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M2_4-1 - Unit 4 How do we model chemical change The central...

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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 2: Tracking Energy Central goal: To make qualitative and quantitative predictions about the amount of energy absorbed or released during a chemical reactions based on the nature of the chemical bonds in the molecules of reactants and products.
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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 Methane Let’s go back to our combustion problem, nut now paying attention to energy issues: Focus on Energy T Why does the temperature increase? Energy is released during the process.
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Chemistry XXI Energy Transfer A common signature of a chemical change is the release or absorption of energy. Surroundings Surroundings System System heat q q > > 0 0 The system absorbs energy ENDOTHERMIC Surroundings Surroundings System System heat EXOTHERMIC q q < < 0 0 The system releases energy Heat
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Chemistry XXI Exothermic or Endothermic Combustion reactions are prototypical examples of exothermic processes: + ENERGY + ENERGY How can we explain the differences
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Chemistry XXI The amount of energy released or absorbed in a chemical reaction is related to the nature and numbers of bonds broken or formed in the process. A Chemical Model Do we need energy to break bonds or do we get energy from bond breaking? Do we need energy to form bonds or do we get energy from bond forming? Let′s think! During a chemical reaction some bonds are broken and some new bonds are formed.
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Chemistry XXI Energy is needed to separate a pair of bonded atoms (“ break ” the bond).
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