1. CHEMICAL REACTIONS Balancing all things Reactants: Zn + I 2 Product: ZnI 2

2.  Chemical reactions occur when bonds between the outermost parts of atoms (valence electrons) are broken then reformed.  Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes.  Symbols represent elements (C, O 2 ), Formulas describe compounds (H 2 O, NaCl), Chemical equations describe a chemical reaction. 2H 2 + O 2 2H 2 OIntroduction

3. Chemical Equations Their Job: a chemical shorthand for the reactants and products and their relative amounts in a reaction. 4 Al (s) + 3 O 2(g) → 2 Al 2 O 3(s) 4 Al (s) + 3 O 2(g) → 2 Al 2 O 3(s) The numbers in the front are called “coefficients” “coefficients” The letters (s) and (g) are the physical states of the compounds during the reaction.

4.  Chemical equations show the conversion of ReactantsReactants - the atoms and molecules on the left of the arrow (ORDER DOES NOT MATTER), into ProductsProducts - the atoms and molecules on the right of the arrow. (ORDER DOES NOT MATTER) A + sign separates molecules on the same side The arrow is read as “yields” or “reacts to form” Example: 2Na (s) + Cl 2(g)  2NaCl (s) This reads “two atoms of solid sodium plus one molecule of chlorine gas react to form two molecules of solid sodium chloride” Note that the coefficients are understood to be 1 when no coefficients are written. Parts of a Reaction Equation Reactants Products

5. Solid = (s) Pure Liquid = (l) Gas = (g) Aqueous solution = (aq) –Dissolved in water Catalyst = Pt Escaping gas = ( ) Precipitate (solid) formed = ( ↓ ) Heat is required = Δ Symbols Used in Equations

6. Because of the principle of the Conservation of Matter, “Cannot create or destroy matter” “Cannot create or destroy matter” Equations must be balanced. It must have the same It must have the same number of atoms on both number of atoms on both reactant and product sides. reactant and product sides. Lavoisier, 1788 Chemical Equations John Dalton, 1790

7. 7 Chemical Equation = Chemical Shorthand 2H 2(g) + O 2(g) → 2H 2 O (l) The subscript 2 on hydrogen tells how many atoms of hydrogen are in one molecule of hydrogen gas The 2 in front of hydrogen is called a coefficient and shows there are two molecules of hydrogen required to maintain conservation of mass in this reaction.

8.  When balancing a chemical reaction you may change the value of coefficients in front of the compounds not  but you may not change the subscripts on the compounds. Changing the subscripts changes the compound. Subscripts are determined by the ratio of charges on the anion and cation in the compound. Balancing Equations

9. Subscripts vs. Coefficients The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

10. 10 The Seven Diatomic Elements N 2, O 2, F 2, Cl 2, Br 2, I 2, H 2 Seven of the elements on the periodic table are diatomic (two atoms) naturally. The theme of the seven diatomics is “7” –There are 7, –They begin with element atomic number 7, Nitrogen –They form a “7” from Nitrogen to Iodine –The 7th is Hydrogen Remember, you will always have a periodic table so just remember the pattern of 7.

11. Chemical Equations 2Al (s) + Fe 2 O 3(s) → Al 2 O 3(s) + 2Fe(s) 2Al (s) + Fe 2 O 3(s) → Al 2 O 3(s) + 2Fe(s) This equation means: This equation means: 2 Al atoms + 1 Fe 2 O 3 molecule produces 2 Al atoms + 1 Fe 2 O 3 molecule produces 1 molecule of Al 2 O 3 and 2 molecules of Fe This reaction is called the thermite reaction and is reported to create a temperature of 3000 C!

12. There are four basic steps to balancing an equation: 1.Write the correct formula for the reactants and the products. DO NOT TRY TO BALANCE IT YET! 2.Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side. 3.Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation. 4.Check your answer to see if: –The numbers of atoms on both sides of the equation are now balanced. –The coefficients are in the lowest possible whole number ratios. (No fractions) Steps to Balancing Equations

13. Some Suggestions to Help You Take one element at a time, working left side to right side. Start with the most complicated and progress to the simplest. Leave free elements in the reaction for last. (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units

14. Balancing Equations Balancing Equations Balancing Equations Balancing Equations ___ Al(s) + ___ Br 2 (l) → ___ Al 2 Br 6 (s) 23

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16. Balancing Equations ___C 4 H 8 (g) + ___ O 2 (g) → ___CO 2 (g) +___H 2 O(g) 6 44 ___C 6 H 12 + ___O 2 → ___CO 2 + ___H 2 O 9 66

17. 17 Balancing Equations ___C 3 H 6 + ___O 2 → ___CO 2 + ___H 2 O __Na 3 PO 4 + __Fe 2 O 3 → __Na 2 O + __FePO 4 __Na 3 PO 4 + __Fe 2 O 3 → __Na 2 O + __FePO 4 3 22 9292 33 ___C 3 H 6 + ___O 2 → ___CO 2 + ___H 2 O 2966 Can’t leave a fraction so multiply by 2x

18. Balancing Equations __B 4 H 10 (g) + ___O 2 (g) → ___B 2 O 3 (g) + ___H 2 O(g) 11 4 2 10 2 5 11 2 Can’t leave a fraction so multiply by 2x __B 4 H 10 (g) + ___O 2 (g) → ___B 2 O 3 (g) + ___H 2 O(g)

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