ib chemistry acids bases, ph, acid dissociation constant ka, ionic product water kw, pka, pkb
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IB Chemistry Acids Bases, pH, Acid Dissociation Constant Ka, Ionic Product Water Kw, pKa, pKbTRANSCRIPT
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http://lawrencekok.blogspot.com
Prepared by Lawrence Kok
Video Tutorial on Acids Bases, pH, Acid Dissociation Constant Ka, Ionic Product Water Kw, pKa and pKb
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Acids and Bases, pH scale, Acid Dissociation Constant Ka, Water Dissociation Constant Kw, pKa
Physical properties of Acids• Electrolytes, produces H3O+, hydronium ion, in water (conduct electricity)
• Sour, pH <7, turns litmus red, phenolphthalein colourless, methyl orange to red
Physical properties of Bases• Bitter, pH > 7, turns litmus blue, phenolphthalein pink, methyl orange to yellow
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Chemical properties of Acids / Bases
• Acid with Metal - Salt + H2 gas2HCI + Mg → MgCI2 + H2
• Acid with Ammonia (aq) - Salt + WaterHCI + NH4OH → NH4CI + H2O
• Acid with Bases (Neutralization) - Salt + Water and other products. Bases are - Metal Hydroxide, Metal Oxide, Metal Carbonates
• Acid HCI reacts with Bases :Metal Hydroxide (Soluble or Alkali ) - Salt + Water LiOH + HCI → LiCI + H2O
Metal Hydroxide (Insoluble) - Salt + WaterCa(OH)2 + HCI → CaCI2 + H2O
Metal Oxide - Salt and WaterCaO + 2HCI → CaCI2 + H2O
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pH measurement of Acidity of solution
pH scale for Acid
• pH is logarithmic scale
• Relationship between pH with Conc H+
• pH = - log [H+], p stand for – log
• pH = Power of Hydrogen, pH 7 = Conc H+ is 10 -7
• Conc H+ = 0.0000001M, pH = -log[0.0000001], pH = 7
• Easier using pH scale than Conc [H+]
• Conc H+ up 10x from 0.0001(10 -4 ) to 0.001(10 -3), pH change by 1 unit from pH 4 to 3
• pH 3 is (10x) more acidic than pH 4
• 1 unit change in pH is 10 fold change in H+ Conc
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pOH for Base• Relationship between pOH with Conc OH
• pOH = -log [OH]
• [OH] = 0.0000001M, pOH = -log[0.0000001], pOH = 7
• pH + pOH = 14, pH + 7 = 14, pH = 7
Calculate pH for 0.1M NaOH1st Method using pOH[OH] = 0.1MpOH = -log[OH] = -log [0.1] = 1pH + pOH = 14pH + 1 = 14, pH = 13.0
2nd Method using Kw[H+] x [OH] = Kw[H+] x [0.1] = 1 x 10 -14
[H+] = 1 x 10 -14/ [0.1][H+] = 10 -13
pH = -log[H+], pH = -log [10 -13], pH = 13.0
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Dissociation Constant or Ionic Product Water, Kw
• H2O dissociate forming H3O+ and OH
• H2O + H2O ↔ H3O+ + OH −
• Kw = [H3O+][OH −]/[H2O]2
• Dissociation H2O is small, so conc [H2O] remains constant giving Kw = [H3O+][OH −]
• [H3O+]= 1.0x10-7, [OH] = 1.0x10-7
• Kw = [H3O+][OH −]
• Kw = [1.0x10-7][1.0x10-7]
• Kw = 1.0x10-14 mol2 dm-6
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Kw is Temp dependent, dissociation water to ions is endothermic.
H2O + H2O ↔ H3O+ + OH − ΔH = +57kJ/molKw = [H3O+][OH −]Temp increases, equilibrium shift to right (endo), so more ions form, Kw increases
• Kw is Temp dependent
• Kw at 25C is 1.0 x 10-14
• Temp up, Kw up, H+ up, pH down
• 25C, Kw is 1.0 x 10-14 and pH is 7 (neutral)
• 50C, Kw is 9.3 x 10-14 and pH is 6.5 (neutral)
• 50C Higher Temp, pH 6.5 (neutral) < 7
Working below.
[H+][OH −]=Kw, [H+][OH −] = 9.3 x 10-14, [H+]2= 9.3 x 10-14, [H+] = √9.3 x 10-14 [H+] = 3.05 x 10-7
pH 6.5 is NEUTRAL, cause number of [H+] and [OH −] are the same = 3.05 x 10-7
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Acid Dissociation Constant, Ka (Ka measure the strength of acid)
HA ↔ H+ + A-
Ka = (H+)(A-) / HA
• Ka High = Stronger Acid
• pKa = -log (Ka)
• pKa Low = Stronger Acid
Base Dissociation Constant, Kb
Ka calculation. Click HERE to viewKb calculation. Click HERE to view
Base (A-) dissolves in water produce OH −
A- + H2O ↔ HA + OH −
Kb = [HA][OH −] / [A-]
• Kb High = Stronger Base
• pKb = - log (Kb)
• pKb Low = Stronger Base
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Acids with High Ka or Low pKa is more acidic
Strong acid -High Ka, Low pKaWeak acid - Low Ka, High pKaAcidicty 0.10M methanoic > 0.10M ethanoic
Methanoic acid, Ka = 1.77 x 10-4
HCOOH ↔ HCOO- + H+
[HCOO-][H+] / [HCOOH] = Ka[H+]2 / [HCOOH] = 1.77 x 10-4
[H+] = √1.77 x 10-4 x 0.10 [H+] = 4.21 x 10-3
pH = -log[H+] = - log [4.21 x 10-3], pH = 2.30
Ethanoic acid, Ka = 1.78 x 10-5
CH3COOH ↔ H+ + CH3COO-
[H+][CH3COO-] / [CH3COOH] = Ka[H+]2 / [CH3COOH] = 1.8 x 10-5 [H+] = 1.33 x 10-3
pH = -log [H+] = -log [1.33 x 10-3], pH = 2.88
Methanoic (pH 2.30) more acidic than Ethanoic acid (pH 2.88) cause has Higher Ka/Low pKa
Click HERE to view Ka for different acidsClick HERE to view more info on acids and bases
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Strong vs Weak Strong Acid/Base - Ionises/ Dissociate completely (100%), Strong electrolyte, Higher conductivity
HCI → H+ + CI- (1 mole) (1 mole H+)
NaOH → Na+ + OH −
(1 mole) (1 mole OH −)
Ba(OH)2 → Ba2+ + 2OH −
(1 mole) (2 mole OH −)
•Strong acids - HI, HBr, HCI, HNO3, H2SO4, HCIO3, HCIO4
•Strong Bases - LiOH, KOH, NaOH, CsOH, Ca(OH)2, Ba(OH)2
Weak acid - Partially dissociate, Most exist in molecules, Poor electrolyte, Lower conductivity
CH3COOH → H+ + CH3COO-
(1 mole) (0.01mole H+)
•Weak Acid - CH3COOH, HF, HCN, H2CO3, H3BO3, HNO2, H3PO4
•Weak Bases - NH3, C2H5NH2 , CH3NH2, (CH3)2NH, C3H5O2NH2
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• Concentrated Acid/Base - high number/moles of solutes per dm3 solution
• Diluted Acid/Bases - low number/moles of solutes per dm3 solution
10M CH3COOH vs 0.01M HCI
•10M CH3COOH is weak but concentrated acid caused not all 10M acid molecule will dissociate fully forming H+ ions
•0.01M HCI is strong but diluted acid cause all 0.01M acid, fully dissociate forming H+ ions
Concentrated Vs Diluted
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Important Formula for IB Calculations
1st Formula[H+][OH ] = 1.0 x10-14 (- log both sides)-{ log[H+] + log [OH] } = - { log 1 + log 10-14 }, pH = - log[H+], pOH = - log [OH]pH + pOH = pKw or pH + pOH = 14
2nd FormulaDissociation Acid, HA = KaHA + H2O ↔H3O+ + A-Ka = [H3O+][A- ]/ [HA]
Dissociation Base, A- = KbA- + H2O ↔ HA + OH-
Kb = [HA][OH-] /[A-]Ka x Kb = Kw (see diagram)
3rd FormulaKa x Kb = Kw(-log both sides)
- log { Ka x Kb } = - log { Kw } - log Ka - log Kb = - log KwpKa + pKb = pKw or pKa + pKb = 14
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Calculation of pH, Ka and Conc of Weak acid Calculate pH of 0.10M CH3COOH, given Ka = 1.8 x 10-5
CH3COOH ↔ H+ + CH3COO-
• [H+] [CH3COO-] / [CH3COOH] = Ka
• [H+]2 / 0.10 = 1.8 x 10-5
• [H+] = 1.33 x 10-3
• pH = -log [H+] = -log [1.33 x 10-3]
pH = 2.88
Calculate Ka of 0.020M CH3COOH given pH is 3.90
CH3COOH ↔ H+ + CH3COO-
• Ka = [H+] [CH3COO-] / [CH3COOH], pH = -log [H+], 3.90 = -log [H+], [H+] = 1.26 x 10-4
• Ka = [1.26 x 10-4] [1.26 x 10-4] / 0.020
Ka = 7.92 x 10-7
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Calculate Conc of CH3COOH given Ka is 4.1 x 10-6 and pH is 4.50
CH3COOH ↔ H+ + CH3COO-
Ka = [H+] [CH3COO-] / [CH3COOH] • [CH3COOH] = [H+] [CH3COO-] / Ka
• pH = -log [H+], 4.50 = -log [H+], [H+] = 3.16 x 10-5
• [CH3COOH] = [3.16 x 10-5] [3.16 x 10-5] / 4.1 x 10-6
[CH3COOH] = 2.44 x 10-4
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Calculation of pH, Kb and Conc of Weak base
Calculate pH of 0.01M NH3 given Kb is 1.8 x 10-5
NH3 + H2O ↔ NH4+ + OH-
[NH4+][OH-] / [NH3] = Kb
• [OH-]2 / [NH3 ] = 1.8 x 10-5
• [OH-] = √1.8 x 10-7, [OH-] = 4.24 x 10-4 pOH = -log [OH], pOH = -log [4.24 x 10-4] = 3.37• pH + pOH = 14• pH = 14 - 3.37 = 10.6
pH = 10.6
Calculate pKb for 0.03M NH3 given pH 10.0 NH3 + H2O ↔ NH4
+ + OH-
Kb = [NH4+][OH-] / [NH3]
• Kb = [OH-]2/ [NH3] • pH + pOH = 14, pOH = 14 - 10 = 4, pOH = - log [OH], 4 = -log [OH], [OH] = 10 -4
• Kb = [10-4]2 / 0.03 = 3.33 x 10-7
• pKb = -log Kb = -log 3.33 x 10-7 = 6.48
pKb = 6.48
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Calculate the Conc base [CH3NH2 ], given pH is 10.8, pKa is 10.64
CH3NH2 + H2O ↔ CH3NH3+ + OH-Kb = [CH3NH3+][OH- ] / [CH3NH2 ]
Convert pH/Ka/pKa to pOH/Kb/pKb cause it is a base
• pKa + pKb = 14, pKb = 14 - 10.64, pKb = 3.36
• pH + pOH = 14, pOH = 14 - 10.8, pOH = 3.2
• Kb = [OH-]2 / [CH3NH2 ]
• [CH3NH2 ] = [OH-]2 / [Kb ] = [10-3.2]2 / 10-3.36
[CH3NH2 ] = 9.13 x 10-4
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Video on Ka calculation using Approximation Method
Key notes from video:For pH Cal and Significant figures
• pH = -log [H+] = -log [1.3 x 10-3] = 2.88
• Conc = 1.3 x 10-3(2 sig fig)
• pH = 2.88 (3 sig fig)
• Number sig fig diff for log calculation
• IB Cal, approximate method is used instead of using quadratic
Click HERE to view approximate calculationClick HERE to view quadratic calculation
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Acknowledgements
Thanks to source of pictures and video used in this presentation
Thanks to Creative Commons for excellent contribution on licenseshttp://creativecommons.org/licenses/
Prepared by Lawrence Kok
Check out more video tutorials from my site and hope you enjoy this tutorialhttp://lawrencekok.blogspot.com