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ENCH 213 (25)
Lecture

# 20activity.pdf

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School
Department
Chemistry
Course
ENCH 213
Professor
Diane Beauchemin
Semester
Fall

Description
Activity • activity is related to the size of hydrated species • larger charge density ⇒ more water molecules bound to ion Effect of electrolyte concentration • increase in electrolyte concentration ◦ → electrostatic effects ◦ → change equilibrium constant ▪ more ions around a given analyte less tendency for central ion to react w/ something else ▪ not a common ion effect, but see a decrease as add KNO 3 ◦ greater ionic strength: higher charge in the ionic Eqmconstant as addKNO nitrate 3 atmosphere, less net charge on ion Ionic strength i=1 Halfthe sumofthe concentrationtimes its charge 1 2 • = 2 ∑ c1Z i n • where c =iconcentration (M) of ion #i • Zi= charge on ion #i • n = total number of different ions Example: ionic strength of 0.05 M KNO + 0.10M 3a SO ? 2 4 • 4 different ions: K , NO , 3a , SO 42- ** CHECKIFTHEREIS ARXNFIRST! 2 2 2 2 • µ = 1/2 × (0.05 × (+1) + 0.05 × (-1) + 0.20 × (+1) + 0.10 × (-2) ) = 0.35 M A solution contains 0.100 M sodium nitrate (NaNO3) and 0.200 M MgCl2. Find the ionic strength of the solution. a) 0.7 M ions in solution:Na , N3 ,Mg , Cl - 2 2 2 2 b) 0.6 M =0.5(0.1x(+1) +0.1x(-1) +0.2(+2) +0.4(-1) ) c) 0.5 M =0.7M Effect of ionic strength Greater effect w/ MgSO b/c each 4 ion has a charge of+/-2 No effect when glucose • increasing ionic strength ◦ reduces attraction between ions added:no charge ◦ increases solubility Decreases b/c ofcommon • ionic strength ion effect ◦ = molarity for 1:1 electrolyte ◦ > molarity for other stoichiometries Activity coefficient • aA= [A] γ A ◦ a =Aactivity of speciesA (=fugacity of a gas) ◦ [A] = molar concentration of A ◦ γ =Aactivity coefficient (= fugacity coefficient in the case of a gast) ◦ → varies with μ • strictly, • γA≈ 1 in very dilute solutions (μ minimal) ◦ → [A] ≈ a A Adding a salt to water affects pH • what is the pH of a solution that is 0.10 M KCl? 2 2 • ionic strength=½(0.1×1 +0.1×1 )= 0.10 M get activities coefficients • K w1 0 × 10 =a-14 H3O+ aOH- =x·0 83·x·0 76 fromtable! + - -14 ◦ where x = [H O ]=3OH]=√(1.0×10 /(0.83×0.76)) ◦ = 1.26 × 10 M -7 -7 -7 • aH3O+ = 1.26×10 ×0.83= 1.05 x 10 M • pH = -log a H3O+= 6.98 • pH change by -0.02 within accuracy limit • [H 3 ] is 26% greater than in pure water What is the pH of water containing 0.0333 M Ca(NO ) ? 3 2 a) 7.00 Mu = 0.5 [ 0.0333M x (+2) + 0.0666M x (-1) ] = 0.0999 = ~0.1M + - b) 6.98 Kw= [H O3] a H3O+[OH]a OH-= same as above c) 7 02 x = [H O ]=[OH ]=√(1.0×10 /(0.83×0.76)) 3 Extended Debye-Hückel equation • γ = activity coefficient of ion • z = charge of ion • µ = ionic strength of the solution • α = effective diameter of ion (hydrated ion) A Factors affecting the activity coefficient for 0 ≤ µ ≤ 0.1 M • as µ (ionic strength) increases , γ decreases • γ → 1, as µ → 0 • if z = O, γ ≈ 1 • at given µ, ◦ as z , γ further departs from 1 ◦ smaller ion: more important activity effects • γ independent of electrolyte → depends only on μ What is the pH of 0.10 M HBr? • [H O3] = 0.10 M • µ = 0.10 M → γ H3O+ = 0.83 + • pH = -log a H3O+ = -log [H O 3 γ H3O+ = -log 0.10 • 0.83 = 1.08 + • as opposed to pH ~ -log [H O ] = 1.30 • activities → more accurate results Calculate the solubility of silver bromate (AgBrO ), using activities, in a saturated solution of silver -5 3 bromate. K forspg(BrO )= 5.5×103. -5 + - -3 K sp.5×10 =[Ag ]γ Ag+ [BrO 3γ BrO3- a) 7.4 × 10 M With γ =1, and x=0.0074M so μ =~0.0074M b) 8.1 × 10 M -3 -3 By interpolation (fromtable):γ Ag+=0.91=γ BrO3- c) 7 8 × 10 M → x =K =(5.5x10 )/ (0.91) 2 sp Systematic treatment of equilibrium • Write the pertinent
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