8th Grade Physical Science By Lisa Anglim Ch. 9 Notes 8th Grade Physical Science By Lisa Anglim

9.1 Forming New Substances

Chemical Reactions- The process by which 1 or more substances undergo change to produce 1 or more different substances Reactions occur when chemical bonds are broken. The atoms rearranged and form new bonds

Clues that a reaction has taken place Formation of a gas Formation of a solid Color Change Energy change – heat, cold, light or sound produced

Conservation of Energy Chemical reactions involve energy It takes a certain amount of energy to start the reaction (activation energy) Some reactions release more energy (exothermic) Other reactions absorb energy (endothermic) Energy is conserved just like mass. The energy is always somewhere, in exothermic reactions it is stored in the reactants and then released. In endothermic reactions it is stored in the products.

Exothermic Reactions: The energy of the reactants is greater than the products. Energy is released during the reaction in the form of light, heat or electricity.

Exothermic Reaction Reactants Products Takes very little activation energy to start the reaction Reactants have more energy stored in their bonds than the products The extra energy will be released in the form of heat and/or light Examples Fire, glow stick, food Activation Energy Reactants Energy given off Products

Endothermic Reactions: The energy of the reactants is less than the products. Energy is absorbed (the reaction requires energy to take place) Photosynthesis

Endothermic Reaction Products Reactants Takes lots of activation energy to start the reaction (hard to start) Products have more energy stored in their bonds than the reactants The extra energy is stored in the bonds of the molecules (like a battery) Examples Photosynthesis (energy absorbed from the sun), photo paper, instant ice pack Products Activation Energy Energy stored (absorbed) Reactants

9.2 Chemical Formula’s and Equations

Chemical Formula A shorthand notation for a compound or a diatomic element using chemical symbols and numbers

Chemical Formula’s The formula gives you the names of the elements and tells you how many atoms of each element are in that substance. The small numbers that are written below and to the right of the chemical symbols are called Subscripts, and notes the number of atoms of that element. A number in front of a chemical formula is a coefficient and it is multiplied by the subscript of all the atoms that are in the formula

Chemical Formula the shorthand notation for a compound using chemical symbols and numbers Symbols Subscript goes with the preceding symbol Coefficient Diatomic molecules O2 N2 Compounds H2O C6H12O6 Has 2 oxygen atoms Has 2 nitrogen atoms CO2 5 Has 2 hydrogen atoms and 1 oxygen Has 6 carbon, 12 hydrogen and 6 oxygen atoms

Counting Atoms AgNO3 2 MgCl2 Mg (HCO3)2 means 1 Magnesium, 2 Hydrogen, 2 Carbon atoms and 6 Oxygen atoms AgNO3 means 1 silver atom, 1 Nitrogen atom and 3 oxygen atoms 2 MgCl2 means 2 magnesium atoms and 4 chlorine atoms

Writing Formulas MgCl2 chloride For example Ionic compounds Magnesium chlorine AlBr3 Aluminum bromide Ionic compounds Remember there must be a metal and a nonmetal Metal comes first Add “ide” to the end of the nonmetal Ignore the subscripts chloride

Examples of ionic compounds (the charges of the ions must be balanced) Lithium fluoride Calcium oxide Lithium oxide Calcium fluoride metal nonmetal metal nonmetal Li Li+ F- F Li+ O-2 LiF Li2O LiO Charges are balanced Charges are balanced Charges are not balanced metal nonmetal metal nonmetal Ca+2 Ca O-2 O Ca+2 F- CaF2 CaF CaO Charges are balanced Charges are balanced Charges are not balanced

CO2 CH4 H2O Covalent compounds For example Only nonmetals are found Use the prefixes to show the subscripts “Mono” is not used for the first elements in the compound Add “ide” to the last nonmetal in the formula For example CO2 Carbon dioxide CH4 Carbon tetrahydride H2O Dihydrogen monoxide Prefixes Mono- 1 Hexa- 6 Di- 2 Hepta- 7 Tri- 3 Octa- 8 Tetra- 4 Nona- 9 Penta- 5 Deca- 10 Caution: This is a lethal substance (kills thousands of people each year), can be used for rocket fuel, causes billions of dollars in damage, found everywhere

Chemical Equations A shorthand description of a chemical reactions using chemical symbols and formulas. C + O2 CO2 (Yields) Reactants Products

Balancing Equations + Law of Conservation of Mass Example 2 1 2 Matter is not created or destroyed in ordinary chemical and physical changes This means that chemical equations must balance. The number and types of atoms that goes into a reaction is the same as what comes out. Example Hydrogen and oxygen mix to form water. H2 + O2 H2O 2 1 2 +

Equations can only be balanced by changing the coefficient, you can not change a subscript.

Balancing Chemical Equations Practice Follow along on your handout as we go through the program. More Review IN OUT