Chapter 9

State Standards:3.b; 3.f; 5.a; 5.b; 5.c

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

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Gas Formation◦ Bubbles

Solid Formation◦ Solid Precipitate

Energy Change◦ Light; heat; absorbed

Color Change4Contreras

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

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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

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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)

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These bonds must break before new ones can be formed

Two new bonds were made between hydrogen atoms and chlorine atoms

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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

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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

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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

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H2OSubscript

2 Hydrogens

No subscript = One Oxygen

OH H

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H2O

Chemical equations use chemical formulas and symbols to describe a chemical reaction

reactants + reactants products + products(starting substances)(new substances)

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C + O2 CO2

separates reactants

“yields” separates reactants from products

H2 + O H2O

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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 *always make sure the number of atoms in the reactants is equal to the atoms in the reactants is equal to the number of atoms in the productsnumber of atoms in the products

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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

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3CO2Coefficient Subscript

H2 + O2 H2O

H2 + O2 2H2O

2H2 + O2 2H2O

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Reactants ProductsH=2 O=2 H=2 O=1

Reactants ProductsH=2 O=2 H=4 O=2

Reactants ProductsH=4 O=2 H=4 O=2

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