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www.apchemsolutions.com Lecture 10 Aqueous Solutions and Chemical Reactions I Tutorial 1) Which solution is the best conductor of electricity (electrolyte)? Justify your answer. a. 1.0 M Li2(CO3), 1.0 M NH3, or 1.0 M LiCl All solutions are 1.0M, meaning that they all contain 1 mole of the formula unit in question per litre of water. NH3 is a covalent compound that does not dissociate into positive and negative ions; and thus, it does not conduct electricity. Li2(CO3) and LiCl are both soluble ionic compounds that dissociate in water; however, Li2(CO3) dissociates into three ions (Li2(CO3)(s) Æ 2 Li+(aq) + CO32-(aq)) whereas LiCl only dissociates into two ions (LiCl(s) Æ Li+(aq) + Cl-(aq)). As a solutions ability to conduct electricity increases as the concentration of ions in the solution increases, 1.0M Li2(CO3) is the best conductor. Write balanced net ionic equations for the reactions in questions 2 through 4. Remember, compounds containing group 1A ions, nitrate, and ammonium are always soluble. 2) A potassium hydroxide solution is mixed with a solution of zinc nitrate. The potassium ion is always soluble as it is a Group 1A element. Nitrate is also soluble with everything. Thus, K+ and NO3- must be spectator ions. Zn2+(aq) + 2 OH-(aq) Æ Zn(OH)2(s) 3) A solution of lead (II) nitrate is poured into a solution of sodium iodide. The sodium ion is always soluble as it is a Group 1A element. Nitrate is also soluble with everything. Thus, Na+ and NO3- must be spectator ions. Pb2+(aq) + I-(aq) Æ PbI2(s) 4) Solutions of ammonium chloride and silver nitrate are mixed. Ammonium and nitrate are soluble with everything. Thus, NH4+ and NO3- must be spectator ions. Ag+(aq) + Cl-(aq) Æ AgCl(s) © 2009, 2008 AP Chem Solutions. All rights reserved. 1 www.apchemsolutions.com 5) Determine the oxidation numbers on each atom in the following compounds. a. CF4 This is a covalent compound that does not contain hydrogen or oxygen atoms. Thus we set the oxidation number of the most electronegative element equal to its charge as an ion. Fluorine is the most electronegative element. As the fluoride ion has a charge of -1. Thus, the oxidation number assigned to fluorine in this molecule is also -1. Because there are four fluorine atoms in the compound and the overall charge on the species is zero, the oxidation number assigned to carbon is +4. b.H2SO3 Oxygen has an oxidation number of -2 and hydrogen has an oxidation number of +1. (3 oxygen atoms) x (-2) = -6 (2 hydrogen atoms) x (+1) = +2 -6 + (+2) = -4 Because there is one Sulfur atom in the compound and the overall charge on the species is zero, the oxidation number assigned to Sulfur is +4. -4 + (+4) = 0 6) Identify the substance that is oxidized and the substance that is reduced in each reaction. a. Zn(s) + H2SO4(aq) Æ ZnSO4(aq) + H2(g) Zn(s) is oxidized. Its oxidation number increases form 0 in Zn(s) to +2 in ZnSO4(aq). ZnSO4(aq) is the same as Zn2+(aq) + SO42-(aq). The oxidation number of a monoatomic ion (Zn2+) is equal to it charge as an ion. H2SO4(aq) is reduced. The oxidation number on hydrogen is reduced from +1 in H2SO4(aq) to 0 in H2. b.Zn(s) + Cu2+(aq) Æ Zn2+(aq) + Cu(s) Zn(s) is oxidized. The oxidation number increases form 0 to +2. Cu2+ is reduced. Theoxidation number is reduced from +2 to 0. © 2009, 2008 AP Chem Solutions. All rights reserved. 2