Introductory Chemistry, 3 rd Edition Nivaldo Tro. Roy Kennedy Massachusetts Bay Community College Wellesley Hills, Maqqwertd ygoijpk[l

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1 Introductory Chemistry, 3 rd Edition Nivaldo Tro Quantities in Car an octane and oxygen molecules and carbon dioxide and water Chemical Reactions Roy Kennedy Massachusetts Bay Community College Wellesley Hills, Maqqwertd ygoijpk[l 2009, Prentice? Hall

2 Outline 8.1 Global Warming : Too much Carbon Dioxide 8.2 Making Pancakes: Relationships between Ingredients 8.3 Making Molecules: Mole to Mole Conversions 8.4 Making Molecule: Mass to Mass Conversions 8.5 More Pancakes: Limiting Reactant, Theoretical Yield, and Percent Yield 8.6 Limiting Reactant, Theoretical Yield, and Percent Yield form Initial Reactants 8.7 Enthalpy: Measure of the Heat Evolved or Absorbed during a Chemical Reaction 2

3 8.1 Global Warming : Too much Carbon Dioxide 3

4 Burning of Fossil Car and molecules from chapter opening Fuels Internal combustion engines produe Carbon dioxide by burning fuels. For example octane, C 8 H 18, in gasoline. 2C 8 H 18 (l) + 25O 2 (g) 16 CO 2 (g) + 18H 2 O(g)

5 Global Warming Average 0.6 C rise in atmospheric temperature since In the same period atmospheric CO 2 levels have risen 25%. Are the two related? Figure 8.1 and Figure 8.2 Showing global tempertature Rise and greenhouse effect. 5

6 Increased CO 2 The primary source of the increased CO 2 levels is combustion of fossil fuels 1860 = Industrial Revolution in the U.S. and Europe. Methane in natural gas CH4(g) 2 O2(g) CO2(g) 2 H2O(g) 6

7 Quantities in Chemical Reactions All substances in a chemical reaction are related. The numerical relationships between compounds in a chemical reaction is called stoichiometry. 7

8 8.2 Making Pancakes: Relationships between Ingredients 8

9 Making Pancakes Pancakes are made with specific amounts of ingredients. Your book forgot buttermilk! Pancake recipe figure on page cup flour + 2 eggs + ½ tsp baking powder +3/4 cup buttermilk 5 pancakes 9

10 Making Pancakes, Continued You can scale the recipe up of down. Notice that for every 2 eggs 5 pancakes What if you have 8 eggs? pancakes 5 pancakes 8 eggs 2 eggs 20 pancakes Graphics illustrating the above equation 10

11 8.2 Making Pancakes: Relationships between Ingredients 11

12 Making Molecules Mole-to-Mole Conversions The balanced equation = chemical recipe 3 H 2 (g) + N 2 (g) 2 NH 3 (g) 3 molecules + 1 molecule 2 molcules H 2 N 2 NH 3 H-H H H H-H + N-N H-N-H H-N-H H-H 12

13 Molecule to Molecule and Mole to Mole conversions 3 H 2 (g) + N 2 (g) 2 NH 3 (g) This equation may be scaled up if the ratio of H 2 :N 2 :NH 3 is kept 3:1:2 Multiply by 4 12 H 2 (g) + 4N 2 (g) 8 NH 3 (g) Multiply by x x(6.022 x10 23 ) H 2 (g) + 1x(6.022 x10 23 )N 2 (g) 2x(6.022 x10 23 )NH 3 (g) This number of molecules can be expressed as moles. 3 mol H 2 (g) + 1 mol N 2 (g) 2 mol NH 3 (g) 13

14 The coefficients in a balanced equation give the number of molecules as well as the number of moles of each substance We can use the ratios of the coefficients to convert between moles of substances in a chemical reaction

15 8.3 Making Molecules: Mole to Mole Conversions 15

16 Molar Ratios 3 H 2 (g) + N 2 (g) 2 NH 3 (g) This equation has 6 molar ratios relating the reactant and products. For example H 2 and N 2 are related by 1 mole N 2 3 mol H 2 or 3 mol H 2 1 mole N 2

17 Mole to Mole Conversions 3 H 2 (g) + N 2 (g) 2 NH 3 (g) 12 moles of H 2 requires how many mole of N 2 The ratio must be H 2 :N 2 is 3:1, so so 12:4 is the same so 4 mole of N 2 is needed. How many moles of N 2 are needed for 1.74 mol of H 2 17

18 Mole to Mole Conversion 3 H 2 (g) + N 2 (g) 2 NH 3 (g) How many moles of N 2 are needed for 1.74 mol of H 2? We need to use the molar ratio. mol H 2 mole N 2 1 mole N mol H 2 x = mole N 2 3 mol H 2 18

19 8.4 Making Molecule: Mass to Mass Conversions 19

20 20

21 Making Molecules Mass-to-Mass Conversions We learned previously to convert between moles and grams using the molar mass. Combining this with moles to moles conversions allows us to related grams of one substrance in an equation to grams of another substance. g of A mol of A mol of B g of B Molar mass of A Coefficients of the Balanced Equation Molar mass of B 21

22 Example 8.2 How Many Grams of Glucose Can Be Synthesized from 58.5 g of CO 2 in Photosynthesis? Photosynthesis: 6 CO 2 (g) + 6 H 2 O(g) C 6 H 12 O 6 (s) + 6 O 2 (g) g of CO 2 mol of CO 2 mol of C 6 H 12 O 6 g of C 6 H 12 O 6 Molar masses needed (sum masses of all atoms) CO 2 = g/mol Glucose = g/mol 22

23 Grams moles moles grams 6 CO 2 (g) + 6 H 2 O(g) C 6 H 12 O 6 (s) + 6 O 2 (g) 58.5 g CO 2 1 mol CO 2 x = 1.33 mol CO g CO mol g CO 2 1 mol C 6 H 12 O 6 x = mol C 6 H 12 O 6 6 mol CO mol g CO g C 6 H 12 O 6 x = 40.0 g C 6 H 12 O 6 1 mol C 6 H 12 O 6

24 Another stoichiometry example How many g of Al 2 (SO 4 ) 3 are required to completely react with 24.7 g of Ba(NO 3 ) 2? Al 2 (SO 4 ) 3 (aq) + 3Ba(NO 3 ) 2 (aq) 3BaSO 4 (s) + 2Al(NO 3 ) 3 (aq) g of mol of mol of g of Ba(NO 3 ) 2 Ba(NO 3 ) 2 Al 2 (SO 4 ) 3 Al 2 (SO 4 ) 3 molar masses needed Ba(NO 3 ) 2 = g/mol Al 2 (SO 4 ) 3 = g/mol

25 Grams moles moles grams Al 2 (SO 4 ) 3 (aq) + 3Ba(NO 3 ) 2 (aq) 3BaSO 4 (s) +Al(NO 3 ) 3 (aq) 24.7 g Ba(NO 3 ) 2 1 mol Ba(NO 3 ) 2 x = mol Ba(NO 3 ) g Ba(NO 3 ) mol Ba(NO 3 ) 2 1 mol Al 2 (SO 4 ) 3 x = mol Al 2 (SO 4 ) 3 3 mol Ba(NO 3 ) mol g Al 2 (SO 4 ) g Al 2 (SO 4 ) 3 x = 10.8 g Al 2 (SO 4 ) 3 1 mol Al 2 (SO 4 ) 3

26 8.5 More Pancakes: Limiting Reactant, Theoretical Yield, and Percent Yield 8.6 Limiting Reactant, Theoretical Yield, and Percent Yield form Initial Reactants 26

27 Note in these two sections, we are only going to cover percent yield Actual; yield % Yield = x 100 Theoretical yield Actual yield amount actually obtained Theoretical yield amount obtained if reaction goes to 100% completion. 27

28 % yield example A is ran with 100 g of Hg reacting with oxygen grams of HgO is isoloted. What is the % yield? Hg(s) + O 2 (g) Molar masses: Hg = g/mol HgO(s) HgO = g/mol 28

29 % yield example (cont) First calculate the theoretical yield. g Hg mol Hg mol HgO g HgO 100 g Hg 1 mol Hg x g Hg mol Hg 1 mol HgO x 1 mol Hg = mol Hg = mol HgO mol Hg g HgO x = 108 g HgO (theoretical 1 mol HgO yield) 29

30 % yield example 75.0 g HgO % Yield = x g HgO = 69.4 % Yield of HgO. 30

31 Enthalpy Change We previously described processes as exothermic if they released heat, or endothermic if they absorbed heat. The enthalpy of reaction is the amount of thermal energy that flows through a process. At constant pressure. DH rxn 31

32 Sign of Enthalpy Change For exothermic reactions, the sign of the enthalpy change is negative when: Thermal energy is produced by the reaction. The surroundings get hotter. DH = For the reaction CH 4 (s) + 2 O 2 (g) CO 2 (g) + 2 H 2 O(l), the DH rxn = kj per mol of CH 4. For endothermic reactions, the sign of the enthalpy change is positive when: Thermal energy is absorbed by the reaction. The surroundings get colder. DH = + For the reaction N 2 (s) + O 2 (g) 2 NO(g), the DH rxn = kj per mol of N 2. 32

33 Enthalpy and Stoichiometry The amount of energy change in a reaction depends on the amount of reactants. You get twice as much heat out when you burn twice as much CH 4. Writing a reaction implies that amount of energy changes for the stoichiometric amount given in the equation. For the reaction C 3 H 8 (l) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(g) DH rxn = 2044 kj So 1 mol C 3 H 8 5 mol O 2 3 mol CO 2 4 mol H 2 O 2044 kj. 33

34 Enthalpy Calculation C 3 H 8 (l) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O(g) DH rxn = 2044 kj How much heat is released when 53.8 g of O2 completely reacts with propane? g O 2 mol O 2 heat (J) 53.8 g of O 2 1 mol O 2 x = 1.68 mol O g O mol of O J x = 687 J of heat 34 5 mol O 2

35 Example 8.7 How Much Heat Is Associated with the Complete Combustion of 11.8 x 10 3 g of C 3 H 8 (g)? Given: Find: Solution Map: Relationships: Solution: x 10 3 g C 3 H 8, heat, kj 1 8 mol C3H g 1 mol C 3 H 8 = kj, Molar mass = g/mol g C g C 3 H 8 3 H 8 1mol C3H g C H mol C 3 H kj 1mol C 3 H kj 1mol C H 3 8 kj kj Check: The sign is correct and the value is reasonable.

36 Practice How Much Heat Is Evolved When a g Diamond Is Burned? (DH combustion = kj/mol C) 36

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