M3_4s-1 - Unit 4 How do we model chemical change The...

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Chemistry XXI Unit 4 How do we model chemical change? M1. Understanding Proportions Determining the amount of substance formed or consumed. M2. Tracking Energy Predicting the amount of energy absorbed or released. M3. Analyzing Rate and Extent Identifying the factors that affect chemical transformations. The central goal of this unit is to help you understand and apply ways of thinking that can be used to model chemical change in a system .
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Chemistry XXI Unit 4 How do we model chemical change? Module 3: Analyzing Rate and Extent Central goal: To explore the factors that determine the rate and extent of chemical reactions.
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Chemistry XXI The Challenge We would like to generate models that allow us to answer questions such as this: What amounts of reactants and products are involved? How much energy will be needed or produced? How fast will the process go and how can I control it? To what extent will the reactants be changed into products? Modeling How do I explain it?
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Chemistry XXI When analyzing chemical reactions we need to pay attention to how substances change and how much energy is transferred, but also to how fast they go . Time Issues T Why do we need a spark? Why is this reaction fast? Let′s think! Let’s go back to our original system : Spark Fast
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Chemistry XXI A Chemical Model All these phenomena can be explained using the following model: 1. For a reaction to occur, the reactant particles must collide. The more collisions in a given time, the faster the process. A + B A B We can control collision frequency changing T , P , V , and N .
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Chemistry XXI Effective Interactions 1. Colliding particles must be positioned so that the reacting groups interact effectively. Ineffective collision NO Effective collision + Consider this reaction: NO + O NO + O The higher the % of configurations that lead to effective collisions ( configuration effectiveness ), the faster the process.
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Chemistry XXI Relative Speeds? Effective Interactions Let’s Think
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Chemistry XXI Reaction Steps It is important to keep in mind that most chemical reactions don’t occur in one step, but several. Despite this fact, the more complex a reacting mixture (#, type, size, geometry, distribution of molecules), the fewer the expected % of effective configurations that can lead to new products.
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Chemistry XXI Activation Energy 3. Colliding particles must have enough energy to reach a transition state that leads to the formation of the new products. The energy required to attain the transition state is called the Activation Energy ( E a ). Products Reactants E p Reaction Coordinate Energy Profile Transition State E a The smaller the Activation Energy , the faster the process.
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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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Chemistry XXI Methane Fast Spark Based on our model, propose hypotheses to explain why the Let’s Think T
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M3_4s-1 - Unit 4 How do we model chemical change The...

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