ELECTROYTES Non-Electrolytes Ions (aq) means dissociated in a water solution

Transcription

1 Chapter 4 Reactions in Aqueous Solutions Aqueous Solutions Electrolytes & Non-electrolytes In aqueous solutions ELECTRYTES conduct electricity Strong conduct well Weak - conduct poorly Water solutions of Non-Electrolytes do conduct electricity poorly Dissociation Ions are formed when an electron is transferred from one atom to another creating a charged species (see Chap. 2) Ionic compounds separate into ions in solution NaCl (s) Na + (aq) + Cl - (aq) (aq) means dissociated in a water solution The electrolytic strength of the solution depends upon the number of ions in solution and the charges on the ions 1

2 Strong Electrolytes Completely Ionized in Solution Strong Acids Strong Bases Soluble salts Acids and Bases Acids - Produce hydrogen ions, +, in aqueous solutions Bases - Produce in hydroxide ions, -, in aqueous solutions This is the Arrhenius definition of acids & bases Strong & Weak Acids Strong Acids completely ionize (separate into hydroxide ions and anions) in aqueous solution Cl (g) + 2 ( ) + (aq) + Cl - (aq) (to completion) Strong acids are strong electrolytes Weak acids partially ionized in aqueous solution C 3 C (aq) + (aq) + C 3 C - (aq) Reversible Weak acids are weak electolytes 2

3 Strong & Weak Bases Strong Bases completely ionize (separate into hydrogen ions and anions) in aqueous solution Na (s) + 2 ( ) Na + (aq) + - (aq) to completion) Strong bases are strong electrolytes Weak bases partially ionized in aqueous solution N 3(aq) + 2 ( ) N + 4 (aq) + - (aq) Reversible Weak bases are weak electrolytes Soluble Salts Soluble Salts completely ionize (separate into cations and anions) in aqueous solution CaCl 2(s) + 2 ( ) Ca +2 (aq) + 2Cl - (aq) Soluble salts are strong electrolytes (to completion) Solubility Not all compounds are soluble in water Solubility (mol/l) << ~ >> Insoluble Moderately Soluble Soluble Insoluble compounds are not electrolytes 3

4 Spontaneity of the Dissolution Process The spontaneity of the dissolution process depends upon 2 factors Change in energy of the system Lower energy system will be favored (energy released) Change in dis-order of the system (entropy) Increased dis-order will be favored Sometimes even though energy is required for the dissolution, the increased dis-order will dominate and cause the substance to dissolve Spontaneity of the Dissolution Process The solvent needs to interact with the solute for dissolving to take place Strong solvent-solute attraction favor dissolution Rule-of-thumb is like dissolves like Nonpolar substances dissolve nonpolar substances Polar substances dissolve polar substances Polar substances will also dissolve ionic substances Ionic substances have full charges and polar substances have partial charges Dissolution of Solids in Liquids Cl - Na + Na + Na + Cl - Na + Na Cl - + Cl - Na Cl - Cl - + Na + Na + Cl - Cl - Na + Cl - Na Cl - + Solvation A process when a solvent surrounds a solute particle ydration: solvation, where water is the solvent 4

5 Solubility Guidelines for Water Generally Soluble Na +, K +, N 4 + cmpds Exceptions No common exceptions F - Insol: MgF 2, CaF 2, SrF 2, BaF 2, PbF 2 Cl - Insol: AgCl, g 2 Cl 2, PbCl 2 Br - Insol: AgBr, g 2 Br 2, PbBr 2 Moderately Sol: gbr 2 I - Many heavy metal iodides are insoluble. S 4 2- Insol: BaS 4, PbS 4, gs 4 Mod. Sol: CaS 4, SrS 4, Ag 2 S 4 N 3- ; N 2 - Mod. Sol: AgN 2 Cl 3- ; Cl - 4 Mod. Soluble; KCl 4 C 3 C - Mod. Soluble; AgC 3 C you will be expected to know these Solubility Guidelines for Water Generally Insoluble Exceptions S 2- Soluble: N 4+, Na +, K +. Mg 2+, Ca 2+ - Soluble: Li, Na, K, Ba() 2 Mod. Sol.: Ca() 2, Sr() 2 xides ( 2-) Reacts with 2 to form hydroxides (resulting is dissolution) Li 2, Na 2, K 2, Ba Na Na (aq) C 3 2- ; P 4 3- ; As 4 3- Soluble: N 4+, Na +, K + Simplified Solubility Rules for Salts in Water Generally soluble, if a salt contains Na+, sodium ion K+, potassium ion N 4 +, ammonium ion N 3-, nitrate ion or acetate ion, C 3 C - All chlorides are soluble, except AgCl, PbCl 2 and g 2 Cl 2 (sometimes written gcl) Most other salts are insoluble There are always exceptions 5

6 Insoluble Bases Many metal hydroxides are NT soluble in water Some examples are: Cu() 2 Zn() 2 Fe() 2 Fe() 3 These are not electrolytes because they don t produce ions in aqueous solutions Precipitation Reactions A precipitate is an insoluble solid AgN 3(aq) + NaCl (aq) AgCl (s) + NaN 3(aq) Total Ionic Equation Ag + (aq)+ N 3 - (aq) + Na + (aq) + Cl - (aq) AgCl (s) + Na + (aq) +N 3 - (aq) A precipitation reaction is a reaction in which results in the formation of an insoluble product The above reaction is also an example of a doubledisplacement (or metathesis) reaction Chemical Equations 3 Ways to Write the Same Thing Formula Unit Equations 2 NaBr (aq) + Cl 2 (g) 2NaCl (aq) + Br 2 ( ) Total Ionic Equations 2[Na + (aq) + Br - (aq)] + Cl 2 (g) 2[Na + (aq) + Cl - (aq)] + Br 2 ( ) Net Ionic Equations (drop the spectator ions) 2Br - (aq) + Cl 2 (g) 2Cl - (aq) + Br 2 ( ) 6

7 Acid & Bases Acids Sour taste Change color of indicators & dyes Turn litmus red Neutralize Bases Electrolytes Corrosive Bases Bitter taste Change color of indicators & dyes Turn litmus blue Slippery feel Neutralize Acids Electrolytes Corrosive Bronsted-Lowry Definition of Acids & Bases Acid is a proton ( + ) donor Base is a proton acceptor 1923 Any substances capable of accepting a proton is base including N 3 (lone pair of electrons on nitrogen can accept the proton) Acid-base reaction is the transfer of a proton from the acid to the base A broader definition of acids and bases ydronium Ion 3 + ydrated ydrogen Ion In aqueous solution + does not exist by itself it is always complexed with water molecules + ( 2 ) n where n = 1,2, 3, + ions (also called protons) are attracted to the oxygen end of the polar water molecule (oxygen has 2 lone pairs of electrons in the water molecule) ydronium ion is typically represented as 3 + [this does NT mean it is complexed with only one water molecule] or + (aq) Aqueous hydroxide ion, - (aq), is similarly hydrated 7

8 Common Strong Acids Formula Name Cl ydrochloric Acid Br ydrobromic Acid I ydroiodic acid N 3 Nitric Acid Cl 4 Perchloric Acid Cl 3 Chloric Acid 2 S 4 Sulfuric Acid Consider other acids as weak acids. Polyprotic Acids ydrochloric Acid Cl is called monoprotic Sulfuric Acid 2 S 4 is called diprotic Phosphoric Acid 3 P 4 is called triprotic Common Strong Bases Group IA Group IIA Li Lithium ydroxide Na Sodium ydroxide K Potassium ydroxide Ca() 2 Calcium ydroxide Rb Rubidium ydroxide Sr() 2 Strontium ydroxide Cs Cesium ydroxide Ba() 2 Barium ydroxide 8

9 Weak Bases Ammonia is the most common weak base N 3(g) + 2 ( ) N 4 + (aq) + - (aq) ther similar nitrogen compounds amines are also weak bases For example: C 3 N 2 or C 6 5 N 2 Acid Base Neutralizations Acid + Base Salt + Water Formula Unit Equations N 3(aq) + K (aq) KN 3(aq) + 2 ( ) Total Ionic Equations + (aq) + N - 3 (aq) + K + (aq) + - K + (aq) + N - 3 (aq) + 2 ( ) Net Ionic Equations (drop the spectator ions) + (aq) ( ) Acid-Base Neutralizations ne-to-one molar ratios Cl + Na NaCl + 2 N 3 + K KN Two-to-one molar ratios Two to one molar ratios 2 Cl + Ca() 2 CaCl ne-to-two molar ratios 2 S 4 + 2Na Na 2 S

10 Acid-Base Reaction with Gas Formation Gas Formation Reactions 2 Cl (aq) + Na 2 C 3(s) C 2(g) + 2 ( ) + 2NaCl (aq) Red-x Equations xidation-reduction ( Redox ) reactions are electron transfer reactions xidation is an increase in oxidation number loss of electrons Reduction is a decrease in oxidation number gain of electrons You can t have one, without the other (conservation of mass) Red-x Equations - xidation I -Is L - Loss f Electrons R - Reduction I - Is G - Gain 10

11 xidation Number Ions created when a element loses or gains electrons The oxidation number (also called oxidation state) is the number of electrons gained or lost by an atom xidation state are assigned according to an arbitrary set of rules The element farther to the right and higher up are is assigned the negative oxidation number. - 1A 2A 3B 4B 5B 6B 7B 8B 1B 2B 3A 4A 5A 6A 7A 8A 1 2 Periodic Table n = 1 e With Atomic Weights n = 2 Li Be B C N F Ne n = 3 Na Mg Al Si P S Cl Ar n = 4 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr n = 5 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe n = 6 Cs Ba La * f Ta W Re s Ir Pt Au g Tl Pb Bi Po At Rn n = 7 Fr Ra Ac ** Rf Db Sg Bh s Mt * Lanthanide Series ** Actinide Series Ce Pr Nd Pm Sm Eu Gd Tb Dy o Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr + Assigning xidation Numbers 1. -N (xidation Number) of free uncombined elements is zero. Polyatomic 2, 2, P 4, S N of a monoatomic ion is equal to the charge on the ion. Fe 2+ -N = 2 3. The sum of -N of all the atoms in a neutral compound is zero. No net charge. 11

12 Assigning xidation Numbers 4. In polyatomic ions, the sum of the -Ns is equal to the charge on the ion. P 3-4 -N = Fluorine has an oxidation number of -1 in all its compounds (except F 2 ) 6. ydrogen has an -N of +1 (C 4 or Cl), unless it is combined with metals in a binary complex, where it is 1, (K or Mg 2 ). Assigning xidation Numbers 7. xygen usually has an -N of 2. Exceptions -1 in peroxides (--- or 2 2 ) -1/2 is superoxides K 2 Combined with F, oxygen has -N +2 (F 2 ) Assigning xidation Numbers 8. Elements position in the periodic table helps assign its -N Group IA (Li, Na ) +1 Group IIA (Mg, Ca ) +2 Group IIIA (B, Al ) +3 Group VA (N, P ) 3 for binary compounds with, metals. Can have other -N for compounds with non-metals (to the right of Group V). 12

13 Assigning xidation Numbers 8. Elements position in the periodic table helps assign its -N Group VIA elements (S, Se, Te) below oxygen have -N of 2 in binary compounds with metals,, N + 4 When combined with oxygen or lighter halogens, lower Group VIA elements can have different -N Assigning xidation Numbers 8. Elements position in the periodic table helps assign its -N Group VIIA elements (Cl, Br, I) have -N of 1 in binary compounds with metals,, N 4+, with a heavier halogen When combined with lighter halogens or oxygen, lower Group VIA elements can have different -N (BrF, Cl 4- ) xidation Numbers Na 2 C 3 C= 4+ C 2 C= 4+ C C= 2+ C 2 C= 0 C 2 6 C= 2- C 4 C= 4-13

14 xidation Numbers N 2 5 N=5+, = 2- S 3 S= 6+ = 2- P -3 4 P=5+ = 2- F 2 =2+ F = 1- U 2 F 2 U=6+ =2- F= 1- F 2 F = 0 SCl 2 S=2+ Cl = 1- xidation Numbers 2 Cr 4 Cr= AsCl 4 As= 2 P 2 7 P = Cl 4 Cl = = = 1A 2A 3B 4B 5B 6B 7B 8B 1B 2B 3A 4A 5A 6A 7A 8A 1 2 Periodic Table with xidation e 1 Numbers Li Be B C N F Ne The more common oxidation Na Mg numbers are in color. Al Si P S Cl Ar 1 2 The oxidation number +3 is common to all lanthanides and actinides in their compounds K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La f Ta W Re s Ir Pt Au g Tl Pb Bi Po At Rn

15 Examples of Red-ox Rxs 2Br - (aq) + Cl 2(g) Br 2( ) + 2Cl - (aq) 2 Ag + 2+ (aq) + Cu (s) Cu (aq) + 2 Ag (s) Fe (aq) Fe 2+ (aq) + 2(g) 4Fe (g) 2Fe 2 3 (s) Types of Redox Reactions Combination Reactions 2 Na (s) + Cl 2(g) 2NaCl (s) P 4(s) + 6Cl 2(g) 4 PCl 3 A combination reaction is a reaction in which 2 or more substances combine to form a single product. Not every combination reaction is a redox reaction. Types of Redox Reactions Decomposition Reactions 2 2 ( ) 2 2(g) + 2(g) ( ) (g) 2 2 ( ) 2 2(g) A decomposition reaction is a reaction in which a single compound reacts to form 2 or more. Not every combination reaction is a redox reaction. 15

16 Types of Redox Reactions Combustion Reactions C 4 (g) + 2 2(g) C 2(g) (l) 2Mg (s) + 2(g) 2 Mg A combustion reaction is a reaction in which a substance reacts with oxygen, usually with release of heat and a flame. Redox Reactions Single Displacement Reactions 2Na (s) (l) 2 Na (aq) + 2(g) Zn (s) + CuS 4 (aq) ZnS 4 (aq) + Cu (s) Cl 2 (g) + 2 NaI (aq) I 2(s) + 2NaCl (aq) Whether the reaction occurs or not can be predicted using the activity series. Activity Series Active metals displace less active metals or hydrogen from their compounds in aqueous solution to form the oxidized form of the more active metal and the reduced (free metal) of less active metal or hydrogen. 16

17 Li K Ca Na Mg Al Mn Zn Cr Fe Cd Co Ni Sn Pb 2 Sb Cu g Ag Pt Au Displaces hydrogen from cold water 2Na ( ) 2Na (aq) + 2(g) Displaces hydrogen from Steam Mg (g) Mg() 2(aq) + 2(g) Displaces hydrogen from non-oxidizing acids Ni + 2Cl Activity Series NiCl 2(aq) + 2(g) Most Active (wants to give away electrons very badly good reducing agent) (wants to hold on to its electrons - poor reducing agents) Least Active Li K Ca Na Mg Al Mn Zn Cr Fe Cd Co Ni Sn Pb 2 Sb Cu g Ag Pt Au Activity Series 1. Each element in the list displaces from a compound any of the elements below it. The larger the interval between elements in the Series, the more vigorous the action. 2. All metals above hydrogen displace hydrogen from hydrochloric acid (Cl) or dilute sulfuric acid ( 2 S 4 ). 3. Metals above magnesium vigorously displace hydrogen from water. Magnesium displaces hydrogen from steam. alogen Activity Series Displacement Reactions Most Active F 2 > Cl 2 > Br 2 > I 2 Least Active Cl 2 + 2NaBr? 2NaCl + Br 2 Br 2 + 2NaCl 2NaBr + Cl 2 you will be expected to know these 17

18 Red-ox Disproportionation Reactions 6K + 3Cl 2 KCl 3 + 5KCl A disproportionation reaction is a reaction in which an element in one oxidation state is simultaneously oxidized and reduced. Classify the Following Reactions Zn (s) + CuCl 2(aq) Cu (s) + ZnCl 2 (aq) heat Na 2 C 3(s) Na 2 (s) + C 2(g) heat 2 Cl (g) 2 2(g) + Cl 2(g) Classify the Following Reactions NaF (aq) + CaCl 2 (aq) CaF 2(s) + NaCl (aq) N 2 5(g) + 2 ( ) 2 N 3(aq) 2 Cl (g) + Ca() 2 (aq) CaCl 2(aq) + 2 ( ) 18

19 Concentration Molarity Molarity (moles/liter) Molarity = moles/liter = mmoles/ml (millimoles/milliliter) 1000 ml = 1 L 1000 mmoles = 1 mole Dilution of Solutions Frequently we have a concentrated solution, and we want to dilute to solution to a specific concentration. For example, e, concentrated ce hydrochloric o c acid (Cl) is 11.6 M. ow do we make 1 L of 0.9 M Cl from concentrated Cl? Dilution of Solutions ow many ml of 11.6 M Cl (hydrochloric acid) are required to prepare 1.00L of M solution of Cl? For dilutions: V M l f l t V M V C x M C = moles of solute = V D x M D 77.6 ml 19

20 Dilution of Solutions Calculate the new concentration of the solution when ml of M 2 S 4 is evaporated to a volume of ml. Gravimetric Analysis Gravimetric analysis is a chemical analysis technique based on the measurement of mass It involves ves the formation, o isolation, o and mass determination of a precipitate (solid formation from a solution). AgN 3 (aq) + NaCl (aq) NaN 3 (aq) + AgCl (s) Gravimetric Analysis Calculate the concentration (M) of barium chloride [BaCl 2 ] in a10.0 ml sample of aqueous solution which has been treated with excess sodium sulfate to precipitate barium sulfate( BaS 4 ). The mass of BaS 4 precipitated was g. The MM of BaS 4 is g/mol. Ba 2+ (aq) + S 2-4 (aq) BaS 4 (s) M 20

21 Titrations Titration is the process in which a solution of one reactant, the titrant, is carefully added to a solution of another reactant, and the volume of the titrant required for complete reaction is measured An indicator is used to indicate the endpoint of the reaction changes the color of the solution - depending on the concentration of the + in solution Equivalence point The point at which the exact stoichiometrically equivalent amounts of the acid and base have reacted Titrations The titrant is usually a standard solution standard solutions have accurately known concentrations Standardization is process of determining the concentration ti of the standard d solution using a Primary Standard The standardized solution is then known as secondary standard Properties of an Ideal Primary Standard Doesn t absorb components of atmosphere (water, C 2, oxygen) Reacts with one invariable reaction igh purity igh formula weight (reduces weighing errors) Soluble Low toxicity & low environmental effects Inexpensive 21

22 Common Primary Standards Sodium Carbonate Na 2 C 3 FW = g/mole Used to standardize acids Reacts with 2 moles of + for every 1 mole of sodium carbonate 2 Cl + Na 2 C 3 2NaCl + C S 4 + Na 2 C 3 Na 2 S 4 + C Common Primary Standards Potassium ydrogen Phthalate (KP) KC 6 4 (C)(C) MM = g/mole P stands for Phthalate (not Phosphorus) Used to standardized base Used to standardized base Supplies 1 + per mole of KP Na + KP NaKP + 2 Primary Standards ml solution of Na reacts with g of KP. Calculate the M of the Na solution Na + KP NaKP

23 Primary Standards ml solution of Na reacts with g of KP. Calculate the M of the Na solution Na + KP NaKP + 2? Mol Na = g x 1 mol KP x 1 mol Na g KP x 1 mol KP = mol Na Moles0.0150mol ML 1L Titration What is the molarity of Cl solution if 38.7 ml of M Na is required to neutralize ml of the Cl solution? Cl + Na NaCl M Acid-Base Calculations Molarity (moles/liter) 50.0 ml of M N 3 are mixed with 50.0 ml of M K what is M of the resulting salt? N 3 + K KN

24 Volume of Acid to Neutralize a Base What volume of M Cl would be needed to neutralize 30.0 ml of M Ca() 2? 20.0 ml Red-x Titrations Mn Fe 2+ Mn Fe What is the molarity of a solution of Fe 2+ ions if ml of it reacts with 26.2 ml of 0.100M Mn 4 - ions? M 24