Chapter 9 State Standards: 3.b; 3.f; 5.a; 5.b; 5.c 1Contreras

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 During chemical reactions, atoms rearrange to form new substances that have different properties than the original substances had 3Contreras

 Gas Formation ◦ Bubbles  Solid Formation ◦ Solid Precipitate  Energy Change ◦ Light; heat; absorbed  Color Change 4Contreras

 The more signs you see, the more likely it is a chemical reaction taking place 5Contreras

 Gas, solid, energy and color changes aren’t always a guarantee of a chemical reaction  Always look for a new substance with new properties to be formed  Reactants – substances going into reaction  Products – new substances formed Reactants + Reactants  Products + Products 6Contreras

 A chemical bond is the force that holds atoms together  Bonds break  rearrange  form new bonds to make new substances Hydrogen + Chlorine Hydrogen Chloride (Diatomic Molecules) 7 These bonds must break before new ones can be formed Two new bonds were made between hydrogen atoms and chlorine atoms Contreras Rearrange

 Energy is needed to break bonds of starting substances  Energy is released when new bonds form in new substances  Exothermic – energy released “exit”; matter becomes warmer  Ex. Gasoline and oxygen in car’s engine  Endothermic – energy absorbed “taken in”; matter becomes cooler  Ex. Photosynthesis, plants absorb light energy 8Contreras

 Law of Conservation of Energy ◦ Energy cannot be created nor destroyed in chemical reactions ◦ Energy can change form and move from reactants to products or visa versa 9Contreras

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 Combination of chemical symbols and numbers to represent a substance  Subscripts ◦ Number of atoms ◦ Written below and to the right of chemical symbol for element 11 H2OH2O Subscript 2 Hydrogens No subscript = One Oxygen O HH Contreras H2OH2O

 Chemical equations use chemical formulas and symbols to describe a chemical reaction  reactants + reactants  products + products (starting substances)(new substances) 12 C + O 2  CO 2 separates reactants“yields” separates reactants from products H 2 + O  H 2 O Contreras

 Mass cannot be created or destroyed in ordinary chemical and physical changes  Atoms are just rearranged in a reaction *always make sure the number of atoms in the reactants is equal to the number of atoms in the products 13Contreras

 Count the atoms!!  Use coefficients to balance an equation ◦ A number placed in front of a chemical symbol or formula ◦ 3 Carbon Dioxide molecules ◦ Coefficient gets multiplied into the formula  3 Carbons  6 Oxygens Contreras14 3CO 2 Coefficient Subscript

 H 2 + O 2  H 2 O  H 2 + O 2  2H 2 O  2H 2 + O 2  2H 2 O 15 ReactantsProducts H=2 O=2H=2 O=1 ReactantsProducts H=2 O=2H=4 O=2 ReactantsProducts H=4 O=2H=4 O=2 Contreras