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UNIT VII

UNIT VII. Stoichiometry. VII.1 Coefficients in a Reaction Equation:. Stoichiometry : the relationship between the amount of reactants used in a chemical reaction and the amounts of products produced by the reaction. VII.1 Coefficients in a Reaction Equation:. 2Mg + O 2 → 2MgO

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UNIT VII

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  1. UNIT VII Stoichiometry

  2. VII.1 Coefficients in a Reaction Equation: Stoichiometry: the relationship between the amount of reactants used in a chemical reaction and the amounts of products produced by the reaction

  3. VII.1 Coefficients in a Reaction Equation: 2Mg + O2 → 2MgO • the coefficient 2, means two magnesium atoms react with one oxygen molecule to produce two molecules of magnesium oxide • if you double the amounts of Mg and O2, what will happen to the amount of MgO??? • if you use 10 times the amount of reactants, what will happen to the amount of MgO???

  4. VII.1 Coefficients in a Reaction Equation: • *the balanced equation describes the ratio in which the substances combine • In terms of MOLES: 2(6.02 x 1023)Mg + (6.02 x 1023)O2 → 2(6.02 x 1023)MgO • 2 mol Mg + 1 mol O2 → 2 mol MgO • The ratio Mg = 22 mol Mg O2 1 1 mol O2

  5. VII.1 Coefficients in a Reaction Equation: Ex: Consider the reaction equation 2Na + C2 → 2 NaCl a.)How many molecules of Na are required to react with 10 molecules of H2? b.) How many moles of NaCl are produced when 15 mol of Na are reacted? • p. 124 #1-5

  6. VII.2 Stoichiometry Calculations involving Moles, Mass, and Gas Volume and Molecules • * A chemical equation is written in terms of moles of reactants and products • To predict the moles of each substance, you can use the mole ratio • Stoichiometry calculations allow you to calculate the amount of CHEMICAL #1 involved in a reaction, based on the amount of CHEMICAL #2

  7. VII.2 Stoichiometry Calculations involving Moles, Mass, and Gas Volume and Molecules • **From UNIT V, we know how to get from L to moles, g to moles, molecules to moles etc.... • MOLES are the way to go from CHEMICAL #1 to CHEMICAL #2 MOLES of MOLES of CHEMICAL CHEMICAL #1 #2 • ALWAYS convert your values into moles ---- then cross the mole bridge!!

  8. VII.2 Stoichiometry Calculations involving Moles, Mass, and Gas Volume and Molecules Ex #1: How many moles of sodium metal would be needed to react with chlorine gas and make 737 g of sodium chloride? 2Na + Cl2 → 2NaCl

  9. VII.2 Stoichiometry Calculations involving Moles, Mass, and Gas Volume and Molecules Ex: #2 How many grams of potassium chloride, KCl, are produced by decomposing118 g of potassium chlorate, KClO3?

  10. VII.2 Stoichiometry Calculations involving Moles, Mass, and Gas Volume and Molecules Ex: #3 C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l) A sample of propane is burned, what mass of H2O(l) is produced if the reaction also produces 50.0L of CO2(g) at STP?

  11. VII.2 Stoichiometry Calculations involving Moles, Mass, and Gas Volume and Molecules Ex: #4 Consider the reaction… 4NH3 + 5O2 → 6H2O + 4NO How many molecules of NH3 are required to react with 3.00 mol of O2(g)? p. 127 # 6-7 ace, 8-12

  12. VII.3 Stoichiometry Calculations Involving Molar Concentration Molar Concentration: c = n/V • only use 22.4L/mol if it is a gas at STP

  13. VII.3 Stoichiometry Calculations Involving Molar Concentration Ex: # 1 If copper wire is put in a solution of silver nitrate, how many grams of copper will react to completely replace silver from 208 mL of 0.100M solution of silver nitrate, AgNO3? • Write the balanced equation for the reaction!

  14. VII.3 Stoichiometry Calculations Involving Molar Concentration Ex: #2 For the reaction: Zn + 2HCl → ZnCl2 + H2 a) What volume of 3.00 M HCl is required to react with 12.35g of zinc?

  15. VII.3 Stoichiometry Calculations Involving Molar Concentration Ex: #2 (continued) For the reaction: Zn + 2HCl → ZnCl2 + H2 b)How many moles of hydrogen are produced when 12.35g of zinc are reacted with the correct amount of HCl?

  16. VII.3 Stoichiometry Calculations Involving Molar Concentration Ex: #3 What volume of CO2(g) at STP is produced if 2.30 L of 0.5 M HCl reacts with an excess of CaCO3?

  17. VII.3 Stoichiometry Calculations Involving Molar Concentration TITRATIONS • Definition: a process used to determine the unknown concentration of a solution (SAMPLE) by reacting it with a measured amount of a solution with known concentration (STANDARD) until a desired equivalence point is reached.

  18. VII.3 Stoichiometry Calculations Involving Molar Concentration Equivalence Point = Stoichiometric Point • point in titration where the ratio of the moles of each species involved exactly equals the ratio of the coefficients of the species in the balanced reaction equation Ratio of COEFFICIENTS = Ratio of MOLES PRESENT • @ this point the indicator changes colour

  19. VII.3 Stoichiometry Calculations Involving Molar Concentration Ex: #1 H3PO4 + 2KOH → K2HPO4 + 2H2O What volume of 0.500 M KOH is required to react with 125mL of 0.300 M H3PO4 in order to produce a solution of K2HPO4?

  20. VII.3 Stoichiometry Calculations Involving Molar Concentration Ex: #2 H3PO4 + 2KOH → K2HPO4 + 2H2O If 19.8 mL of H3PO4 with an unknown molarity react with 25.0 mL of 0.500 M KOH, according to the above reaction, what is the molarity of the H3PO4? • p. 131 #17-24 - try #25!

  21. VII.4 STOICHIOMETRY OF EXCESS QUANTITIES • Often in a reaction, not all of the reactants are used up – frequently one or more reactants are in excess • LIMITING REAGENT: is the reactant you run out of first • EXCESS REAGENT: is the one you have left over

  22. VII.4 STOICHIOMETRY OF EXCESS QUANTITIES • ex: you are given one dozen loaves of bread, a gallon of mustard and three pieces of salami – how many salami sandwiches can you make??? • Limiting reagent = Salami • Excess reagents: bread and mustard • the limiting reagent determines how much product you can make

  23. VII.4 STOICHIOMETRY OF EXCESS QUANTITIES In order to FIND OUT which reactant is limiting…you do TWO stoichiometry problems: • the one that makes the least product is the limiting reagent!!

  24. VII.4 STOICHIOMETRY OF EXCESS QUANTITIES Ex: #1 Copper reacts with sulfur to form copper (I) sulfide. If 10.6g of copper reacts with 3.83g of S how much product will be formed?

  25. VII.4 STOICHIOMETRY OF EXCESS QUANTITIES Ex: #2 Mg(s) + 2HCl(g) → MgCl2(s) + H2(g) If 10.1 mol of magnesium and 4.87 mol of HCl gas are reacted, how many moles of gas will be produced? How much excess reagent remains?

  26. VII.4 STOICHIOMETRY OF EXCESS QUANTITIES Ex: #3 If 10.3 g of aluminum are reacted with 51.7g of CuSO4 how much copper will be produced? How much excess reagent will remain?

  27. VII.4 STOICHIOMETRY OF EXCESS QUANTITIES Ex: #4 When 79.1 g of zinc are reacted with 1.05L of 2.00M HCl, to produce ZnCl2, and H2gas, which reactant will be in excess and by how much? • p. 133 # 26-32

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