Chapter 18 - Chapter 18 Chemical reactions begin when the...

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Chapter 18 Chemical reactions begin when the reactants come into contact and continue until at least one reactant, (the limiting reactant), is completely used up A significant portion of world energy resources is used to cause desirable reactions to occur – reactions that transform inexpensive, readily available substances into new substances with more useful properties, such as polymers and medicines It is important to differentiate between a reaction that is so slow that it appears not to occur, such as air oxidation of gasoline, and one that cannot take place of its own accord, such as decomposition of sodium chloride 18.1 Product-favored process – When products predominate over reactants (Ex: The reaction of bromine with aluminum, rusting or iron, and combustion of gasoline) Reactant-favored process – When reactants predominate over products if equilibrium has been reached A reactant-favored product is always exactly the opposite of a product-favored process The designations “product-favored” and “reactant-favored” indicate the direction in which a chemical reaction will take place – either forward or backward based on a given equation 18.2 Energy will spread out (disperse) unless it is hindered from doing so (Ex: One-way transfer of energy from a hotter sample to a colder sample) Most exothermic reactions are product-favored at room temperature for a similar reason Chemical potential energy that has been stored in bonds between relatively few atoms, ions, and molecules (the reactants) spreads over many more atoms, ions, and molecules as the surroundings (as well as the products) are heated Dispersal of energy occurs because the probability is much higher that energy will be spread over many particles than that it will be concentrated in a few The probability that energy will become dispersed becomes overwhelming when large numbers of atoms or molecules are involved If energy can be dispersed over a very much larger number of particles, it will be Energy becomes more dispersed when a system consisting of atoms or molecules expands to occupy a larger volume This kind of energy dispersal is associated with a characteristic property of gases: A gas expands until it fills a container
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That dispersal of atoms and molecules also involves dispersal of energy seems obvious, because energy accompanies the particles as they disperse
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Chapter 18 - Chapter 18 Chemical reactions begin when the...

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