction. CH 4 (g) + O 2 (g) CO 2 (g) + 2 H 2 O (g)
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Chemistry XXI Reaction Steps In the mechanistic model , the overall reaction is viewed as the result of multiple elementary reactions or steps occurring simultaneously in the system. A + A B + C Bimolecular 2 A 2 C + E For example, the overall reaction: 2 A 2 C + E may involve two elementary steps: Unimolecular B C + E B is an intermediate Detecting intermediates is an important means of investigating reaction mechanisms.
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Chemistry XXI Overall Rate Order In the mechanistic model , the overall rate of the reaction is an “emergent property” of the rates of the individual steps. For many reactions, one step is slow enough to limit the rate of the overall reaction: A + A B + C Slow 2 A 2 C + E Fast B C + E Overall Rate = k [ A ][ A ] = k [ A ] 2
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Chemistry XXI Mirror Images Consider the following problem of central relevance for our understanding of the origin of life: Most amino acids found on Earth appear in only one of two possible mirror-image forms, called enantiomers or optical isomers . L D These isomers have most of the same properties, but react differently with L and D isomers of other “ chiral ” substances. Non-superimposable
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Chiral Centers Molecular chirality is commonly caused by the presence of carbon atoms in a molecule attached to four different groups: Chiral carbon Non-Chiral carbons Let′s think! Identify the chiral carbons in
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M4_5 - Unit 5 How do we predict chemical change? The...

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