Chemical Reactions

Evidence of a chemical reaction (Unexpected) color change Formation of a precipitate Formation of a gas Evolution of heat energy Evolution of odor

Chemical Equations Provide information about the reaction: ◦Formulas of reactants and products ◦States of reactants and products ◦Relative numbers of reactant and product molecules that are required ◦Can be used to determine mass of reactants used and of products that can be made

Writing equations Use proper formulas for each reactant and product Proper equation should be balanced - obey the Law of Conservation of Mass ◦all elements on reactants side also on product side ◦equal numbers of atoms of each element on reactant side as on product side

Symbols used in equations Symbols used after chemical formula to indicate state ◦(g) = gas; (l) = liquid; (s) = solid ◦(aq) = aqueous, dissolved in water ◦ ↑ - gives off or produces gas ◦ ↓ - produces a precipitate (ppt)

Writing Balanced Equations Steps: 1.Determine the reactants and products. 2.Write the reactants on the left side, and the products on the right. 3.Balance the equation by placing coefficients where needed (NEVER change subscripts in a formula in order to balance an equation). 4.Use the lowest whole number coefficients possible.

Example – Recognizing Reactants and Products When iron metal (steel wool) burns in air it produces a red compound iron (III) oxide ◦Burning in air means reacting with O 2 ◦Metals are solids, except for Hg which is liquid ◦Ionic compounds are solid unless they are dissolved in water (aq)

¬write the equation in words ◦identify the state of each chemical iron(s) + oxygen(g)  iron(III) oxide(s) ­write the equation in formulas ◦identify diatomic elements ◦identify polyatomic ions – count as 1 “unit” when balancing IF it does not change ◦determine formulas 4 Fe(s) + 3 O 2 (g)  2 Fe 2 O 3 (s)

Practice For each of the following, write (including states) and balance the chemical equation: 1.Aqueous aluminum nitrate reacts with aqueous sodium hydroxide to produce aluminum hydroxide as a precipitate and aqueous sodium nitrate. 2.Iron reacts with sulfuric acid (H 2 SO 4 ) to produce aqueous iron(III) sulfate and hydrogen.

Practice 3. Ethyne (C 2 H 2 ) gas reacts with oxygen to produce carbon dioxide and water. 4. Magnesium reacts with nitrogen to produce magnesium nitride.

Classifying Chemical Reactions - Reactions can be classified into 5 types. - Recognizing these types can be used to predict the products or reactants of a reaction. - Some reactions fit into more than 1 class

1. Single replacement = one element or ion replaces another element in a compound. 2 Al (s) + 3 CuCl 2 (aq)  2 AlCl 3 (aq) + 3 Cu (s)

2.Double replacement = positive & negative portions of two compounds interchange. Pb(NO 3 ) 2 (aq) + 2 KI (aq)  PbI 2 (s) + 2 KNO 3 (aq)

3. Decomposition = substance breaks down into simpler substances. 2 H 2 O 2 (l)  2 H 2 O(g) + O 2 (g)

4. Combination = two or more substances combine to form one new substance (Synthesis) 2 Mg (s) + O 2 (g)  2 MgO (s)

5. Combustion = oxidation of an organic compound; normally will form carbon dioxide and water. CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O(l)

Predicting products For each of the following, write (including states) and balance the chemical equation and give the reaction type: Ethane (C 2 H 6 ) gas burns in the presence of oxygen. Zinc metal reacts with aqueous copper(II) sulfate. Aqueous silver nitrate reacts with aqueous potassium chloride.

Aluminum metal reacts with (aqueous) hydrochloric acid. Aqueous sodium carbonate reacts with (aqueous) nitric acid. (Aqueous) nitric acid reacts with aqueous potassium hydroxide