Ch. 5 - Chemical Reactions I. Chemical Changes in Matter Chemical Reactions Law of Conservation of Mass Chemical Equations

Chemical Reaction A change in which one or more substances are converted to different substances. A+B  C+D REACTANTSPRODUCTS

Chemical Reaction Another name for a chemical change (1)New properties when you are done (2)No new atoms are made (3)Atoms are rearranged and new compounds can be made (4)Old bonds are broken; New bonds are formed

Indications of Chemical Reactions (1) Fizzing or bubbling (formation of a gas) (2)Change in color, odor, or properties (3) Temperature change( colder or hotter) (4)Formation of a precipitate Precipitate- solid formed in a liquid Precipitate- solid formed in a liquid ( turns cloudy) ( turns cloudy)

Starting a Reaction Always takes a little energy Energy goes into breaking bonds in the reactants Can use different forms of energy (1) Heat (2)Electricity (3)Light

Chemical reactions can be represented in two ways: (1) Word equations Liquid water yields gaseous oxygen and gaseous hydrogen (2) Chemical equation (skeletal) 2H 2 0(l)  O 2 (g) + 2H 2 (g)

Chemical Equations Aqueous lead(II) nitrate plus two units of aqueous potassium iodide produces solid lead(II) iodide and two units of aqueous potassium nitrate. Pb(NO 3 ) 2 (aq)+2KI(aq)  PbI 2 (s)+2KNO 3 (aq) Coefficient - # of units of each substance

Chemical Equations Describing Coefficients: individual atom = “atom” 2Mg  2 atoms of magnesium covalent substance = “molecule” 3CO 2  3 molecules of carbon dioxide ionic substance = “unit” 4MgO  4 units of magnesium oxide

Parts of a chemical equation Reactants Products 2Na(s)+ Cl 2 (g)  2NaCl(s) coefficient subscript yields state (1)Reactants- the substances you start with (2)Products- the substances you make (3)coefficient-number in front of formula that changes the number of atoms, (3)coefficient-number in front of formula that changes the number of atoms, (4)Subscript- number below symbol that indicates the number of atoms.

C. Chemical Equations

Law of Conservation of Mass Matter is not created or destroyed. Atoms can only rearrange. Discovered by Lavoisier. 4H 2O 4H 2O

Balancing Equations You must have the same number of each element on each side of the equation (must obey the law of conservation of mass- matter can not be created or destroyed.)

A. Balancing Steps 1. Write the unbalanced equation. 2. Count atoms on each side. 3. Add coefficients to make #s equal. Coefficient  subscript = # atoms 4. Reduce coefficients to lowest possible ratio, if necessary. 5. Double check atom balance!!!

  2 3  6   6  Al + CuCl 2  Cu + AlCl 3 Al Cu Cl B. Balancing Example Aluminum and copper(II) chloride form copper and aluminum chloride.

Moles and Reactions The coefficients of balanced equations tell how many particles react and how many moles of particles We can make ratios with those moles 2 Mg + O 2  2 MgO If 2 moles of Mg react, 1 mole of O 2 will be required 2 mol Mg or 1 mol O 2 1 mol O 2 2 mol Mg

III. Types of Reactions There are five types of reactions: *1.Synthesis *2. Decomposition *3.Double displacement *4. Single displacement 5. Combustion.

1. Synthesis (Composition) the combination of 2 or more substances to form a compound only one product forms A + B  AB 2H 2 + O 2  2H 2 O

2. Decomposition a compound breaks down into 2 or more simpler substances only one reactant AB  A + BAB  A + B 2NaCl  2Na + Cl 2

3. Single Replacement one element replaces another in a compound –metal replaces metal (+) –nonmetal replaces nonmetal (-) A + BC  AC + B Zn + CuSO 4  ZnSO 4 + Cu

4. Double Replacement ions in two compounds “change partners” cation(+) of one compound combines with anion(-) of the other AB + CD  AD + CB MgO + BaCl 2  MgCl 2 + BaO

C x H y +O 2  CO 2 +H 2 O Combustion CH 4 + 2O 2  CO 2 + 2H 2 O the burning of a substance in O 2 to produce heat hydrocarbons (C x H y ) always form CO 2 and H 2 O

Energy & Chemical Reactions Energy Changes Exothermic Reactions Endothermic Reactions

Energy Changes During a chemical reaction… –energy is used to break bonds –energy is released when new bonds are formed breaking bonds making bonds

(1)Exothermic Reaction reaction that releases energy in the form of light or heat. energy released by making new bonds outweighs energy required to break old bonds H 2 (l) + O 2 (l)  H 2 O(g) + energy –reaction that powers the space shuttle lift-off –Reaction by animals that glow, jellyfish, lightning bugs ( bioluminescence) Examples:

C. Endothermic Reaction reaction that absorbs energy, may feel cold. energy required to break old bonds outweighs energy released by making new bonds –process used to obtain aluminum from aluminum ore –Photosynthesis –CO 2 +H energy  C 6 H 12 O 6 + O 2 2Al 2 O 3 + energy  4Al + 3O 2 examples:

Substances that speed up a reactions are catalysts. Our bodies have catalysts called enzymes. Substances that slow down reactions are called inhibitors. Preservatives are inhibitors.

Molecular and Formula Mass The mass of all atoms in the formula of a compound or molecule. To calculate: 1. Write correct formula 2. Identify the number of different atoms in the formula 3. Multiply the number of atoms by their atomic mass. 4. Add all the masses together

Example: Magnesium Nitrate Mg(NO 3 ) 2 Mg = 1 x 24 = 24 N = 2 x 14 = 28 O = 6 x 16 = amu