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Understanding Chemical Reactions and Energy

Explore chemical reactions, conservation of mass, balancing equations, reaction types, energy changes, catalysts, and inhibitors in this comprehensive guide.

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Understanding Chemical Reactions and Energy

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  1. Chapter 24 Chemical Reactions

  2. Section 1-Chemical Changes • Chemical reactions are taking place all around you and even within you • Chemical reaction-a change in which one or more substances are converted into new substances • Reactants-substances that react • Products-new substances produced

  3. Conservation of Mass • Lavoisier-famous scientist who discovered that masses of reactants and products were the same in an experiment • SO, he concluded: • Law of Conservation of Mass-the starting mass of reactants equals the final mass of the products OR • Matter is neither created nordestroyed during chemical reactions

  4. Unit Managers • Atoms in a chemical reaction are rearranged, but never lost or destroyed. • Coefficients- represent the number of units of each substance taking part in a reaction • (can be thought of as unit managers) • Knowing the number of units of reactants enables chemists to add correct amounts of reactants to a reaction

  5. Section 2-Chemical Equations • Balancing an equation doesn’t change what happens in a reaction-it simply changes the way the reaction is represented. • The balancing process involves changing coefficients in a reaction • Balanced chemical equation-has the same number of atoms of each element on both sides of the equation

  6. Choosing Coefficients • Finding out which coefficients to use to balance an equation is often a trial-&-error process, but with practice the process becomes easier. • Steps to Balancing Equations: • Write a chemical equation for the reaction using formulas and symbols • Check the Equation for atom balance • Choose coefficients that balance the equation • Recheck the numbers of ea/atom on ea/side and adjust coefficients if necessary

  7. Section 3-Classifying Chemical Reactions • Combustion Reactions-(burning) occurs when a substance reacts with oxygen to produce energy in the form of heat and light Synthesis Reaction- two or more substances combine to form another substance. A + B ---AB

  8. Decomposition Reaction- occurs when one substance breaks down, or decomposes, into two or more substances AB --- A + B Single Displacement-when one element replaces another element in a compound A + BC --- AC + B OR D + BC --- BD + C

  9. The Activity Series • A metal will replace any less active metal • Notice: copper, silver and gold-least active metals • SO: that is why these elements often occur as deposits of the relatively pure element.

  10. More on types of reactions • Double Displacement- the positive ion of one compound replaces the positive ion of the other to form two new compounds • Precipitate- an insoluble compound that comes out of solution during a double displacement reaction • AB + CD --- AD + CB

  11. Section 4-Chemical Reactions and Energy • All chemical reactions release or absorb energy • This energy can take many forms, such as heat, light, sound and electricity • Chemical bonds are the source of this energy

  12. When chemical reactions take place, some chemical bonds in the reactants must be broken • Breaking these bonds takes energy • Bond formation releases energy

  13. Exergonic Reactions- Chemical reactions that release energy • Less energy is required to break the original bonds than is released when new bonds form • Exothermic Reaction- when the energy given off in a reaction is primarily in the form of heat • *Exothermic reactions provide most of the power used in homes and industries

  14. Endergonic Reactions- a chemical reaction that requires more energy to break bonds than is released when new ones are formed • The energy absorbed can be in the form of light, heat, or electricity • Endothermic reaction- when the energy needed is in the form of heat • (Endothermic can also refer to phase/physical changes as well)

  15. Catalysts • Catalyst- a substance that speeds up a chemical reaction without being permanently changed itself • The mass of the product that is formed remains the same • The catalyst remains unchanged and can be reused

  16. Inhibitors • Inhibitors— substances that are used to combine with one of the reactants in a reaction. • The inhibitor ties up the reactant and prevents it from undergoing the original reaction. • Ex: food preservatives BHT and BHA are inhibitors that prevent spoilage of certain foods, s

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