CChemical Reactions are occurring all around you at all times. ›E›Examples: Digestion, Photosynthesis

AA chemical reaction occurs when elements join, separate or rearrange themselves. AA chemical equation is a description of a chemical reaction using the chemical formulas of the reactants and the products. ReactantsProducts The arrow can be read as “produces” or “forms.” Starting SubstancesEnding Substances

Example #1 The reaction of Hydrogen gas plus oxygen gas produces water. H2H2 + O2O2 H2OH2O A skeleton equation only shows the bare bones. It doesn’t show the ratios of the reactants or products. Practice Writing a Skeletal Equations

Example #2 Dicarbon hexahydride reacts with oxygen to form carbon dioxide gas and water. C2H6C2H6 + O2O2 H2OH2OCO 2 + Always remember: The reactants react to produce the products. Practice Writing a Skeletal Equations

Example #3 Aluminum sulfate reacts with Barium chloride to produce aluminum chloride and barium sulfate. Al 2 (SO 4 ) 3 + BaCl 2 BaSO 4 AlCl 3 + Practice Writing a Skeletal Equations

Additional Symbols: State of Matter  Indicate the state of matter of a reactant or product by using the following symbols after the formula. (s) = solid (l) = liquid (g) = gas (aq) = aqueous Practice Writing a Skeletal Equations

Example #4: Solid Zinc reacts with aqueous hydrochloric acid to produce hydrogen gas and aqueous zinc ( II ) chloride. Zn(s) + HCl(aq) H 2 (g) + ZnCl 2 (aq) Practice Writing a Skeletal Equations

Additional Symbols: Added Heat or Catalyst  Added heat or a catalyst are required for many reactions to either take place or to speed up the reaction.  For heat this is symbolized by placing the word “heat”, or the symbol ∆, over the →.  For a catalyst, the symbol or formula of the catalyst is placed over the →.  A Catalyst is a substance that speeds up the rate of a chemical reaction but is not used up in the reaction. It is neither a reactant nor a product. Practice Writing a Skeletal Equations

Al 2 O 3 ∆ AlO 2 + H 2 O 2 MnO 2 H 2 O+O 2 Example #5: Added Heat Example #6: Catalyst

C 6 H 12 O 6 + O2O2 H2OH2O+CO 2 (s)(g) (l)(g) ∆ Practice Writing a Skeletal Equations Example #7 Solid glucose (C 6 H 12 O 6 ) reacts with oxygen gas in the presence of heat to produce liquid dihydrogen monoxide and carbon dioxide gas.

Lets look the equation needed to make bicycles. Assume a bicycle is made of a just 4 parts: frame, wheels, pedals, and handlebars. The skeletal equation: frame wheelpedalhandlebarbicycle ++ However, this equation is incomplete because the ratios of the reactants are not correct. 2 2 In order to make the equation correct, we need to add coefficients in front of the wheel and pedal to indicate the correct number of parts needed. Balancing Equations +

Rules for Balancing Equations 1. Write the correct skeletal equation. 2. Count the number of atoms of each element in the reactants. 3. Count the number of atoms of each element in the products.  If a polyatomic ion appears on both sides of the equation, you may count the ion as a group.

Rules for Balancing Equations 4. Balance the elements one at a time by changing the coefficients. Never change subscripts. › Changing the coefficient will change the number of atoms of other elements in the compound. Therefore, may need to figure new coefficients several times.

H2H2 +O2O2 H2OH2O # of H = # of O = Example #1 Hydrogen plus Oxygen produces water Examples for Balancing Equations

AgNO 3 +CuCu(NO 3 ) 2 Ag + Ag = NO 3 = Cu = NO 3 = Ag = Cu = Examples for Balancing Equations Example #2: Silver ( I ) nitrate reacts with Copper metal to produce copper ( II ) nitrate and solid silver.

Zn(OH) 2 +H 3 PO 4 Zn 3 (PO 4 ) 2 +H2OH2O Zn = Zn = OH = HOH OH = H = H = PO 4 = PO 4 = *Helpful Hint* - if you have the hydroxide ion (OH) on one side of the equation and water on the other side, then re-write water as HOH or H(OH). Examples for Balancing Equations

More Examples for Balancing  Li + H 2 O → H 2 + LiOH  H 2 + O 2 → H 2 O  Al 2 O 3 → Al + O 2  Li + FeBr 2 → LiBr + Fe  C 2 H 6 + O 2 → CO 2 + H 2 O  P 4 + O 2 → P 4 O 10  Na 2 SO 4 + CaCl 2 → CaSO 4 + NaCl  Fe 2 O 3 + H 2 SO 4 → Fe 2 (SO 4 ) 3 + H 2 O  CaCl 2 + AgNO 3 → AgCl + Ca(NO 3 ) 2

Why balance equations? The reason is because of the Law of Conservation of Mass. Remember the Law of Conservation of Mass states that matter cannot be created or destroyed. The number of atoms of each element in the reactants must be equal to the number of atoms of each element in the products.

Mg + O 2  MgO If 48.6 grams of magnesium react to produce 80.6 grams of magnesium oxide, according to the law of conservation of mass, how much oxygen gas must have been consumed? Law of Conservation of Mass In the following examples, balance the equation. Then, answer the question that follows.

NO + H 2  NH 3 + H 2 O If 60 grams of nitrogen oxide react to produce 34 grams of nitrogen trihydride and 36 grams of water, according to the law of conservation of mass, how much hydrogen gas must have been consumed? Law of Conservation of Mass

If grams of calcium chloride react with 98.1 grams of sulfuric acid to produce grams of Calcium sulfate, according to the law of conservation of mass, how much hydrochloric acid must have been produced? CaCl 2 + H 2 SO 4  CaSO 4 + HCl