Exp 4A: Conductivity Of Aqueous Solutions Purpose –Study conductivity of a series of solutions to determine the difference between strong electrolytes, weak electrolytes and nonelectrolytes –Use conductivity to distinguish between strong and weak acids and strong and weak bases –Use conductivity to study the effects of ion concentration

Conductivity of Solutions The conductivity (or specific conductance) of an electrolyte solution is a measure of its ability to conduct or allow the passage of electricity. Conductivity units The SI unit of conductivity is siemens per meter (S/m) Conductivity vs Conductance Consider a piece of wire. Electrical conductivity is a property of the material of the wire, and its value changes only with temperature (it decreases linearly with temperature). On the other hand conductance is the measure of the ease with which current can flow in the wire. It depends on the physical parameters of the wire (length,area of cross section) as well as the conductivity of the material of the wire. conductance=conductivity x area of cross section / length

APPLICATIONS and CONCEPT CONNECTIONS Conductivity measurements are used routinely in many industrial and environmental applications as a fast, inexpensive and reliable way of measuring the ionic content in a solution. For example, the measurement of product conductivity is a typical way to monitor and continuously trend the performance of water purification systems. For water quality- Conductivity is linked directly to the total dissolved solids (T.D.S.). High quality deionized water has a conductivity of about 5.5 μS/m, Typical drinking water in the range of 5-50 mS/m Sea water about 5 S/m (one million times higher than deionized water). Conductivity is traditionally determined by measuring the AC (alternating currect) resistance of the solution between two electrodes. Dilute solutions follow Kohlrausch's Laws of concentration dependence and additivity of ionic contributions. Onsager gave a theoretical explanation of Kohlrausch's law by extending the Debye–Hückel theory.Kohlrausch's LawsOnsagerDebye–Hückel theory

Exp 4A: Conductivity Of Aqueous Solutions Electrolytes –Aqueous solutions of ionic compounds Ionic compounds dissolve and dissociate in water NaCl(s)  Na + (aq) + Cl - (aq) Formation of positive and negative ions in solution Solution conducts electricity Strong electrolytes conduct electricity easily –Strong electrolytes are completely dissociated (100%) Weak electrolytes conduct electricity poorly –Weak electrolytes are only partially dissociates (<100%) –Mostly undissociated = molecular form CH 3 COOH(l) H + (aq) + CH 3 COO - (aq) Nonelectrolytes Compounds that do not conduct electricity in solution Compounds that do not form ions in aqueous solution

Exp 4A: Conductivity Of Aqueous Solutions NaCl(s) + H 2 O(l)  Na + (aq) + Cl - (aq) –1 positive charge + 1 negative charge 2 ionic charges MgCl 2 (s) + H 2 O(l)  Mg 2+ (aq) + 2 Cl - (aq) –1 ion with +2 charge, 2 ions with –1 charge 4 ionic charges Fe(NO 3 ) 3 + H 2 O(l)  Fe 3+ (aq) + 3 NO 3 - (aq) –1 ion with +3 charge, 3 ions with –1 charge 6 ionic charges Conductivity depends on –Concentration of ions –Charge of ions –(Size of ions (mobility in solution: large ions move more slowly))

Exp 4A: Conductivity Of Aqueous Solutions Electrolytes conduct electricity /4_1_Stg_Wk_Nonelelytes.swf+

Exp 4A: Conductivity Of Aqueous Solutions Strong electrolytes –Conduct electricity easily –Electrolyte (almost) completely dissociated in solution NaCl(s) + H 2 O(l)  Na + (aq) + Cl - (aq) Weak electrolytes –Poorly conducting solutions –Electrolytes mostly in molecular form with few ions NaH 2 PO 4 (s) + H 2 O(l) Na + (aq) + H 2 PO 4 - (aq) Magnitude of conductance of a solution –proportional to the number and type of ions in solution More ions (higher ion concentration): more conductivity More ionic charges: more conductivity

Exp 4A: Conductivity Of Aqueous Solutions Strong Acids –HCl(g) + H 2 O(l)  H + (aq) + Cl - (aq) –HNO 3 (l) + H 2 O(l)  H + (aq) + NO 3 - (aq) Strong Bases –KOH(s) + H 2 O(l)  K + (aq) + OH - (aq) Weak Acids –CH 3 COOH(l) + H 2 O(l) H + (aq) + CH 3 COO - (aq) Weak Bases –NH 3 (g) + H 2 O(l) NH 4 + (aq) + OH - (aq)

Exp 4A: Conductivity Of Aqueous Solutions Strong and Weak electrolytes Strong Electrolyte Weak Electrolyte

Exp 4A: Conductivity Of Aqueous Solutions Mixtures of electrolytes –Conductance shows additive effect if electrolytes do not react with each other more ions, more charges, more conductivity –If a chemical reaction occurs between the electrolytes, the properties of the new substance(s) determine conductivity typically reaction between a weak acid and a base or a weak base and an acid NH 3 (aq) + HCl(aq)  NH 4 + (aq) + Cl - (aq)

Exp 4A: Conductivity Of Aqueous Solutions Dilution A certain amount of a solution added to an amount of solvent to lower the concentration –Add 10 mL of 1.0 M NaCl to 90 mL of H 2 O –Final volume = 10 mL + 90 mL = 100 mL Dilution = 1:10 Concentration = (10 mL x 1.0 M NaCl)/ 100 mL = 0.10 M NaCl The concentration changes. Does the total amount of NaCl particles change? –Dilution formula: initial molarity (mol/L) x initial volume (L) = final molarity (mol/L) x final volume (L) = M i x V i = M f x V f = mol/L x L = mol

Exp 4A: Conductivity Of Aqueous Solutions Prelab Question 4a How will you prepare 10 mL of M HCl from 0.10 M HCl? Answer Use dilution formula V 1 x M 1 = V 2 x M 2 –M 1 = 0.10 M –M 2 = M –V 2 = 10 mL –V 1 = V 2 x M 2 /M 1 = 10 mL x M/0.10M = 5.0 mL of 0.10 M HCl Take 5.0 mL of 0.10 M hydrochloric acid in a 10.0-mL graduated cylinder and dilute to 10 mL with dH 2 O

Exp 4A: Conductivity Of Aqueous Solutions Prelab Question 5a How will you prepare 80 mL of 0.10 M CH 3 COOH from 6.0 M acetic acid? Answer Use dilution formula V 1 x M 1 = V 2 x M 2 –M 1 = 6.0 M –M 2 = 0.10 M –V 2 = 80 mL –V 1 = V 2 x M 2 /M 1 = 80 mL x 0.10 M/6.0M = 1.3 mL of 6.0 M acetic acid Take 1.3 mL of 6.0 M acetic acid in a 5.0-mL graduated cylinder or use a 5.0 mL pipet. Add ~ 50 mL dH 2 O to a 100-mL graduated cylinder, then add the 1.3 mL of acetic acid. If you use a 5.0 mL cylinder for the acetic acid, rinse it out with dH2O and add it to the 100-mL cylinder. Repeat this process 2 more times. Add dH 2 O to a total volume of 80 mL. Prelab Question 5b Same as 5a

Exp 4A: Conductivity Of Aqueous Solutions Part 1: Compare conductance of 20 mL dH 2 O, 20 mL tap water, 20 mL ethanol in 50 mL beaker Why is there a difference in conductance, if any, between distilled water and tap water? Part 2: Measure conductance of hydrochloric acid solutions 20 mL 0.10 M HCl M HCl: dilute 10 mL 0.10 M HCl + 10 mL dH 2 O M HCl: dilute 10 mL M HCl + 15 mL dH 2 O How do you make dilutions?  See prelab assignment 4!

Exp 4A: Conductivity Of Aqueous Solutions Part 3: Measure conductivity in different solutions SolutionConcentrationConductance 110 mL 0.10 M HNO mL dH 2 O 210 mL 0.10 M KOH + 10 mL dH 2 O 310 mL 0.10 M KCl + 10 mL dH 2 O 410 mL 0.10 M KNO mL dH 2 O 510 mL 0.10 M Ca(NO 3 ) mL dH 2 O 610 mL 0.10 M NH mL dH 2 O 710 mL 0.10 M HC 2 H 3 O mL dH 2 O 810 mL 0.10 M HCl + 10 mL 0.10 M KNO 3 Calculate new concentrations 910 mL 0.10 M HNO mL 0.10 M KCl Calculate new concentrations 1010 mL 0.10 M HCl + 10 mL 0.10 M KOH Calculate new concentrations 1110 mL 0.10 M NH mL 0.10 M HC 2 H 3 O 2 Calculate new concentrations

Exp 4A: Conductivity Of Aqueous Solutions Measure conductivity in different solutions SolutionConcentrationConductance 110 mL 0.10 M HNO mL dH 2 O mL 0.10 M KOH + 10 mL dH 2 O mL 0.10 M KCl + 10 mL dH 2 O mL 0.10 M KNO mL dH 2 O mL 0.10 M Ca(NO 3 ) mL dH 2 O mL 0.10 M NH mL dH 2 O 710 mL 0.10 M HC 2 H 3 O mL dH 2 O 810 mL 0.10 M HCl + 10 mL 0.10 M KNO mL 0.10 M HNO mL 0.10 M KCl 1010 mL 0.10 M HCl + 10 mL 0.10 M KOH 1110 mL 0.10 M NH mL 0.10 M HC 2 H 3 O 2

Exp 4A: Conductivity Of Aqueous Solutions Next week: No Formal report. Turn in the following, stapled in this order On paper: Type or write 1)a summary of the experiment and 2)your conclusions about the relationship between conductivity and concentration Data and Calculations sheets for Exp 4A: Conductivity Of Aqueous Solutions Answers to post-lab questions on the lab manual sheets Prelab Assignment for Exp 4B: Ionic Reactions in Aqueous Solutions –Avoid getting definitions from Google or Wikipedia. Use a textbook or lab manual or a science dictionary –Read Prelab preparations and protocol –Answer Prelab questions 1a-d, 2a-h, 3, 4