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Chapter 8 Chemical Equations and Reactions 8-1 Describing Chemical Reactions Chemical reaction – process by which one or more substances are changed into one or more different substances Reactants – starting substances Products – ending substances Conservation of mass tells us that the mass of the reactants must be equal to the mass of the products 8-1 Chemical Equations Chemical equation – represents (with symbols and formulas) the identities and relative amounts of the reactants and products in a chemical reaction 8-1 Signs of a Chemical Reaction Evolution of heat and light Production of a gas/bubbles Formation of a precipitate Color change 8-1 Requirements for Properly Written Chemical Equations All reactants and products must be represented Correct symbols and formulas must be used for all substances in the equation (remember diatomics!) Reactants on left, products on right Arrow means “yields” or “forms” Law of conservation of matter must be satisfied – equations must be BALANCED 8-1 Balancing Chemical Equations Equations must be balanced to show the same number of atoms of each type on both sides of the equation Balance equations by writing coefficients Coefficient – a small whole number that appears in front of a formula in a chemical equation NEVER change subscripts! 8-1 Balancing Equations Write a balanced chemical equation for the formation of ammonia from its elements. 8-1 Balancing Chemical Equations Write a balanced chemical equation for the decomposition of sodium chloride to its elements. 8-1 Word and Formula Equations It is often helpful to begin by writing a word equation that represents all the facts about a chemical reaction. In a word equation, reactants and products are represented by words. methane + oxygen carbon dioxide + water 8-1 Word and Formula Equations A formula equation represents the reactants and products of a chemical reaction by their symbols or formulas. CH4 + O2 CO2 + H2O (unbalanced) 8-1 Additional Symbols Used in Chemical Equations (s) (l) (aq) (g) Yields Solid Liquid Aqueous gas 8-1 Reversible Reactions A reversible reaction is one in which products can re-form the original reactants 3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g) 8-1 Reactions Requiring Heat Some reactions will only proceed if the reactants are heated. 2HgO(s) 2Hg(l) + O2(g) 8-1 Practice Translate the following into a sentence: 2NaOH(aq) + MgCl2(aq) 2NaCl(aq) + Mg(OH)2(s) NaCl(aq) + AgNO3(aq) NaNO3(aq) + AgCl(s) 8-1 Practice Write a balanced chemical equation for the following processes: Solid calcium reacts with solid sulfur to produce solid calcium sulfide. Hydrogen gas reacts with fluorine gas to produce hydrogen fluoride gas. 8-2 Types of Chemical Reactions Using generalizations about the ways various substances interact with each other, we can predict the products of reactions There are five main types of chemical reactions. 8-2 Types of Chemical Reactions Synthesis Decomposition Single replacement Double replacement Combustion (of hydrocarbons) 8-2 Synthesis Reactions Two or more substances react to form one new compound A + X AX Recognize by SINGLE PRODUCT 8-2 Synthesis Reactions Reactions of elements with oxygen or sulfur FORM OXIDES OR SULFIDES Magnesium reacts with oxygen Barium reacts with sulfur Rubidium reacts with sulfur 8-2 Synthesis Reactions Reactions of metals with halogens FORM METAL HALIDES Sodium reacts with chlorine Potassium reacts with iodine Magnesium reacts with fluorine 8-2 Synthesis Reactions Metal oxides react with water to form metal hydroxides Calcium oxide reacts with water Magnesium oxide reacts with water 8-2 Synthesis Reactions Nonmetal oxides react with water to form acids Sulfur dioxide reacts with water Carbon dioxide reacts with water 8-2 Decomposition Reactions A single compound breaks down to produce two or more simpler substances AX A + X Recognize by single reactant Often require energy in the form of heat, light or electricity. 8-2 Decomposition Reactions Decomposition of binary compounds produces elements Decomposition of water Decomposition of sodium chloride Decomposition of magnesium mercury (II) oxide 8-2 Decomposition Reactions Decomposition of metal carbonates FORMS METAL OXIDE AND CARBON DIOXIDE Decomposition of calcium carbonate Decomposition of magnesium carbonate 8-2 Decomposition Reactions Decomposition of metal hydroxides FORMS METAL OXIDES AND WATER Decomposition of calcium hydroxide Decomposition of copper (II) hydroxide 8-2 Decomposition Reactions Decomposition of metal chlorates FORMS METAL CHLORIDES AND OXYGEN Decomposition of potassium chlorate Decomposition of sodium chlorate 8-2 Decomposition Reactions Decomposition of acids PRODUCES NONMETAL OXIDES AND WATER Decomposition of carbonic acid Decomposition of sulfuric acid 8-2 Single Replacement Reactions One element replaces another similar element in a compound A + BX AX + B OR Y + BX BY + X 8-2 Single Replacement Reactions Replacement of a metal by another metal Aluminum reacts with lead nitrate Sodium reacts with iron (II) chloride Calcium reacts with aluminum phosphate 8-2 Single Replacement Reactions Replacement of hydrogen by a metal Sodium reacts with water Magnesium reacts with hydrochloric acid Calcium reacts with nitric acid 8-2 Single Replacement Reactions Replacement of Halogens Chlorine reacts with potassium bromide Fluorine reacts with sodium chloride Fluorine reacts with potassium iodide 8-3 Activity Series of the Elements An activity series is a list of elements organized according to the ease with which the elements undergo certain chemical reactions For metals, greater activity means greater ease of LOSS OF ELECTRONS to form cations. For nonmetals, greater activity means greater ease of GAIN OF ELECTRONS to form anions. An activity series can be used to determine whether or not a single replacement reaction will take place. 8-3 Activity Series More active metals are HIGHER on the list. A more active metal can replace a less active metal on the activity series. Example: Na + CoCl2 Example: Cu + AlCl3 Example: Cr + HCl Example: Au + MgSO4 8-3 Activity Series Example: Cl2 + NaBr Example: Br2 + KF Example: Cl2 + MgBr2 8-2 Double Replacement Reactions The ions of two compounds exchange places to form two new compounds One of the products is usually a precipitate, gas or molecular compound (like water) AX + BY AY + BX 8-2 Double Replacement Reactions Formation of a precipitate Potassium iodide reacts with lead (II) nitrate Magnesium chloride reacts with sodium hydroxide 8-2 Double Replacement Reactions Formation of a gas Iron (II) sulfide reacts with hydrochloric acid 8-2 Double Replacement Reactions Formation of water An acid reacts with a base – also called a NEUTRALIZATION reaction Hydrochloric acid reacts with sodium hydroxide Sulfuric acid reacts with potassium hydroxide 8-2 Combustion Reactions A substance combines with oxygen, releasing a large amount of energy in the form of light and heat Hydrocarbons burn in oxygen A hydrocarbon is a compound composed of hydrogen and carbon 8-2 Combustion Reactions Combustion of hydrocarbons When a hydrocarbon reacts with oxygen, the products are carbon dioxide and water Combustion of butane Combustion of heptane 8-2 Simple Hydrocarbons (CnH2n+2) CH4 C2H6 C3H8 C4H10 C5H12 C6H14 C7H16 C8H18 C9H20 C10H22 methane ethane propane butane pentane hexane heptane octane nonane decane It’s over!