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M3_2-student - Unit 2 How do we analyze chemical change...

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Chemistry XXI Unit 2 How do we analyze chemical change? Module 3: Measuring Time Central goal: To make qualitative predictions about the influence of diverse factors, such as temperature, concentration and reaction mechanism, on reaction rates.
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Chemistry XXI The Challenge Modeling How do I explain it? All chemical changes involve the transformation of matter and energy into new forms. Some of these changes occur in matter of seconds, while other make take years. What factors determine the speed ( rate ) of a chemical reaction? How can we alter or control the reaction rate?
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Chemistry XXI Fast and Slow Consider, for example, some of the many reactions in which oxygen is involved in the atmosphere. C (s) + O 2 (g) CO 2 (g) Needs a push, but fast. C 8 H 18 (l) + 12.5 O 2 (g) 8 CO 2 (g) + 9 H 2 O (g) Needs a push, but fast. NO 2 (g) + O 2 (g) NO (g) + O 3 (g) Needs sunlight, but fast. 4 Fe (s) + 3 O 2 (g) 2 Fe 2 O 3 (s) Continuous, but slow. Cl (g) + O 3 (g) ClO (g) + O 2 (g) Continuous and fast. Let’s Think How do you explain the differences?
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Chemistry XXI A Chemical Model The differences in speed of chemical reactions can be explained using the following model: 1. For a reaction to occur, the reactant particles must collide. 2. Colliding particles must be positioned so that the reacting groups interact effectively. Ineffective collision NO Effective collision +
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Chemistry XXI The energy required to attain the transition state is called the Activation Energy ( E a ). A Chemical Model 3. Colliding particles must have enough energy to reach a transition state that leads to the formation of the new products. Products Reactants E p Reaction Coordinate H rxn Energy Profile Transition State E a
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Chemistry XXI Activation NO 2 (g) + CO (g) NO (g) + CO 2 (g) Consider the exothermic reaction: For the reverse endothermic process:
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Chemistry XXI Let’s Explore The proposed model allows us to explain and predict the effect of activation energy, temperature, and concentration on reaction rates. Go to: http://www.chem.arizona.edu/chemt/C21/sim Use this molecular dynamics simulation to investigate the effect of E a , T , V , and N on the speed of a reaction, Activation
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Chemistry XXI Let’s Explore
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Chemistry XXI Activation Energy Typical kinetic energy distribution among particles at a fixed temperature The lower E a , the more particles have enough energy to react E a Rate
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Temperature The kinetic energy distribution among particles changes with changing temperature. The higher
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This note was uploaded on 04/09/2009 for the course CHEM 151 taught by Professor Staff during the Fall '08 term at Arizona.

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M3_2-student - Unit 2 How do we analyze chemical change...

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