c101_topic-6 - 10/24/11 Aqueous Solu�ons Molarity  ...

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Unformatted text preview: 10/24/11 Aqueous Solu�ons Molarity   Pure substances account for many interes�ng and important rxns but chemical rxns most o�en take place in solu�on. Substances dissolved in liquid solu�on are called solutes. The liquid used to dissolve the solutes is called the solvent. Chemists use many different solvents but we will focus on water. When water is the solvent the solu�on is said to be aqueous.   Any solu�on contains at least two different chemical species; solvent and one or more solutes. To answer ques�ons such as “How much is there?” about solu�ons, we need to know the amount of each solute present in a known quan�ty of solu�on. The amount of a solute in a solu�on is given by the concentra�on which is the ra�o of the amount of solute to the amount of solu�on. We use the term molarity (M) which is the number of moles of solute divided by the total volume of the solu�on in liters. Molarity (M) = (moles of substance) = mol / L (total volume of solu�on) Problems Ionic Solu�ons   Dissolve 20.5 g of sugar (C12H22O11) in water and add enough water to a final volume of 250 mL. What is the molarity of the solu�on?   To understand the chemical behavior of solu�ons we must “think molecules.” Begin with the ques�on, “What chemical species are present in the solu�on?” For example, a solu�on of NaCl is best viewed as a solu�on containing three species: Na+, Cl-­‐ and H2O.   All salts dissolve according to their stoichiometric ra�o of ca�ons and anions. Thus, a salt that contains an equal number of ca�ons and anions gives a solu�on in which the molari�es of the ca�on and the anion are equal.   Zea�n is a plant growth hormone. How much zea�n would you need to make up a 500 mL solu�on at 0.1 M? Ionic Solu�ons Ionic Solu�ons   A 1.0 M solu�on of NaCl is 1.0M each in Na+ and Cl-­‐. One formula unit of NaCl breaks up into on Na+ ca�on and one Cl-­‐ anion.   On the other hand, ZnCl2, one of the ionic metal halides, contains 2 mol of Cl-­‐ anions for every 1 mol of Zn2+ ca�ons. This ra�o is maintained when zinc chloride dissolves in water: 1.0M solu�on of ZnCl2 is 1.0 M in Zn2+ and 2.0 M in Cl-­‐.   When a salt containing polyatomic ions dissolves in water, ca�ons separate from anions, but each polyatomic ion remains intact. For example, ammonium nitrate is composed of NH4+ polyatomic ca�ons and NO3-­‐ polyatomic anions. So ammonium nitrate dissolves in water to give a solu�on containing NH4+ ca�ons and NO3-­‐ anions. 1 10/24/11 Problems Dilu�ons   Find the molari�es of the ionic species present in 250 mL of an aqueous solu�on containing 1.75 g of ammonium sulfate, (NH4)2SO4.   A solu�on of high molarity is diluted by adding more solvent. In a dilu�on the quan��es of solutes remains the same but the volume increases. When the volume of a solu�on is increased without increasing the amounts of solutes, the result is a solu�on of lower molarity.   The quan�ta�ve aspects of dilu�ons can be summarized by remembering that moles of solute do not change during dilu�on. Thus,   How many grams of the blue crystalline solid, copper(II) sulfate pentahydrate, are required to prepare 750 mL of aqueous solu�on whose concentra�on is 0.255 M? Dilu�ons   Molessolutes,ini�al = Molessolutes,final   Because Moles = (Molarity) (Volume), this leads to the simple equa�on: MiVi = MfVf which can be used for dilu�on calcula�ons if we know any of the three quan��es. Stoichiometry of Reac�ons in Aqueous Solu�on   Precipita�on rxns can be treated quan�ta�vely. It is important that you understand everything that we have talked about to this point.   How much Fe(OH)3 will form when 50.0 mL of 1.50 M NaOH is mixed with 35.0 mL of 1.00 M FeCl3 solu�on? Problem   When aqueous solu�ons are used for intravenous infusions, it is essen�al that the molarity of the solu�on match the molarity of blood. Blood is approx. 0.31 M, so a solu�on of NaCl for infusion, called saline, must have a conc. of 0.155 M. Calculate the mass of NaCl that should be used to prepare 5.00 L of a 1.00 M stock solu�on and then calculate how many milliliters of the resul�ng stock solu�on must be diluted to give 455 mL of 0.155 solu�on for infusion. Problem   Silver bromide is a major component of photographic films and paper. When a film manufacturer mixed 75.0 L of a 1.25 M solu�on of silver nitrate with 90.0 L of a 1.5 M potassium bromide solu�on, 17.0 kg of silver bromide solid was obtained. What was the percent yield of this precipita�on rxn? 2 10/24/11 Problem Acid-­‐Base Reac�ons   A white precipitate forms when 200 mL of 0.200 M potassium phosphate solu�on is mixed with 300 mL of 0.250 M calcium chloride solu�on. Write the net ionic equa�on that describes this process. Calculate the mass of the precipitate that forms and iden�fy the ions remaining in solu�on.   One of the most fundamental and important chemical reac�ons is the combina�on of a hydroxide ion and hydronium ion to produce two molecules of water: OH-­‐(aq) + H3O+(aq) 2H2O(aq) Acid-­‐Base Reac�ons Acid-­‐Base Reac�ons Proton Transfer and Strong Acids Proton Transfer and Strong Acids   A H ca�on is a H atom that has lost its single electron. Removing the electron leaves a hydrogen nucleus, which is just a proton, so a hydrogen ca�on is the same thing as a proton. Thus, any rxn in which H+ moves from one species to another is called a proton transfer reac�on. Protons are unstable by themselves. In aqueous solu�on, they associate with water molecules to form hydronium ions.   The produc�on of water from hydroxide and hydronium ions is the most fundamental example of an acid-­‐base rxn. Any rxn in which a proton is transferred from one substance to another is an acid-­‐base rxn. In an acid-­‐base rxn an acid molecule donates a proton and a base molecule accepts the proton. Acid: A substance that donates protons Base: A substance that accepts protons. 3 10/24/11 Acid-­‐Base Reac�ons Acid-­‐Base Reac�ons   The previous rxn is called a neutraliza�on rxn. Keep in mind that not all acid-­‐base rxns are neutraliza�on rxns.   Acids and bases are abundant in chemistry. Any species that can give up a proton to another substance is classified as an acid; any substance that can accept a proton from another substance is classified as a base. Of the top 12 industrial chemicals, 3 are acids-­‐sulfuric acid, phosphoric acid and nitric acid-­‐and 3 are bases-­‐ammonia, calcium oxide and sodium hydroxide.   A solu�on of HCl is produced when HCl dissolves in water. In water, HCl acts as an acid because it donates a proton to a water molecule giving a hydronium ion and a chloride ion. In this rxn water acts as a base accep�ng a proton from an acid. Acid-­‐Base Reac�ons Weak Acids   HCl quan�ta�vely produces hydronium ions and chloride ions when it dissolves in water. The species present in an aqueous solu�on of HCl are Cl-­‐ and H3O+ and, of course, water. Any acid that undergoes quan�ta�ve rxn with water to produce hydronium ions and appropriate anion is called a strong acid.   All of the major six strong acids can donate a proton to hydroxide ions or water molecules. There are many other substances that donate protons to the hydroxide ion but cannot readily donate protons to water molecules. These compounds are called weak acids.   There are many weak acids and many are biochemical substances produced by living organisms. There are four major weak acids among the top 50 industrial chemicals: phosphoric acid, ace�c acid, terephthalic acid and adipic acid.   Each contains up to three acid H atoms that can be transferred as H+ to a strong proton acceptor such OH-­‐. Acid Nomenclature Acid Nomenclature   1. Halogen acids are named by using the prefix hydro-­‐ and the suffix –ic. In aqueous solu�on, HCl is hydrochloric acid.   2. An acid that contains a polyatomic anion whose name ends in –ate has a name ending in –ic. For example, H2CO3 contains the carbonate polyatomic anion, so H2CO3 is carbonic acid. Likewise HClO4 is perchloric acid.   3. An acid that contains a polyatomic anion whose name ends in –ite has a name ending in –ous. For example, HNO3 contains nitrite and is nitrous acid and H2SO3 is sulfurous acid.   4. In the chemical formulas of acids that are not carbon based, acidic hydrogen atoms are listed first: H2SO4, HCl and so on.   5. The names of acids containing CO2H all end in –ic and their corresponding anions end in –ate. The chemical formulas of these acids usually contain the CO2H group at the end of the formula: CH3CO2H is ace�c acid and C6H5CO2H is benzoic acid. 4 10/24/11 Bases Bases   Solu�ons with strong bases react readily with strong or weak acids because hydroxide anions are very good proton acceptors. NaOH(s) Na+(aq) + OH-­‐(aq) CaO(s) + 2H2O(l) Ca2+(aq) + 2OH-­‐(aq)   Ammonia, on the other hand, is an example of a weak base. A weak base does not readily accept protons from water molecules but it does accept protons from hydronium ions. Ammonia reacts quan�ta�vely with hydronium ions to generate ammonium ions. NH3(aq) + 2H3O+(aq) NH4+(aq) + H2O(l) Net Ionic Equa�ons Net Ionic Equa�ons   Aqueous solu�ons contain ions that may combine to form neutral solids. The balanced chemical eq. for such a rxn has ions as the reactants and neutral ionic solid as the product. Although one reactant carries posi�ve charge and the other carries nega�ve charge, the sum of all charges on the reactant side is the same as the sum of all charges on the product side.   Suppose we mix aqueous solu�ons of KOH and iron(III) chloride (FeCl3) to form a mixed solu�on that contains K+ and Fe3+ ca�ons, Cl-­‐ and OH-­‐ anions. A precipitate forms which analysis shows to be iron(III) hydroxide. Iron(III) ca�ons combine with hydroxide anions for form neutral Fe(OH)3 solid. This precipita�on rxn can be expressed with a chemical eq. Iron(III) ca�ons have +3 charges so the neutral solid must contain three OH-­‐ anions for every iron(III) ca�on. This balanced chemical eq. is called the net ionic eq. Fe3+(aq) + 3OH-­‐(aq) Fe(OH)3(s) Net Ionic Equa�ons Solubility Guidelines   A net ionic eq. contains all species that par�cipate in a chemical rxn. No�ce that neither K+ nor Cl-­‐ appears in the eq. for the precipita�on of Fe(OH)3. Although they are present in the solu�on these two ions undergo no change during the precipita�on rxn. Similarly, neither K+ nor NO3-­‐ appears in the eq. for the precipita�on of PbI2. Ions that are not involved in the chemical change are referred to as spectator ions. Spectator ions are omi�ed from the net ionic eq.   1. Salts that contain the following ca�ons are soluble: NH4+ and Group I metal ca�ons.   2. Salts that contain the following anions are soluble: nitrate, chloride, bromide, iodide, sulfate, hydrogensulfate, acetate and perchlorate.   3. Any salt not covered by guidelines 1 or 2 is insoluble.   4. There are several excep�ons to guildeline 2: AgX, PbX2, Hg2X2 (where X = Cl, Br or I), BaSO4 and PbSO4 are insoluble.   5. A few compounds not covered by guidelines 1 and 2 are nevertheless soluble: Ba(OH)2, MgS, CaS and BaS are soluble. 5 10/24/11 Reduc�on-­‐Oxida�on Reac�ons Reduc�on-­‐Oxida�on Reac�ons   Reduc�on-­‐oxida�on rxns occur when electrons from one chemical substance are transferred to another. Also known as redox rxns they include the rus�ng of iron, the bleaching of hair and the burning of gasoline. Before Reac�on A�er Reac�on Mg(s) Mg2+(s) H3O+(aq) H2(g) Cl-­‐ (aq) Cl-­‐ (aq) H2O(l) H2O(l) A H3O+(aq) (excess reagent)   Solid Mg has been transformed into Mg2+ ions and hydronium ions have decomposed to give H2 gas. Measurements reveal that for every mole of Mg consumed, 1 mol of H2 is produced and 2 mol of H3O+ are consumed.   The rxn of Mg metal with aqueous strong acid illustrates the fundamental principles of redox chemistry. When a piece of Mg is dropped into 6 M HCl solu�on, a vigorous rxn starts almost immediately. The metal dissolves and gas bubbles from the solu�on. Analysis of the gas shows that it is H2 and analysis of the solu�on reveals the presence of Mg2+ ions. Reduc�on-­‐Oxida�on Reac�ons   Mg(s) + H3O+(aq) + 3OH-­‐(aq) Mg2+(aq) + H2(g) 2H2O(l)   The loss of electrons by Mg atoms to form Mg2+ ca�ons indicates that this rxn between Mg metal and H3O+ is a redox process. An atom of Mg forms a Mg2+ ca�on by losing two electrons. Because electrons must be conserved in every chemical process, the electrons lost by Mg must be gained by some other species. In this example, the electrons lost by Mg are gained by H3O+ to form H2 and H2O. Reduc�on-­‐Oxida�on Reac�ons   Conserva�on of electrons is the basis of oxida�on and reduc�on. Gains and losses of electrons always occur together.   Oxida�on is the loss of electrons from a substance.   Reduc�on is the gain of electrons by a substance.   When Mg reacts with acid, Mg atoms are oxidized and hydronium ions are reduced.   Hydrogen is an oxidizing agent because it accepts electrons from a reducing agent. Reduc�on-­‐Oxida�on Reac�ons Reduc�on-­‐Oxida�on Reac�ons   Redox rxns are more complicated than proton transfer rxns or precipita�on rxns because the electrons transferred in redox chemistry do not appear in the balanced chemical eq. Instead, they are “hidden” among the star�ng materials and products. We can keep track of electrons by wri�ng two half-­‐reac�ons that describe the oxida�on and the reduc�on separately.   A half-­‐rxn is a balanced chemical eq. that describes either the oxida�on or reduc�on but not both. Thus, a half-­‐rxn describes half of the redox rxn. Mg(s) + H3O+(aq) + 3OH-­‐(aq) Mg2+(aq) + H2(g) + 2H2O(l) Mg(s) Mg2+(aq) + 2e-­‐ (oxida�on) 2H3O+(aq) + 2e-­‐ H2 + 2H2O (reduc�on) _____________________ 6 10/24/11 Reduc�on-­‐Oxida�on Reac�ons   A half-­‐rxn is a balanced chemical eq. that describes either the oxida�on or reduc�on but not both. Thus, a half-­‐rxn describes half of the redox rxn. Mg(s) + H3O+(aq) + 3OH-­‐(aq) Mg2+(aq) + H2(g) + 2H2O(l) Mg(s) Mg2+(aq) + 2e-­‐ (oxida�on) 2H3O+(aq) + 2e-­‐ H2 + 2H2O (reduc�on) _____________________ Mg(s) + 2H3O+(aq) Mg2+(aq) + 2H2O(l) Oxida�on Numbers Oxida�on Numbers   The oxida�on number of an atom is the apparent or real charge that an atom has when all bonds between atoms of different elements are assumed to be ionic.   Oxida�on numbers are useful because in every redox rxn, some atoms change their oxida�on numbers. Therefore, redox rxns can be iden�fied by no�ng changes in oxida�on numbers.   Guidelines   1. Each atom in a pure element has an oxida�on number of O.   2. The sum of the oxida�on numbers of all atoms equals the net charge on the species.   3. The halogens are almost always -­‐1.   4. When bonded to a nonmetal, H has an oxida�on number of +1.   5. The oxida�on number of O is -­‐2 in most compounds.   6. The most electronega�ve atom in a polyatomic species has an oxida�on number equal to its number of valence electrons minus eight. Problems Titra�ons   In class problems.   Analysis of acids and bases is o�en carried out by adding one to the other un�l the amount of acid exactly matches the amount of base. The solu�on to be analyzed is placed in a beaker or flask. Then the other solu�on, called the �trant, is added slowly by means of a calibrated measuring vessel called a buret. This process if called a �tra�on. CH3CO2H + OH-­‐ CH3CO2-­‐ + H2O 7 10/24/11 Titra�ons Titra�ons   Hydroxide ion is slowly added which accepts one proton from a molecule of acid. As the �tra�on proceeds, fewer and fewer acid molecules remain in the beaker, but the solu�on is nevertheless acidic. At the stoichoimetric or equivalence point, just enough hydroxide ions have been added to react with every transferable proton present in the beaker before the �tra�on was started. The hydroxide ion in the next drop of �trant do not react because acid molecules are no longer present in the solu�on. Before the stoichiometric point, the solu�on contains excess acid. A�er passing the stoichiometric point, the solu�on contains excess OH-­‐. Titra�ons Titra�ons   Stoichiometric point: mol OH-­‐ added = mol acidic hydrogen present   If we know the M and volume of the �trant, we can compute the number of moles of hydroxide required to react with all the acid.   An indicator is added to aid us in determining when the stoichiometric point has been reached. It is a molecule whose color depends on the concentra�on of OH-­‐ ions. Problem #1 Problem #2   Vinegar is acidic because it is a solu�on of ace�c acid in water. A 5.000 mL sample of vinegar is diluted to 150.0 mL with water and �trated with 0.1250 M NaOH. It takes 38.65 mL to reach the stoichiometric point. What is the molarity of ace�c acid in the vinegar?   A biological chemist needed to standardize a solu�on of KOH. A sample of potassium hydrogenphthalate (KHP) weighing 0.6745 g was dissolved in 100.0 mL of water. A drop of indicator was added to the KHP solu�on, which was then �trated with the KOH solu�on. The �tra�on required 41.75 mL of base to reach the stoichiometric point. Find the molarity of the KOH solu�on. 8 ...
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This note was uploaded on 11/17/2011 for the course CHEM 101 taught by Professor Scottnickolaisen during the Fall '11 term at California State University Los Angeles .

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