Presentation on theme: "Chapter 4: Types of Chemical Reactions and Solution Stoichiometry"— Presentation transcript:
1Chapter 4: Types of Chemical Reactions and Solution Stoichiometry Mr. Redmond
2Aqueous Solutions dissolved in water Properties: Electrical ConductivityStrong Electrolytes- conducts wellWeak Electrolytes- conducts littleNonelectrolytes- does not conduct
3Strong ElectrolyteSubstances that completely ionize when dissolved in waterTypes:Soluble SaltsStrong AcidsStrong Bases
4Soluble Salts NaCl dissolved in water Practically all of the sample becomes Na+ and Cl- ions
5Acids (Sour)Arrhenius states: an acid is a substance that produces H+ ions (protons) when it is dissolved in waterHCl, HNO3, and H2SO4 all virtually ionize when placed in water, therefore are considered strong acids.
6Bases (Bitter) When dissolved in water it produces OH-, hydroxide. Strong bases: NaOH, KOH
7Weak Electrolytes When placed in water they produce few ions. Weak AcidWeak Base
8Weak AcidAcetic Acid, dissociates only to a slight extent in an aqueous solution. Only about 1% of its molecules ionize.HC2H3O2(aq) H+(aq) + C2H3O2-(aq)
9Weak Base Ammonia solution is basic because it produces some OH- ions NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)
10NonelectrolytesSubstances that dissolve but do not dissolve into ions that can conduct electricityEthanol, C2H5OH and sucrose, C12H22O11
11Molarity Molarity= moles of solute per volume of solution in liters M = Molarity = (moles of solute)/(liters of solution)Solute- a substance dissolved in a liquidSolution- a homogeneous mixture
12Sample Ex. 4.2You dissolve 1.56 g of gaseous HCl in enough water to make 26.8 mL of solution. Find the molarity.
13Sample Ex. 4.2 cont.First we calculate the number of moles of HCl......Change volume from mL to LNow divide and find Molarity
14Sample Ex. 4.4Calculate the number of moles of Cl- ions in 1.75 L of 1.0 x 10-3 M ZnCl2Formula: ZnCl2(s) Zn2+(aq) + 2Cl-
15Sample Ex. 4.4 cont. You have 1.0 x 10-3 M ZnCl2 After it dissociates you have 1.0 x 10-3 M Zn2+ and 2.0 x M Cl-Use the known information to calculate moles of Cl-
16Sample Ex. 4.5Typical blood serum is about 0.14 M NaCl. What volume of blood serum contains 1.0 mg NaCl?
17Sample Ex. 4.5 cont. How many moles of NaCl are in 1 mg NaCl? What volume of 0.14 M NaCl solution contains the number of moles of NaCl in 1 mg NaCl?
18Sample Ex. 4.6To analyze the alcohol content of a certain wine, a chemist needs 1.0 L of M K2Cr2O7 (potassium dichromate) solution. How much solid K2Cr2O7 must be weighed out to make this solution?
19Sample Ex. 4.6 cont.Determine the moles of potassium dichromate required.Convert moles into grams
20DilutionsMost times when you purchase chemicals for a lab they come in a concentrated form call stock solutions.Water is added to these to give the desired molarity of the particular solution
21DilutionImportant to understand that: moles of solute after dilution = moles of solute before dilutionFor example: if we need 500 mL of 1.00 M acetic acid from a 17.4 M stock solution of acetic acid, we would need to dilute it.
22DilutionFirst, determine the number of moles of acetic acid in the final solution......We then use the volume of 17.4 M acetic acid that contains the calculated amount of molesWe then use this information to solve for the volume
23Sample Ex. 4.7What volume of 16 M sulfuric acid must be used to prepare 1.5 L of a 0.10 M H2SO4 solution?
24Sample Ex. 4.7 Determine moles of H2SO4 in 1.5 L of a 0.10 M H2SO4 Find the volume of 16 M H2SO4 that contains 0.15 mol H2SO4
25DilutionThe equation: M1V1 = M2V2 , may help you remember that the moles before dilution is equal to the moles after dilutionRepeat the last example using this equation
26DilutionM1V1 = M2V2M1=16M, M2= 0.10M, V2=1.5 LSolve for V1
27Types of Reactions Precipitation reactions Acid-base reaction Oxidation-reduction reaction
28Precipitation Reactions Sometimes when two solutions are mixed an insoluble substance is formed, this is called a precipitation reactionThe solid substance that separates from the solution is called the precipitate
29Precipitation Reactions Say we perform a precipitation reaction with an aqueous solution of potassium chromate K2CrO4(aq), which is yellow, and mix it with a colorless aqueous solution containing barium nitrate Ba2(NO3)2(aq)When these are mixed a yellow precipitate forms
30Precipitation Reactions What is the yellow solid forming?
31Precipitation Reactions Remember in virtually every case, when a solid containing ions dissolves in water, the ions separateFrom this we know that we have K+, CrO42-, Ba2+, and NO3-
32Precipitation Reactions The reaction is: K2CrO4(aq) + Ba(NO3)2(aq) productIt may be easier to see what is happening if we write the equation a little different K+(aq)+CrO42-(aq)+Ba2+(aq)+2NO3-(aq) productWhat are the different ways these ions could combined?
33Precipitation Reactions Here are our choices: K2CrO4, KNO3, BaCrO4, or Ba(NO3)2We can quickly exclude K2CrO4 and Ba(NO3)2 since they are the reactantsIt takes a little background knowledge to figure the rest out
34Precipitation Reactions K+ ion and the NO3- ion are both colorless, thus KNO3 would precipitate white.CrO42- ion is yellow as noted earlier thus the yellow solid must be BaCrO4But what happened to K+ and NO3-?
35Precipitation Reactions The other ions are left dissolved in the solution. If we removed our precipitate and then dissolved the water we would find KNO3(s) left in the beaker
37Sample Ex. 4.8Use the table just given to solve the problems. Predict what will happen when you mix the following pairs.KNO3(aq) and BaCl2(aq)Na2SO4(aq) and Pb(NO3)2(aq)KOH(aq) and Fe(NO3)3(aq)
38Sample Ex. 4.8Split up the ions and pair them. The table show that KCl and Ba(NO3)2 are soluble in water, thus no precipitate forms.NaNO3 is soluble but PbSO4 is not.K+ and NO3- salts are soluble but Fe(OH)3 is only slightly soluble, thus it will precipitate
39Describing Reactions in Solutions formula equationcomplete ionic equationnet ionic equation
40Formula Equation The typical equation we normally write K2CrO4(aq) + Ba(NO3)2(aq) BaCrO4(s) + 2KNO3(aq)
41Complete Ionic Equation In a complete ionic equation, all the substances that are strong electrolytes are represented as ions2K+(aq)+CrO42-(aq)+Ba2+(aq)+2NO3-(aq) BaCrO4(s) + 2K+ + 2NO3-
42Net Ionic EquationA net ionic equation only includes those solution components directly involved in the reaction. We do not write the spectator ions (K+ and NO3-)Ba2+(aq) + CrO42-(aq) BaCrO4(s)
43Sample Ex. 4.9For the following reaction, write the formula equation, the complete ionic equation, and the net ionic equation.Aqueous potassium chloride is added to aqueous silver nitrate to form a silver chloride precipitate plus aqueous potassium nitrate
44Stoichiometry of Precipitation Reaction Just like in Chapter 3, we first convert all quantities into molesSecond, we form the proper molar ratio using the coefficients of the balanced equationUse the limiting reactant when necessary
45Stoichiometry of Precipitation Reaction Special attention needs to be given to what reaction actually occurs. Use the complete ionic formula and your background knowledge of the reactants to determine the reactionWe must use the molarity and volume to determine the moles of reactant
46Sample Ex. 4.10Calculate the mass of solid NaCl that must be added to L of a M AgNO3 solution to precipitate all the Ag+ ions in the form of AgCl.
47Sample Ex. 4.10 Write the complete ionic equation Refer to the solubility tableDetermine what the spectator ions are and what compound is the precipitateConvert molarity and volume to molesUse the proper molar ratio then convert to grams
48Acid-Base Reactions Arehenius’ concept of acids and bases Acids produce H+ ions when dissolved in waterBases produce OH- ions when dissolved in waterNot all acids or bases have H+ or OH- so a more general definition is needed
49Acid-Base Reactions Bronsted-Lowery Model Acids are proton donors Bases are proton acceptorsHow do we predict acid-base reactions?
50Acid-Base ReactionsTake the example of mixing solutions of HCl and NaOH. We know that HCl is a strong acid and NaOH is a strong base.Write the ions that are present and look at the solubility table, will NaCl precipitate?Because water is a nonelectrolyte, large quantities of H+ and OH- ions cannot coexist in the solution. What happens?
51Acid-Base Reactions They will react to for H2O. H+(aq) + OH-(aq) H2O(l)This is the net ionic equation for the reaction that occurs when aqueous HCl and NaOH are mixed.
52Acid-Base ReactionsWhat if we now mix solutions of HC2H3O2 and KOH, acetic acid is a weak acid while KOH is a strong base.What ions are available?OH- is a proton acceptor and has such a great affinity for protons that it can strip them from the acetic acid
53Acid-Base Reactions The net ionic equation for the reaction is OH-(aq) + HC2H3O2(aq) H2O(l) + C2H3O2-(aq)What do we learn from this? The hydroxide ion is such a strong base that for purposes of stoichiometric calculations it can be assumed to react completely with any weak acid that we will encounter in this course.
54Acid-Base ReactionsList the ions present and decide what reaction will occurWrite the balance net ionic equationCalculate moles of reactantDetermine limiting reactantCalculate required moles of reactant or productConvert to grams or volume as required
55Sample Ex. 4.13In a certain experiment, 28.0 mL of M HNO3, and mL of M KOH are mixed. Calculate the amount of water formed in the resulting reaction. What is the concentration of H+ or OH- ions in excess after the reaction goes to completion?
56Sample Ex. 4.13 What ions are available to react? Check solubility table and write net ionic equationCompute the amount of H+ and OH- presentWhat is the molar ratio between H+ and OH-?
57Sample Ex. 4.13 What is the limiting reactant? What amount of OH- or H+ are in excess?What is the total volume of the solution after mixing the two solutions?Use the new information to calculate the concentration.
58Acid-Base TitrationsVolumetric analysis is a technique for determining the amount of a certain substance by doing a titration.A titration involves delivery (from a buret) of a measured volume of a solution of known concentration (the titrant) into a solution containing the substance to be analyzed (the analyte)
59Acid-Base TitrationsWhen you have added just enough titrant to react exactly with the analyte you have reached the equivalence point, also called the stoichiometric pointThis point is often marked by an indicator, a substance that is added at the beginning of the titration that changes color at or very near the equivalence point
60Acid-Base TitrationsThe point where the indicator actually changes color is called the end point of the titrationThe goal is to choose an indicator such that the end point is exactly at the equivalence point
61Acid-Base TitrationsTo have a successful titration you need to meet three requirements:The exact reaction between titrant and analyte must be known and rapidThe equivalence point must be marked accuratelyThe volume of titrant required to reach the equivalence point must be known exactly
62Acid-Base TitrationsWhen the analyte is base or an acid, the required titrant is a strong acid or base respectively. We call this an Acid-Base Titration.Phenolphthalein is a common indicator for acid-base titration which is clear in an acidic solution but pink in a basic solution
63Acid-Base TitrationsThe end point here occurs one drop of base beyond the equivalence point
64Sample Ex. 4.14A student carries out an experiment to standardize (determine the exact concentration of) a sodium hydroxide solution. To do this, the student weighs out a g sample of potassium hydrogen phthalate (KHC8H4O4, or KHP) which has a molar mass of g/mol. KHP has one acidic hydrogen. KHP is dissolved in distilled water and phenolphthalein is added as an indicator.
65Sample Ex. 4.14He then titrates the solution with sodium hydroxide to phenolphthalein’s end point. The difference between the final and initial buret readings indicates mL of sodium hydroxide solution is required to react exactly with g KHP. Calculate the concentration of the sodium hydroxide solution.
66Sample Ex. 4.14 What ions are initially present Now consider a solution with those ions, what will the OH- do to the HC8H4O4-?Write the net ionic equation and determine the molar ratio.Determine moles of KHP and use the molar ratioCalculate Molarity of NaOH
67Sample Ex. 4.15An environmental chemist analyzed the effluent from an industrial process know to produce the compounds carbon tetrachloride and benzoic acid (HC7H5O2), a weak acid that has one acidic hydrogen atom per molecule. A sample of this effluent weighing g was shaken with water, and the resulting aqueous solution required mL of M NaOH for neutralization. Calculate the mass percent of HC7H5O2 in the original sample.
68Sample Ex. 4.15 What ions are present? Write the net ionic equation What happens when you have a weak acid in a solution with OH-?What is the molar ratio?Find moles of OH-From the molar ratio calculate the mass of HC7H5O2 then calculate mass percent
69Oxidation-Reduction Reaction Also called redox reactions, occurs when one or more electrons are transferedExamples, reaction to produce NaCl, photosynthesis, combustion reaction, etc.
70Redox ReactionOxidation state (oxidation number)- provides a way to keep track of electrons in redox rxns, particularly those containing covalent substances.Oxidation state of atoms in covalent compounds are obtained by arbitrarily assigning the electrons which are normally shared to an atom
71Assigning ElectronsIf the two atoms are identical then the electrons are shared equallyIf two different atoms are involved, the electrons are assigned to the atom that has a stronger attraction to electrons, for example H2ONote that the oxidation state is an imaginary charge
73Sample Ex. 4.16 Assign oxidation states to all atoms in the following CO2SF6NO3-
74Non-Integer Oxidation States Apply our rules to Fe3O4We first assign O a -2So the three iron atoms must give us +8We find it to be +8/3Although not common, we need to realize that it is possible to have non-integer oxidation states
75Characteristics of Redox Rxns Redox rxns are characterized by the transfer of electronsSometimes it is less obvious though, CH4(g) + 2O2(g) CO2(g) + 2H2O(g)Note the oxidation state of O2 is 0 because it is in elemental form. Notice that Carbon goes from -4 in CH4 to +4 in CO2, this is accounted for by the loss of 8 e-
76Characteristics of Redox Rxns Also, each O changes from oxidation state of 0 to -2 in H2O and CO2We can show this by using the equations CH CO2 + 8e O2 + 8e CO2 + 2H2O
77Characteristics of Redox Rxns Oxidation is an increase in oxidation state (the loss of an electron)Reduction is a decrease in oxidation state (the gain of an electron)A helpful reminder LEO GER
78Characteristics of Redox Rxns Looking at the example Na(s) + Cl2(g) NaCl(s)We see that sodium is oxidized while chloride is reducedWe say that Cl2 is the oxidizing agent (electron acceptor) and Na is the reducing agent (electron donor)
79Sample Ex. 4.17When powdered aluminum metal is mixed with pulverized iodine crystals and a drop of water is added to help the reaction get started, the resulting reaction produces a great deal of energy. The mixture bursts into flames, and a purple smoke of I2 vapor is produced from the excess iodine. The reaction is:2Al(s) + 3I2(s)-->2AlI3(s)
80Sample Ex. 4.17Identify which atoms are oxidized and reduced and what are the oxidizing and reducing agents?
81Sample Ex. 4.18Metallurgy, the process of producing a metal from its ore, always involves oxidation-reduction reactions. In the metallurgy of galena (PbS), the principal lead containing ore, the first step is the conversions of the lead sulfide to its oxide:2PbS(s) + 3O2-->2PbO(s) + 2SO2(g)
82Sample Ex. 4.18The oxide is then treated with carbon monoxide to produce the free metal:PbO(s) + CO(g)-->Pb(s) + CO2(g)For each reaction identify the atoms that are oxidized and reduced, and specify the oxidizing and reducing agents.
83Balancing Oxidation-Reduction Rxns Sometimes it is difficult to balance redox rxns in aqueous solutions.We can use a method called the half reaction method to make balancing easier
84Half Reactions We separate the reaction into two half reactions One involves oxidation while the other involves reduction
85Half Reactions Example: Ce4+(aq) + Sn2+(aq)--> Ce3+(aq) + Sn4+(aq) We separate the unbalanced reaction into two half reactions:Ce4+(aq)--> Ce3+ (reduced)Sn2+(aq)--> Sn4+ (oxidized)
86Half ReactionsYou will generally write the two half reactions, balance them, and then add them together.However, the procedure differs slightly depending on whether you are in an acidic or basic solution
87Half Reactions in Acidic Solutions Write the two half reactionsFor each half reaction:Balance all elements except hydrogen and oxygenBalance Oxygen using H2OBalance Hydrogen using H+Balance charges using electrons
88Half Reactions in Acidic Solutions If needed, multiply one or both balanced half reactions by an integer to equalize the number of electrons transferred in the two half reactionsAdd the half reactions and cancel identical speciesCheck that atoms and charges are balanced
89Half Reactions in Acidic Solutions Follow the steps for balancing the reaction MnO4-(aq)+Fe2+--acid-->Fe3+(aq)+Mn2+(aq)
90Sample Ex. 4.19Potassium dichromate (K2Cr2O7) is a bright orange compound that can be reduced to a blue-violet solution of Cr3+ ions. Under certain conditions, K2Cr2O7 reacts with ethyl alcohol (C5H5OH) as follows:H+(aq)+Cr2O72-(aq)+C2H5OH(l)-->Cr3+(aq)+CO2(g)+H2O(l)Use half reaction method to balance the equation.
91Half Reactions in Basic Solutions Follow the same steps as an acidic solution to find a balanced equationAdd OH- to both sides to cancel out any H+ ionsForm H2O with the H+ and OH- and eliminate any H2O appearing on both sides of the equationCheck that elements and charges balance
92Sample Ex. 4.20Silver is sometimes found in nature as large nuggets; more often it is found mixed with other metals and their ore. An aqueous solution containing cyanide ions is often used to extract the silver using the following reaction that occurs in a basic solution:Ag(s)+CN-(aq)+O2--basic-->Ag(CN)2-(aq)