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Balancing Chemical Equations

I’m coming back to haunt you!. Balancing Chemical Equations. Section 5.3. But why?. “We have been practicing balancing.” Yes, for a reason. “So we know mass is conserved.” Yes, but there’s more. “Uh oh…”. Other reasons to balance….

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Balancing Chemical Equations

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  1. I’m coming back to haunt you! Balancing Chemical Equations Section 5.3

  2. But why? • “We have been practicing balancing.” • Yes, for a reason. • “So we know mass is conserved.” • Yes, but there’s more. • “Uh oh…”

  3. Other reasons to balance… • Finding how many moles of reactants will be needed to make a certain number of moles of products • Mole Ratios • Law of Definite Proportions • Calculating relative masses of reactants and products from a chemical equation! • In other words: “putting it all together!”

  4. Let’s Balance an Equation! • Mg + O2 MgO • Left: 1 Mg, 2 O • Right: 1 Mg, 1 O • Mg + O2  2MgO • Left: 1 Mg, 2 O • Right: 2 Mg, 2 O • 2Mg + O2  2MgO • Left: 2 Mg, 2 O • Right: 2 Mg, 2 O

  5. 2Mg + O2 2MgO = + • What can we learn from this? • Two Magnesium atoms and one oxygen molecule are required to produce Two MgO molecules. • Two dozen magnesium atoms and one dozen oxygen molecules are required to produce two dozen MgO molecules. • 2 moles magnesium atoms and 1 mole oxygen molecules are required to produce 2 moles of MgO molecules.

  6. A math problem: • 2a + b = 2c • What do you get if you multiply both sides of the equation by 2? • 4a + 2b = 4c • One more time, using chemical equations: • 2Mg + O2 2MgO • Multiply both sides of the chemical equation by 2 • 4Mg + 2O2  4MgO • 4 moles of Mg and 2 moles of O2 will yield 4 moles of MgO

  7. Graphically: + = 4Mg + 2O2  4MgO

  8. Sample problem: • How many moles of oxygen molecules are required to react with (unlimited) hydrogen to produce only 6 moles of water? • Step 1: Write the equation • H2 + O2 H2O • Step 2: Balance it • 2H2 + O2 2H2O • Step 3: Find the mole ratio of the reactants and products • H2 to O2 to H2O = 2:1:2 • Step 4: What do I have to multiply the ratio by so that water has a 6 in front of it? • 2:1:2 multiplied by 3: 6:3:6 • Step 5: Pick off the correct number for oxygen: • 3 moles of oxygen molecules are required to produce 6 moles of water

  9. One more: • How many moles of oxygen molecules are required to react with magnesium to produce 24 moles of Magnesium Oxide? • 2Mg + O2 2MgO • Mole ratio of Mg to O2 to MgO is 2:1:2 • How do I find the correct ratio??? • 2:1:2 times 12 = 24:12:24 • 12 moles of oxygen molecules are required.

  10. One last piece of the puzzle: • What mass of MgO can be produced with only 1 mole of O2 in the reaction we’ve been studying? • Mole ratio of Mg to O2 to MgO is 2:1:2 • Only 2 moles MgO can be produced. • Periodic Table to the Rescue: • Mg: 24.3 g/mol O: 16.0 g/mol • MgO: 24.3 + 16.0 g/mol = 40.3 g/mol. • 40.3 g/mol * 2 mol MgO = 80.6 g MgO

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