CHM1045 Lecture 10 Outline of Last Lecture I Chemical i Chemical reaction definition ii Chemical equation definition II How to read Chemical Equations III Balancing Chemical Equations i Step 1 ii Step 2 iii Step 3 iv Step 4 v Step 5 vi Example 1 Outline of Current Lecture I Amount of Reactants and Products i Example 1 ii Example 2 II Limiting Reagent i Example 3 ii Example 4 III Reaction Yield i Example 5 Current Lecture Amounts of Reactants and Products Mass g of compound A use molar mass g mol of compound A Mass of compound B use a mole ratio of A B from balanced equation Use molar mass of compound B Moles of compound A Moles of compound B 1 Write balance Equation 2 Convert Quantities of known substances into moles 3 Use coefficients in balanced equations to calculate the of moles of the sought quanitity 4 Convert moles of sought quantity into desired units Example 1 The food we eat is degraded or broken down in our bodies to provide energy for growth and function A general overall equation for this very complex process represents the degradation of glucose C6H12O6 to carbon dioxide CO2 and water H2O If 856 g of C6H12O6 is consumed by a person over a certain period what is the mass of CO2 produced Answer 856 C6H12O6 X 1 mol C6H12O6 4 750 mol C6H12O6 180 2g C6H12O6 4 750 molC6H12O6 X 6 mol CO2 28 50 mol CO2 1 mol C6H12O6 28 50 mol cO2 X 44 01g CO2 1 25 10 3 g CO2 1 mol CO2 Example 2 All alkali metals react with water to produce hydrogen gas and the corresponding alkali metal hydroxide A typical reaction is that between lithium and water How many grams of Li are needed to produce 9 89 g of H2 9 89g H2 X 1 mol H2 X 2 mol Li X 6 941g Li 68 1g Li 2 016 gH2 1 mol H2 1 mol Li Limiting Reagent Reactants used up first in the reaction 2NO O2 2NO2 NO is the limiting reagent O2 is the excess reagent Example 3 Urea NH2 2CO is prepared by reacting ammonia with carbon dioxide In one process 637 2 g of NH3 are treated with 1142 g of CO2 a Which of the two reactants is the limiting reagent b Calculate the mass of NH2 2CO formed c How much excess reagent in grams is left at the end of the reaction a moles of NH2 2CO 637 2g NH3 1 mol NH3 X 1 mol NH2 2CO 17 03 g NH3 2 mol NH3 18 71 mol NH2 2CO moles of NH2 2CO 1142gCO2 X 1 mol CO2 X 1 mol NH2 2CO 44 01 g CO2 1 mol CO2 25 59 mol NH2 CO2 NH3 must be the limiting reagent because it produces a smaller amount of NH2 2CO b mass of NH2 2CO 18 71 mol NH2 2CO x 60 06g NH2 2CO 1 mol NH2 2CO 1124g NH2 2CO c mass of CO2 reacted 18 71 mol NH2 2CO x 1 mol CO2 x 44 01gCO2 1 mol NH2 2CO 1 mol CO2 823 4 g CO2 The amount of CO2 remaing is the difference between the initial amount 1142g and the amount reacted 823 4g Mass of CO2 remaining 1142g 823 4g 319g Example 4 The reaction between alcohols and halogen compounds to form ethers is important in organic chemistry as illustrated here for the reaction between methanol CH 3OH and methyl bromide CH3Br to form dimethylether CH3OCH3 which is a useful precursor to other organic compounds and an aerosol propellant This reaction is carried out in a dry water free organic solvent and the butyl lithium LiC4H9 serves to remove a hydrogen ion from CH3OH Butyl lithium will also react with any residual water in the solvent so the reaction is typically carried out with 2 5 molar equivalents of that reagent How many grams of CH3Br and LiC4H9 will be needed to carry out the preceding reaction with 10 0 g of CH3OH Grams of CH3Br 10 00gCH3OH x 1 mol CH3OH x 1 mol CH3Br x 94 93g CH3Br 32 04g CH3OH 1 molCH3OH 1 mol CH3Br 29 6g CH3Br Grams of Li C4H9 10 00gCH3OH x 1 mol CH3OH x 2 5 mol LiC4H9 x 64 05g LiC4H9 32 04g CH3OH 1 molCH3OH 1 mol LiC4H9 50 0g LiC4H9 Reaction Yield Theoretical Yield is the amount of product that would result if all the limiting reagent reacted Actual Yield is the amount of product actually obtained from a reaction Yield Actual Yield Theoretical Yield X 100 Example 5 Titanium is a strong lightweight corrosion resistant metal that is used in rockets aircraft jet engines and bicycle frames It is prepared by the reaction of titanium IV chloride with molten magnesium between 950 C and 1150 C In a certain industrial operation 3 54 107 g of TiCl4 are reacted with 1 13 107 g of Mg a Calculate the theoretical yield of Ti in grams b Calculate the percent yield if 7 91 106 g of Ti are actually obtained A Moles of TI 3 54 x10 7g TiCl4 X 1 mol TiCl4 X 1 mol Ti 189 7 g TiCl4 1 mol TiCl4 1 87 10 5 mol Ti moles of Ti 1 13 10 7 gMg X 1 mol Mg X 1 mol Ti 23 31 gMg 2 mol Mg 2 32 10 5 mol Ti 1 87 10 5 mol Ti X 47 88g Ti 1 mol Ti B yield 8 95 10 6g Ti actual yield X 100 Theoretical yield 7 91 10 6 X 100 8 95 10 6g 88 4
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