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Praxis II Chemistry prep Karen M. Morris Univ. of Notre Dame [email protected] 631-6945 The Atom What we observe vs. What we represent 6 C Carbon 12.011 12 6 C C C 13 6 14 6 Let’s Practice 1. Explain why atomic weights of elements often differ markedly from whole numbers. 3. In what way do the isotopes of a given element differ? 2. Determine the number of protons, electrons and neutrons in each: 235 92 U 22 10 Ne 59 28 Ni 1. Explain why one of the following is not a property of an element: a. boiling point b. density c. volume The Periodic Table What we observe vs. What we represent 115 Ds Rg Cn Uut Uuq Uup 116 Uuh 117 118 Uuo Let’s Practice 1. Compare the atomic size of row 4 elements to row 5 elements. Describe the general trend as one goes from left to right across the periodic table. 1. Arrange the following ions in order of increasing size: Mg2+, Na+, Al3+ 1. In the following pairs of ions, identify which is most likely to exist and explain why you think so. Ca2+ or Ca2S2+ or S24. Why is the radius of a positive ion smaller than the radius of its parent atom? Chemical Formulae and Equations What we observe vs. What we represent Forming Chemical Compounds Ionic Covalent Writing Chemical Equations Law of Conservation of Matter Let’s Practice 1. The chemical formula of the compound dopamine is C8H11O2N Would you expect this to be an ionic or covalent compound? Explain… 1. Identify the pure compound as ionic or covalent: HCl CuBr2 F2 Na3PO4 CCl4 AgCl Let’s Practice 1. Balance the following chemical equations a. H2 + Cl2 HCl b. Na + H2O NaOH + H2 c. Zn + AgCl ZnCl2 + Ag d. KClO3 KCl + O2 Observations of Matter 1. Representations can be atoms or large quantities of atoms 2. Experimentally discovered that all large quantities represented by one number (discovered by Avogadro) and measured as 6.022 x 1023 atoms 3. This is DEFINED as “1 mole” of matter 4. You measure this mass quantity in grams per mole. 1 mole of C contains 6.022 X 1023 atoms and masses 12.011 g 1 mole of H contains 6.022 X 1023 atoms and masses 1.008 g 1 mole of U contains 6.022 X 1023 atoms and masses 238.029 g Compounds 1 mole of carbon dioxide, CO2, contains 6.022 x 1023 molecules of CO2 and masses the sum of 1 Carbon plus 2 Oxygen: (12.011 g) + 2(15.99 g) = 43.99 g A.K.A. as a “molar mass” (mass of 1 mole) 1 mole of calcium carbonate, CaCO3, contains _________ molecules of CaCO3 and masses the sum of __________ Compounds We can manipulate the molar mass of CO2 to determine the mass percent of each element in the sample. This idea is the basis of quantitative analysis of materials. Molar Mass CO2 = (12.011 g) + 2(15.99 g) = 43.99 g Mass % of C in the sample = [(12.011)/(43.99)] * 100 = 27.3% Mass % of O in the sample = [2(15.99)/(43.99)]* 100 = 72.7% or 100% - 27.3% = 72.7% (the easy way ;-) Let’s Practice H2O H2 + O2 1. Something is “wrong” with this equation. Fix it. 2. After fixing it, determine the mass of each compound. 3. How many moles of hydrogen and oxygen do we get if we break down 3 moles of water? 4. How many grams of hydrogen to we get if we break down 3 moles of water? 5. What is the mass percent of hydrogen in water? Let’s Practice The chemical formula of the compound dopamine is C8H11O2N What is the mass percent of carbon, C, in dopamine? What is the molar mass of dopamine? What is the mass of 0.15 moles of dopamine? How many moles of C are in 0.15 moles of dopamine? How many grams of C are in 0.15 moles of dopamine? Let’s Practice a) H2 + Cl2 2 HCl a) Na + 2 H2O 2 NaOH + H2 a) Zn + 2 AgCl ZnCl2 + 2 Ag a) 2 KClO3 2 KCl + 3 O2 Physical Properties of Matter What we OBSERVE vs. what we REPRESENT 1. Ice cube melting 2. Melted puddle of water evaporating Phases of matter 1. Solid 2. Liquid 3. Gas Let’s Practice 1. Draw representations of solid, liquid and gas at the atomic level. How are your drawings different? How the same? 1. What happens to a gas volume when it is compressed? What happens to a liquid volume when it is compressed? What happens to a solid volume when it is compressed? 1. What happens to a gas volume when it is heated? What happens to a liquid volume when it is heated? What happens to a solid volume when it is heated? Chemical Changes What we OBSERVE vs. what we REPRESENT 1. Observed: a. Sometimes NOTHING!! Need instrumentation to measure b. Sometimes solid is formed, bubbles (gas), heat, light c. Reactant materials will not easily return. 2. Represented: a. Physical rearrangement of atoms b. Balance in number of atoms in reactants and products. Let’s Practice a) H2 (g) + Cl2 (g) 2 HCl (g) a) Na (s) + 2 H2O (l) 2 NaOH (aq) + H2 (g) a) Zn (s) + 2 AgCl (aq) ZnCl2 (aq) + 2 Ag (s) a) 2 KClO3 (s) 2 KCl (s) + 3 O2 (g) General Types of Reactions Reaction Types: What we OBSERVE vs. what we REPRESENT Precipitation: MgCl2 + 2 NaOH Mg(OH)2 + 2 NaCl 2 clear and colorless solutions poured together and a white solid appears Acid-Base Neutralization: HCl + NaOH H2 O + NaCl a very acidic and a very basic solution poured together to form a clear and colorless neutral solution (salt + water)