Physical Science - Ch 11 Notes

PSCI 170: Physical Science I

Spring 2004

Solutions to Homework •

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Chapter 11: You need to know the differences between ionic and covalent compounds. Be able to describe the differences based on the things you know about how these different compounds act when dissolved in water – what elements make up each type of compound – examples of each type of compound. Remember that the conductivity detector activity that we performed in laboratory class was another way of distinguishing metals and nonmetals and ionic compounds and covalent compounds. Be able to determine the ions that will be formed by the main group elements by using the periodic table. You are able to do this because the roman numeral portion of the main group (A) elements tells you the number of outer shell or valence electrons. You should be able to use the crossover technique to write the empirical formula of an ionic compound. You should be able to look at the formulas and say whether the substances are ionic or covalent.

Be able to answer all of the Applying the Concepts questions. These are the multiple choice questions at the end of the chapter. You will see a few of these, or questions very similar to these, on your exam. You were to Hand in Questions for Thought: 2,3,7,9,11,13,14. 2. What is the difference between a polar covalent bond and a non-polar covalent bond? Answer: A polar covalent bond is between two nonmetal atoms of significantly different electronegativities and a nonpolar covalent bond is between two nonmetal atoms of similar electronegativities. 3.What is the difference between an ionic and a covalent bond? Answer: An ionic bond is formed between a cation and an anion, between a metal cation and a nonmetal anion. Covalent bonds are formed between nonmetal atoms. Ionic bonds are formed by transfer of electrons from the metal to the nonmetal. Covalent bonds are formed by sharing electrons. 7. Sodium fluoride is often added to water supplies to strengthen teeth. Is sodium fluoride ionic, nonpolar covalent or polar covalent? Explain the basis of your answer. Answer: Sodium fluoride, formed from a metal, sodium and a nonmetal, fluorine. Therefore, this would be ionic. 9. What kinds of elements are found in (a) ionic compounds with a name ending with an “-ide” suffix? (b) covalent compounds with a name ending with an “ide” suffix? Answer: ‘ide would mean anions, so these are the nonmetal parts of the substances. Such as chlorides and fluorides. For the covalent compounds, the second element will end in the ide. Can be some of the same elements as for the anions of the ionic substances. 11. What are variable charge ions? Explain how variable-charge ions are identified in the modern systematic system of naming compounds. Answer: Variable charged ions are ions of the transition metals. They can have more than one charge. For example, copper can form Cu2+ or Cu+. They are indicated by putting a roman numeral in parentheses that indicates the charge on the ion. So Cu(II) and Cu(I). 13. Write the formula for magnesium hydroxide. Explain what the parentheses mean. Answer: Mg(OH)2. Need to use the parentheses to show that there are 2 hydroxide ions – two of the polyatomic ion. So you have to use parentheses when you have more than one polyatomic ion.

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14. What is a double bond? A triple bond? Answer: A double covalent bond is formed when two atoms share two pairs of electrons. A triple bond is formed when two atoms share three pair of electrons. Hand in Group B Parallel Exercises: 1,3,4,5,7. 1. Use electron dot symbols in quations to predict the formulas of the ionic compounds formed between the following. While this question asks you to use electron dot structures of the elements to predict the formulas of the ionic compounds that are formed, use the rules that I gave you with the periodic table. You wrote down those rules from lecture classes. IA elements form ions of +1 charge; IIA elements form ions of +2 charge; IIIA elements form ions of +3 charge; VA elements form ions of – 3 charge; VIA elements form ions of – 2 charge; VII elements form ions of – 1 charge. a) Li forms the ion Li + and F forms the ion F-. Therefore these ions form the compound LiF b) b) Be forms the ion Be 2+ and S forms the ion S2-. By the crossover technique this would mean that this would form Be2S2. However, this is not the simplest ratio so each of these subscripts can be divided by 2, so the empirical formula is BeS. c) Li forms the ion Li+ and O forms the ion O2-. By the crossover technique when these come together they would form a substance with the empirical formula Li2O. d) Al forms the ion Al3+ and S forms the ion S2-. Therefore by the crossover technique when these two come together they would form a substance with the empirical formula Al2S3. 3. Name the following polyatomic ions. Answer: You can name these polyatomic ions by looking at table 11.4. If you are asked anything about polyatomic ions the formulas and charges will be given to you. Except, for the hydronium ion, H3O+ and the hydroxide ion, OH-. Those you should know because they are involved with the properties of acids and bases. So look at these from the table. 4. Use the crossover technique to write formulas for the following compounds: a) Aluminum hydroxide , Al3+ and OH- , Al(OH)3 b) Sodium phosphate Na+ and PO43, Na3PO4 c) Copper(II)chloride Cu2+ and Cl- , CuCl2 d) Ammonium sulfate NH4+ and SO42 so the formula would be (NH4)2SO4e) Sodium hydrogen carbonate , Na+ and HCO3- so the formula would be NaHCO3 f) Cobalt (II)chloride, Co2+ and Cl-, so the formula would be CoCl2. 5. Write formulas for the following covalent compounds. a) Silicon dioxide, SiO2 b) Dihydrogen sulfide, H2S c) Boron trifluoride, BF3 d) Dihydrogen dioxide, H2O2 e) Carbon tetrafluoride, CF4 f) Nitrogen trihydride, NH3 7. I would not expect you to be able to answer questions like these for a quiz or an exam. I would only expect you to recognize ionic versus covalent bonds. a) Si and C, covalent, fairly nonpolar b) Cl and Cl, covalent c) S and O, covalent – probably not very polar d) Sr and F, ionic e) O and H, covalent

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f) K and F, ionic

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