Stoichiometry (the math of chemical reactions) Unit 4 – Finally Part 3 Stoichiometry (the math of chemical reactions)

The Way of the Mole

Practice Make Perfect! – Molar Mass Example Calculate the molar mass of potassium sulfate.   Calculate the molar mass of ammonium chromate.

Grams to Moles & Moles to Grams Example What is the mass of 0.247 mol of calcium nitrate?   How many moles are in 26.7g of sodium sulfate?

Moles to Grams & Grams to Moles Example How many grams are in 3.04 mol of ammonia?   How many moles are in 12.4g of copper(II) phosphate?

Atoms/Molecules to Moles or Grams Example How many atoms are in 2.3 mol of Cu?   How many molecules are in 3.4mol of NaCl?

Atoms/Molecules to Moles or Grams Example How many atoms are in 45.6g of Al?   How many molecules are in 56.2g of AlCl3?

Atoms/Molecules to Moles or Grams Example How many grams are in 5.6 x 1024 molecules of BCl3?   How many moles are in 6.7 x 1034 atoms of bromine?

Learning to Use Mole Ratios! Get them from the BALANCED EQUATION Convert the number of moles Substance A to moles of Substance B 2H2 + O2 → 2H2O 2 mol 1 mol 2 mol Example of MOLE RATIOS 2 mol H2 2 mol H2 1 mol O2 1 mol O2 2 mol H2O 2 mol H2

How many moles of Br2 can be made from 5 moles of MgBr2? Sample Problems MgBr2 + Cl2  MgCl2 + Br2 How many moles of Br2 can be made from 5 moles of MgBr2?  

How many moles of H2O2 are needed to make 0.98mol of O2? Sample Problems 2 H2O2  2 H2O + O2 How many moles of H2O2 are needed to make 0.98mol of O2?  

How many grams of NaH can be made from 1.4g of H2 Sample Problems 2 Na + H2  2 NaH How many grams of NaH can be made from 1.4g of H2  

2 AgNO3 + ZnCl2  2AgCl + Zn(NO3)2 How many grams of Zn(NO3)2 Sample Problems 2 AgNO3 + ZnCl2  2AgCl + Zn(NO3)2 How many grams of Zn(NO3)2 can be made from 2.45g of ZnCl2?  

2 AgNO3 + ZnCl2  2AgCl + Zn(NO3)2 Sample Problems 2 AgNO3 + ZnCl2  2AgCl + Zn(NO3)2 How many molecules of ZnCl2 are needed to produce 6.45g of AgCl?  

2 AgNO3 + ZnCl2  2AgCl + Zn(NO3)2 Sample Problems 2 AgNO3 + ZnCl2  2AgCl + Zn(NO3)2 How many molecules of ZnCl2 are needed to make 3.2g of AgCl?  

Limiting Reactants + CB + 4 T CT4 plus 16 tires excess 8 car bodies http://www.brettschmidt.com/3d_images/garage/samples/car_body.jpg 8 car bodies 48 tires 8 cars CB + 4 T CT4

Limiting Reactants + C + 2 H2 CH4 plus 8 hydrogen molecules excess Methane, CH4 + plus 8 hydrogen molecules excess plus 16 hydrogen atoms excess http://www.brettschmidt.com/3d_images/garage/samples/car_body.jpg 8 carbon atoms 24 hydrogen molecules 8 methane molecules C + 2 H2 CH4

Container 1 Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 269

Before and After Reaction 1 N2 + H2 NH3 3 2 Before the reaction After the reaction All the hydrogen and nitrogen atoms combine. Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 269

Container 2 Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 270

Before and After Reaction 2 N2 + H2 NH3 3 2 excess limiting Before the reaction After the reaction LIMITING REACTANT DETERMINES AMOUNT OF PRODUCT Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 270

Real-World Stoichiometry: Limiting Reactants Fe + S FeS Ideal Stoichiometry Limiting Reactants S = Fe = excess LeMay Jr, Beall, Robblee, Brower, Chemistry Connections to Our Changing World , 1996, page 366

Limiting Reactants Limiting Reactant Excess Reactant used up in a reaction determines the amount of product Excess Reactant added to ensure that the other reactant is completely used up cheaper & easier to recycle If one or more of the reactants is not used up completely but is left over when the reaction is completed, then the amount of product that can be obtained is limited by the amount of only one of the reactants A limiting reactant is the reactant that restricts the amount of product obtained. The reactant that remains after a reaction has gone to completion is present in excess. Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem

Limiting Reactants aluminum + chlorine gas  aluminum chloride Al(s) + Cl2(g)  AlCl3 2 Al(s) + 3 Cl2(g)  2 AlCl3 100 g 100 g ? g A. 200 g B. 125 g C. 667 g D. 494 g

Limiting Reactants aluminum + chlorine gas  aluminum chloride 2 Al(s) + 3 Cl2(g)  2 AlCl3 100 g 100 g x g How much product would be made if we begin with 100 g of aluminum? How much product would be made if we begin with 100 g of chlorine gas?

Limiting Practice Problems Find the limiting reagent when 1.22g O2 reacts with 1.05g H2 to produce H2O. How many grams of water are produced?

4Al + 3O2  2 Al2O3 What mass of aluminum oxide is formed when 10.0 g of Al is burned in 20.0 g of O2? Be sure to identify the limiting and excess reagents.

Calculating the Amount of Excess – Bonus Points Silver nitrate and sodium phosphate are reacted in equal amounts of 137g each. Write the balanced equation. How many grams of silver phosphate are produced? How much of the excess reagent in #1 is left?

Limiting Reactant: Recap You can recognize a limiting reactant problem because there is MORE THAN ONE GIVEN AMOUNT. Convert ALL of the reactants to the SAME product (pick any product you choose.) The lowest answer is the correct answer. The reactant that gave you the lowest answer is the LIMITING REACTANT. The other reactant(s) are in EXCESS. To find the amount of excess, using the limiting determine how much of the excess you need then subtract the amount used from the given amount to find out how much you have left over. If you have to find more than one product, be sure to start with the limiting reactant. You don’t have to determine which is the LR over and over again!

Types of Reactions

Synthesis Reactions Two substances combine to make a single new substance Also called a combination reaction. A + B  AB HINT: If there is only one product – it is likely a synthesis. Examples Na + Cl2 → H2 + O2 →

Decomposition Reactions Single substance broken down into two or more substances. AB  A + B HINT: If there is only one reactant – it is likely a decomposition reaction. Examples CaCl2  Ca + Cl2 2H2O2  2H2O + O2

Single Displacements (Replacement) A reaction occurs when a single element takes the place of one of the elements in a compound. AB + Z  ZB + A Metals displace metals while nonmetals displace nonmetals. Examples – Are metals replacing metal or nonmetals replacing nonmetals? Fe + CuSO4  FeSO4 + Cu 2KI + Br2  2KBr + I2 2CuF + Ba  BaF2 + 2Cu

Not So Fast There… The lone element doesn’t always break up the couple! We can use a tool called the activity series to predict if the compound will stay together or break up. The activity series is a list of metals and hydrogen that are arranged in order of reactivity. (inside your periodic table) Elements high on the list can replace any element below it on the list. Be sure to use the correct side of the list.

Double Displacements (Replacement) A double displacement reaction always involves two ionic compounds that switch partners with each other. Again, positive ions switch with positive ions (and/or vice-versa). AB + XY  AY + XB HINT: Two couples switch partners at the dance. Example Pb(NO3)2 + 2KI  PbI2 + 2KNO3

Not So Fast There…Again! Must learn how to predict solubility! Use solubility rules (inside periodic table)

Combustion Reactions A combustion reaction occurs when a substance (the “fuel”) reacts very rapidly with oxygen to form carbon dioxide and water. Combustion reactions release a good deal of energy in a very short period of time. Hydrocarbon + O2  CO2 + H2O HINT: Something combines with oxygen to produce carbon dioxide and water.