Formulas, Equations, and Moles. I

Chemical Reactions and Equations Chapter 3 z z Formulas, Equations, and Moles: I z z What is a chemical reaction? A chemical equation uses symbols to represent what happens at the atomic level. What happens when you boil water? What happens during the electrolysis of water? Chemical equations follow the Law of Conservation of Mass Reaction  Conservation of atoms, not conservation of Equation molecules (molecular structures can change) 2 Balancing Equations z z z Balancing Equations Adjust coefficients, not subscripts, to balance an equation Coefficients are whole-number integers in the lowest ratio Reactants Æ Products z H2 + O2 Æ Coefficients vs. Subscripts H2O 3 Balancing Equations z z z z z z z Draw structures for the following substances: H2, Br2, HBr, CO, CO2, Cl2, AlCl3, N2, H2, NH3 Use your drawings to balance the equations b l below. Add more substances b t as needed d db by adding coefficients. H2 (g) + Br2 (l) Æ HBr (g) CO (g) + O2 (g) Æ CO2 (g) Al (s) + Cl2 (g) Æ AlCl3 (g) N2 (g) + H2 (g) Æ NH3 (g) Guidelines to Balancing Equations z z z z z Write correct formulas for reactants and products. Begin balancing with the most complex formula. formula Balance polyatomic ions as a single unit. Check each reactant and product to verify the coefficients. Mg(NO3)2 + Cr2(SO4)3 Æ MgSO4 + Cr(NO3)3 6 1 Practice Balancing Equations z Balance the following equations:          z Practice Balancing Equations Gummy Bear z Na + H2O Mg + O2 Æ MgO H2 O 2 Æ H2 O + O 2 Alkali metals in H2O NaN3 Æ Na + N2 CaCO3 Æ CaO + CO2 Mg3(PO4)2 + NaOH Æ Mg(OH)2 + Na3PO4 KClO3 Æ KCl + O2 Na + H2O Æ NaOH + H2 AgNO3 + MgCl2 Æ AgCl + Mg(NO3)2 Burn methanol CH3OH + O2 Æ CO2 + H2O Balanced equations:  2 Mg + O2 Æ 2 MgO         Worked Ex. 3.1 - 3.3; Problems 3.1 - 3.3 2 H2 O 2 Æ 2 H2 O + O 2 2 NaN3 Æ 2 Na + 3 N2 CaCO3 Æ CaO + CO2 Mg3(PO4)2 + 6 NaOH Æ 3 Mg(OH)2 + 2 Na3PO4 2 KClO3 Æ 2 KCl + 3 O2 2 Na + 2 H2O Æ 2 NaOH + H2 2 AgNO3 + MgCl2 Æ 2 AgCl + Mg(NO3)2 2 CH3OH + 3 O2 Æ 2 CO2 + 4 H2O 7 Avogadro’s Number and the Mole z z z We need to be able to determine amounts of chemicals to use so they react in the exact ratio needed. For example example, we need 2 molecules of H2 and 1 molecule of O2 to make 2 molecules of H2O. Do you want to count individual atoms and molecules? 8 Avogadro’s Number z Who wants to count: Peppers? Water Molecules? Size of atoms Grains of rice? The Mole Avogadro’s Number and the Mole z z z z We can measure the mass of a pure sample but how do we know how many atoms or molecules are in that sample? Mole: amount of a substance that contains a specific number of particles. A mole (mol) is a quantity that tells us two things: z z 1 12C atom weighs 12 amu, 1 mole of 12C atoms weighs 12 grams. 1 dozen = 12 objects; 1 mole = 6.022 x 1023 objects This is Avogadro’s objects. Avogadro s number number. The moral: 1 mole of carbon-12 weighs 12 grams and contains 6.022 x 1023 atoms.  the number of particles in a substance  the mass of that substance (containing a specific number of particles) 2 Avogadro’s Number and the Mole z How big IS Avogadro’s number?  An Avogadro's number of standard soft drink cans would cover the surface of the earth to a depth of over 200 miles.  If you had Avogadro's number of unpopped popcorn kernels, and spread them across the US, the country would be covered in popcorn to a depth of over 9 miles.  If we were able to count atoms at the rate of 10 million per second, it would take about 2 billion years to count the atoms in one mole. Avogadro’s Number and the Mole z z z z Demo: 1 mole of different substances What do these substances have in common? Atoms measured in amu, moles measured in grams amu and g are the same value, different units  1 atom 12C = 12 amu; 1 mol 12C atoms = 12 g  1 atom 4He = 4 amu; 1 mol 4He atoms = 4 g 14 Avogadro’s Number and the Mole z 1 mole contains 6.0221415 x 1023 particles Molar Masses z  Particles can be atoms, ions, molecules, or formula units z Determine what type of particle Avogadro’s Avogadro s number will describe for the following:  Kr  C6H12O6  MgCO3  SF6  CaBr2 P Calculate Molar Masses of the following: z z z z z z z z z Al CO2 Cl2 SO2 CaCl2 H2SO4 AlCl3 Mg3(PO4)2 Na2CO3 z z z z z Atomic, molecular, and formula masses are not practical in lab. We can scale up to calculate the mass of a mole of substances. Molar mass is the same value as atomic mass, just with different units. Molar mass (g/mol: (mass per 1 mole of substance) What is molar mass of C6H6? What is molar mass of Mg(OH)2? Molar masses have 6.022 x 1023 particles in the substance. Worked Ex. 3.4; Problems 3.4 - 3.6 Avogadro’s Number and the Mole z Conversions:  If we know the mass of a substance, how can we calculate number of moles? z mass Æ moles (use molar mass)  If we know how many moles of a substance, how can we calculate number of particles? z moles Æ number of particles (use Avogadro’s number) 1 mol molar mass (in g) 1 mol 6.02 x 10 23 particles 3 Mole Diagram Practice – Group Work 1. 2. Mass 3 3. 4. Mole 5. 6. Particles 7. 8. How many moles are in a 2.67 g sample of Ag? How many grams are in a 0.45 mol sample of Mg3(PO4)2? How many atoms are in a 16 16.3 3 g sample of S? How many formula units are in 0.81 g of MgCl2? How many ions are in 0.81 g of MgCl2? (Hint: How many ions are in 1 formula unit of MgCl2?) How many moles of ions are in 0.81 g of MgCl2? How many C atoms are in 3.64 g of C6H12O6? How many O atoms are in 0.0765 g of CO2 Courtesy: www.unit5.org/christjs/Stoichiometry/Mole%20Island%20Diagram.ppt Practice – Group Work z z z z Calculate the number of atoms that are in a 6.00 g sample of carbon. Calculate the number of moles in 12.04 x 1023 atoms of uranium. Calculate the mass of 12.04 x 1023 atoms of uranium. If you have 1 g samples of NaCl, CO2, and H2O, which sample will contain the fewest particles? 4