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Chapter 10 Chemical Reactions Evidence of Chemical Change A chemical reaction is a process by which the atoms of one or more substances are rearranged to form different substances. 4 Indicators Change in color Change in temperature or energy Production of a gas Formation of a precipitate Representing Balanced Chemical Equations Reactants Product 2 H 2( g ) O2( g ) 2 H 2O(l ) Coefficient Subscript Physical state Symbols + Separates two or more products or reactants Separates reactants from products (s) Solid (l) Liquid (g) Gas (aq) Aqueous Writing Chemical Equations Iron combines with oxygen to form solid iron (III) oxide 4Fe( s ) 3O2( g ) 2Fe2O3( s ) All metals are represented by their symbol in the periodic table with NO subscripts. All elemental gases are diatomic (BrINClHOF) Use your ion table or molecular name to figure out the chemical formula of compounds in your equation Balance the equation Note the physical state of each substance in the equation Practice Problem Aqueous calcium chloride combines with aqueous silver nitrate to form aqueous calcium nitrate and solid silver chloride. Another Practice Problem Aluminum combines with aqueous hydrochloric acid (HCl) to form aqueous aluminum chloride and hydrogen gas. Hint: Write the formula for each substance as if it were independent of the other substances in the equation. Balancing Chemical Equations A written chemical equation must abide by the law of conservation of matter Make sure that both sides of an equation have an equal number of each type of atom represented. Balance equations by adding coefficients. Never mess with subscripts Examples __ NaCl __ MgBr2 __ MgCl2 __ NaBr Na Cl Mg Br __ Mg __ FeCl3 __ MgCl 2 __ Fe Mg Fe Cl Strategies for Balancing Chemical Equations 1. The even/odd combination Make the subscript of one, the coefficient of the other and vice versa. Iron (III) oxide Iron + Oxygen 2Fe2 03 4Fe 3O2 Strategies Continued 2. Balance groups as a whole if the elements of the group are only represented in the group on each side of the equation. __ Mg (OH ) 2 __ Al __ Al (OH )3 __ Mg Mg OH Al Strategies Continued 3. Balancing Hydrocarbon Combustion Reactions The first step is to add a coefficient to the hydrocarbon so that the total number of hydrogens in the molecule is divisible by four. Then, balance the product side of the equation before finishing up with oxygen. __ CH 5 __ 02 __ CO2 __ H 2 0 C H O Strategies Continued 4. Splitting Water When hydroxide shows up on one side of an equation and water is on the other side, split water into H(OH) to balance out hydroxide. __ Mg (OH ) 2 __ H 2 SO3 __ MgSO3 __ H 2O Things to Keep in Mind Again, do not mess with subscripts In the final solution, the coefficients have to be in the lowest whole number ratio. 4H 2 2O2 4H 2O 2H 2 O2 2H 2O If at first you don’t succeed, try, try again! Most of the solutions to these problems come from trial and error. Practice Problem Write the equation Note the physical state of each substance Balance the equation Potassium chromate combines with lead (II) nitrate to form potassium nitrate and lead (II) chromate Types of Chemical Reactions – 5 General Formulas Combustion CxHy + O2 CO2 + H20 Always the products of these combustion reactions Synthesis A+B AB (Two elements combine to form a compound) Decomposition AB A+B (One compound splits to form two compounds) Reactions Cont. Single Replacement (Single Displacement) A + BC AC + B Metals replace metals Non-metals replace non-metals Double Replacement (Double Displacement) AB + XY AY + XB Compounds switch partners Pre-class Question 3/25 Combustion Decomposition Synthesis Single replacememt Double Replacement Identify the type of reaction 3Ba ( s ) Fe2 ( SO4 ) 3( aq) 2 Fe( s ) 3BaSO4 ( s ) 2C4 H10( g ) 13O2 ( g ) 8CO2 ( g ) 10 H 2 0( l ) 2 Mg ( s ) O2 ( g ) 2 MgO( s ) CaCl2 ( aq) Li2CrO4 ( aq) CaCrO4( s ) 2 LiCl( aq) 2 Fe2O3( s ) 4 Fe( s ) 3O2 ( g ) Predicting Products of Hydrocarbon Combustion Reactions With hydrocarbons, the products are always CO2 (g) and H2O(l) _ C8 H18( g ) _ O2( g ) Predicting Products of Synthesis Reactions For Synthesis Reactions For metals that only form one cation, determine the charge on the ion of each element (metallic and nonmetallic) and form a compound from the two ions. If one of the elements forms more than one cation or 2 nonmetals are combined, the products can only be predicted if the ratio of elements in the compound are given Ag( s ) Cl2 ( g ) Predicting Products of Decomposition Reactions HCl (aq ) Predicting Products of Double Replacement Reactions • Rearrangement of cations and anions to form 2 new compounds. • Reaction occurs when a solid or water is produced from aqueous reactants MgCl2( aq) Li3 PO4( aq) Predicting Products of Single Replacement Reactions Refer to the activity series chart to determine if the reaction will occur. Metals replace metals. Nonmetals replace nonmetals Cl2 ( g ) CaBr2 ( aq) Na( s ) AlCl3( aq) Physical States Atoms Metals – All solid except mercury (liquid) Nonmetals – all diatomic gases except bromine (liquid) and iodine (solid) Compounds Covalent compounds – usually a liquid or gas, but not predictable Acids – always aqueous (aq) Ionic compounds – refer to solubility table Soluble (dissolves) – aqueous (aq) Insoluble (doesn’t dissolve) – solid (s) Using a solubility Table for Ionic Compounds NaCO3 General Rules Table • Look up the solubility for a compound’s anion Common Ion Solubility Exceptions CO32- insoluble Group IA and NH4+ • If the substance is insoluble then it is a solid • If the substance is soluble it is aqueous • Check to see if the cation in your compound is in the exception column. Final Answer: NaCO3(aq) Determine the physical states CaSO4 Solid (NH4)2S Aqueous K3PO4 Aqueous CuC2H3O2 Aqueous