GENERAL CHEMISTRY I
GENERAL CHEMISTRY I CHEM 111
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This 0 page Class Notes was uploaded by Dean Jacobs on Sunday November 1, 2015. The Class Notes belongs to CHEM 111 at New Mexico State University taught by Staff in Fall. Since its upload, it has received 12 views. For similar materials see /class/233227/chem-111-new-mexico-state-university in Chemistry at New Mexico State University.
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Date Created: 11/01/15
4 Chapter 4 Reactions in Aqueous Solution 41 Lecture 1 060502 Exam Notes Class average 716 very good for this exam since we covered some difficult material The material gets harder not easier Last spring my class averages started at 718 on the rst exam and dropped down to the low 60 s on the rest of the exams That s the bad news The good news is this exam harder than last spring s rst exam we covered all of chapter 3 So you re doing well Rule of thumb The average on the rst exam is roughly the average for the course Sub sequent exams drop the exam average but lab and quiz scores raise it So if your grade on this test is what you want in the course you re in good shape If you re reasonably close you have lots of opportunity to improve Two more midterms plus the nal One of the most important habits you can develop in college is to learn from your mis takes On Monday I will give you a closedbook inclass quiz taken directly from this exam I ll post a copy of the exam and the key online later this morning you should review your exam and make sure you understand what you missed Then you should get some points on Monday Bring a calculator Some speci c trouble spots on the exam See Overhead Chapter 4 Most chemistry we are interested in and almost all of chemistry related to life takes place in water in aqueous solution A solution is de ned a homogeneous mixture of two or more substances The substance present in the largest quantity is called the solvent The substances present in the smallest quantity is called the solute A solution can be in any phase gas liquid or solid In this chapter water is the solvent Solutes in water may be gases liquids or solids Airv ethanol and sugar or salt All solutes that dissolve in water can be categorized as either electrolytes or nonelectrolytes electrolyte A substance that when dissolved in water results in a solution that conducts electricity nonelectrolyte A substance that when dissolved in water does not affect the conductivity The origin of this behavior is in the formation of ions Write an equation for dissolving NaCl in water and discuss transport of ions The conductivity aparatus has electrodes that are positively and negatively charged Ions in the solution are attracted to the electrode with the charge opposite the ionic charge This motion of ions leads to electrical conductivity Show Overhead of strong and weak electrolytes and nonelectrolytes Why are some substances strong electrolytes and others weak Compare HCl and HF HCl when dissolved in water yields H and Cl HF when dissolved in water yields H and F They are very similar except in one important point HCl dissociates completely in water while HF does not We say that the HF dissociation process is reversible and indicate this with a second arrow HFF HF39 The ions formed by this reaction may recombine to form HF a neutral molecule that doesn t contribute to the solution conductivity Thus the overall conductivity for HF is less than that for H0 since HF does not completely dissociate Strong electrolytes are either strong acids ones that completely dissociate strong bases soluble hydroxides or other soluble ionic compounds Weak electrolytes are weak acids weak bases like ammonia Slightly soluble ionic com pounds behave like weak electrolytes also lonic compounds are listed under electrolytes however since we are focusing on species that do dissolve in water Water is itself a weak electrolyte since it can dissociate into ions H200 xi Haq OHaq Nonelectrolytes are usually molecular compounds that donlt dissociate at all Unlike most solvents water is quite effective at dissolving ions W hy It is surprising that any solvent dissolves ionic substances The oppositely charged ions in the ionic crystal are strongly attracted yet they can be separated in water This is because water is a polar solvent It is a bent molecule with a net negative charge on the oxygen and a net positive charge on the hydrogen The negative charge on water interacts favorably with the cations The positive charges on water interact favorably with the anions The net effect are very stable hydrated ions The stability of the hydrated ions is great enough that the ionic compound can sacri ce its ionic bonds to form these ion water species In sections 2 4 in this chapter we will introduce three classes of chemical reactions in solution These arenlt the only types of reactions possible but give us a very good start understanding solution chemistry Section 2 Precipitation reactions Section 3 Acid Base reactions Section 4 Oxidation Reduction reactions When you see a reaction written try to recognize which type it is I ll show you how as we go Precipitation reactions are characterized by the formation of an insoluble product a precipitate when solutions are mixed together PbN3gaq 2NaIaq gt PbIg 2NaN3aq To predict whether a precipitate will form or not you need to learn some solubility rules See overhead of Table 4 2 Note that these rules are phrased in a yes no fashion either soluble 0r insoluble In fact all salts are soluble to at least a very small extent and no salt is in nitely soluble We will quantify this in a later chapter 1 All alkali metal compounds are soluble NaI and sodium nitrate in the above example 2 All ammonium compounds are soluble 3 All compounds containing nitrate chlorate and perchlorate are soluble 4 Most hydroxides are insoluble Exceptions are the alkali metal hydroxides and the heavy alkalineearth hydroxides We consider BaOHg to be soluble and CaOH2 to be slightly soluble 5 Most compounds containing the halides Cl Br and I are soluble Exceptions are those with Ag39 Hg and Pb2 6 All carbonates phosphates and sul des are insoluble except those with alkali metals and the ammonium ion 7 Most sulfates are soluble lalcium sulfate and silver sulfate are slightly soluble Barium sulfate mercuryII sulfate and lead sulfate are insoluble 44 Chapter 4 Lecture 4 Review question Oxidation number is de ned the charge an atom would have in a molecule or ionic compound if electrons were transferred completely for binary ionic compounds like l39aCl the oxidation number for each atom is equal to the charge that we learned about previously For more complicated compounds we need some rules to determine oxidation numbers The species being reduced is the species whose oxidation number is reduced This occurs through the gain of electrons Haq in this case The species being oxidized is the species whose oxidation number is increases This occurs through the loss of electrons Zns in this case The species being reduced is also called the oxidizing agent The species being oxidized is also called the reducing agent To answer questions like this one for more complicated reactions we need some rules for assigning oxidation numbers See Handout 41 Note Ox quiz due on Monday has some compounds you don t know how to do based on the rules given F20 CN I ll give you 100 on the quiz if you get at least 90 correct lomplete the handout show the balancing method for redox reactions in their lab For the reaction in the following unbalanced equation Haq Fe2 MnO aq gt Mn2 Fe3aq H200 1 Determine the oxidation number of each element in each species 2 Label the species being oxidized and reduced Which is the oxidizing agent 3 Determine the number of electrons that are transferred in the oxidation and reduction processes 4 Balance the number of electrons transferred 5 Balance the equation In your lab you were told to balance the charge well the other species That procedure yields the same answer If you balance the number of electrons transferred you don t need to balance the charge Handout 42 Due at end of class Trends in oxidation numbers these t the rules also but it s nice to just know the trends Overhead of Fig 48 Metals have only positive oxidation numbers Transition metals have several possible oxida tion numbers Nonmetals may have positive or negative oxidation numbers Negative numbers down to their charge we discussed previously Positive numbers can go high their column number 6 in column 6 7 in column 7 etc Activity Series Many metals are readily oxidized Some downright insist upon being oxidized if given even the slightest chance We can order the metals in terms of their tendency to be oxidized Show overhead of Fig 411 The review question at the beginning is an example of a reaction of a metal with hydrogen ions from an acid Write out an example of the reaction with water to form hydrogen gas and the metal hy droxide 21Vas 211200 gt 21Vaaq H2g 20H aq Metals can also react with metal ions in the series For example if zinc metal is placed in contact with copper ion the zinc dissolves to form le lC ion and the copper is plated out Zns Cu2aq gt Zn2aq Cus You used this reaction in one of your labs Question Why is gold a good metal to use in jewelry besides being pretty and easily shaped It is very stable