Chapter 8 Chemical Reactions We will not cover Section 8 5 Periodic Trends in Reactivity of the Main Group Elements Chemical Equations Definitions A chemical reaction is a process in which atoms are rearranged to form chemical compounds In a chemical reaction neither energy nor matter is created or destroyed Chemical equations represent the change from reactants to products by the use of chemical formulas Interpreting and Writing Chemical Equation Reactants Products Reactants Product 2 moles of hydrogen react with 1 mole of oxygen to form 2 moles of water The physical state of the substance is indicated as g gas liquid s solid aq aqueous solution 2 KNO3 s 2 KNO2 s O2 g CO2 g H2O H2CO3 aq Writing and Balancing Chemical Equations 1 I A 1 2 B 1 2 C 1 2 3 Write the correct formula for the reactants and the products Because matter is neither created nor destroyed in a chemical reaction the total mass of the reactants must equal the total mass of the products Balance the equation by placing coefficients in front of the formulas of the reactants and products While there are no specific rules for balancing chemical equations there a few guidelines a b c d Balance elements other than hydrogen and oxygen first Treat polyatomic ions as a unit Balance hydrogen and oxygen after other elements have been balanced Check to be sure that you have the same number of atoms of each kind on both sides of the equation 4 Balance the following equations NH4NO3 N2O H2O Fe H2O Fe3O4 H2 HCl CaCO3 CaCl2 H2O CO2 AgNO3 Ca3 PO4 2 Ag3PO4 Ca NO3 2 C2H6 O2 CO2 H2O D 1 Patterns of Chemical Reactivity Types of Chemical Reactions Combination two or more reactants combine to form a single product N2 g 3 H2 g 2 NH3 g 2 Decomposition the breaking up of a complex reactant into simpler products CaCO3 s CaO g CO2 g 3 Combustion burning of a substance in oxygen CH4 g 2 O2 g CO2 g 2 H2O II Combustion Analysis 2 Earlier we calculated the empirical formulas from mass and percentage data In this section we will calculate the empirical formula from the combustion products A 1 2 a b c 2 Determination of Empirical Formula In a combustion reaction the mass of carbon dioxide and water can be measured How to use the information from the combustion products Calculate the mass of the desired element in the combustion products From the mass of CO2 the mass of C can be calculated and from the mass of H2O the mass of H can be calculated The mass of the remaining element which is usually oxygen can be calculated by subtracting the mass of C and H from the total mass of the sample Covert the mass of each element to moles then convert moles to small whole numbers and that is the empirical formula Sample calculation Menthol a strong smelling substance used in cough drops is a compound of carbon hydrogen and oxygen When 0 1595 g of menthol was burned 0 449 g of CO2 and 0 184 g of H2O formed What is the molecular formula of menthol if the molar mass of menthol is 156 g mol a Find the mass of each component C H and O mass of C mass of H mass of O b Convert mass to moles and to small whole numbers 3 moles of C moles of H moles of O empirical formula mass of empirical formula molecular formula molar mass mass of empirical formula Molecular formula III Calculations Based on Balanced Chemical Equations Balanced chemical equations can be used to predict the amount of product that will form if the amount of reactant is known or the percent yield for a given reaction or how much reactant is needed to form a given amount of product The use of chemical equation in this manner is referred to as stoichiometry A 1 Moles of Reactants and Products Give the molar relationship of FeS2 to all the reactants and products in the chemical equation 4 FeS2 s 11 O2 g 2 Fe2O3 s 8 SO2 g 2 Consider the reaction 4 4 FeS2 s 11 O2 g 2 Fe2O3 s 8 SO2 g How many moles of SO2 forms when 5 27 moles of oxygen react with excess FeS2 If 3 45 moles of SO2 form how many grams of Fe2O3 form B Mass of Reactants and Products Most frequently amounts of reactants are given in terms of grams rather than moles and since a chemical equation relates reactants and products in moles the masses must be converted to moles in order to determine the amount of product s The general sequence for calculating the amount of product in grams is Reactant Product mass moles molar relationship in equation moles mass Consider the equation Fe2O3 s 3 CO g 2 Fe s 3 CO2 g How many grams of iron III oxide must react with carbon monoxide to produce 125 grams of iron How many grams of carbon monoxide are needed to react with 97 4 grams of iron III oxide C Limiting Reagent 5 a b Consider the recipe 2 cups flour 1 cup sugar 2 eggs 12 cupcakes 2 cups flour 1 cup sugar 1 egg cupcakes Needed Available The egg limits the number of cupcakes that can be made It is the limiting reactant The other two ingredients are in excess A limiting reagent problem is one in which two starting amounts we have to decide which will determine the maximum amount of product that can form are given and To determine the limiting reagent calculate the amount of product that would form from each reactant and the one that forms the smaller amount of product is the limiting reagent Consider the reaction 4 FeS2 11 O2 2 Fe2O3 8 SO2 What is the maximum amount of Fe2O3 that can form from 57 8g of FeS2 and 92 5 g of O2 FeS2 O2 D Reaction Yield The amount of product in a chemical reaction can range from a large amount 90 or more to a fraction of a percent Generally the reaction yield is expressed in terms of percentage In any chemical reaction the amount of product recovered is always less than the theoretical yield The theoretical yield is the maximum possible yield based on the chemical reaction that is the calculated amount theoretical yield yield actual yield x 100 What is the percent yield for the above reaction if 32 7 grams of Fe2O3 formed actual yield theoretical yield 6 E Review Calculation Limiting reagent maximum amount of product percent yield amount of excess reagent remaining Consider the reaction SrH2 s 2 H2O Sr OH 2 s 2 H2 g Calculate the maximum amount of hydrogen in grams that can form from the reaction of 5 00g SrH2 and 4 80g of H2O Which is the limiting reactant What is the percent yield if 0 183 grams of hydrogen are produced What is the maximum amount in grams of Sr OH 2 that can form in this reaction How many …