17.1 Common Ion Effect

Transcription

1 17.1 Common Ion Effect Buffer Solutions The resistance of ph change Dr. Fred Omega Garces Chemistry 201 Miramar College 1 Common Ion Effect

2 Common Ion Effect Ionization of an electrolyte, i.e., salt, acid or base is decreased when a common ion is added to that solution. i) What is the % ionization for M acetic acid? (Pure) HC 2 H 3 O 2 + H 2 O D C 2 H 2 O 2 + H 3 O + K a = M Solving the ice problem: k a = = M = [C 2 H 3 O 2 ] [H 3 O + ] /0.10 M g [H 3 O + ]= M % a = ( / 0.10 ) * 100 = 1.34 % ph = 2.87 ii) What is % a if M HC 2 H 3 O 2 is mix w/ 0.100M NaC 2 H 3 O 2? (Buffer) HC 2 H 3 O 2 + H 2 O D C 2 H 2 O 2 + H 3 O + K a = M i Lots C x x +x +x [c] 0.100x Lots x x k a = M = [ x ] [x] /( x) [0.100] [x] /( ) x = [H 3 O + ]= M ph = 4.74 % a = ( / 0.10 ) * 100 = % Ionization % decrease in presence of common ion!! 2 Common Ion Effect

3 Common Ion Effect Equation Consider the previous problem in which a common ion is in the same solution. HC 2 H 3 O 2 + H 2 O D C 2 H 3 O 2 + H 3 O + K a = M i Lots C x x +x +x [c] 0.100x Lots x x or [c] [HC 2 H 3 O 2 ] Lots [C 2 H 3 O 2 ] [H 3 O + ] k a = [C 2 H 3 O 2 ] [H 3 O + ] rearrange the equation [H 3 O + ] = k a [HC 2 H 3 O 2 ] [HC 2 H 3 O 2 ] [C 2 H 3 O ] Taking the log of both side log [H 3 O + ] = log (k a [HC 2 H 3 O 2 ] / [C 2 H 3 O 2 ] ) or ph = log k a log( [HC 2 H 3 O 2 ] / [C 2 H 3 O 2 ] ) let C a = [HC 2 H 3 O 2 ] and C b = [C 2 H 3 O 2 ] ) therefore ph = pk a log C a / C b or ph = pk a + log C b / C a This is the Henderson Hasselbach Equation: ph = pk a + log C b / C a or poh = pk b + log C a / C b 3 Common Ion Effect

4 HendersonHasselbach Equation ph of a solution can be calculated using a useful equation: ph = pk a + log [A ] / [HA] Where HA & A are the weak acid and its conjugate and K a is for HA Similarly, poh = pk b + log [HA] / [A ] Where HA & A are the weak base and its conjugate and K b is for A 4 Common Ion Effect

5 HendersonHasselbach Equation: Example Consider the common ion effect problem and lets see how the HendersonHasselbach equation can be used to simplify this problem. What is ph if M HC 2 H 3 O 2 is mix w/ 0.100M NaC 2 H 3 O 2? HC 2 H 3 O 2 + H 2 O D C 2 H 2 O 2 + H 3 O + K a = M i Lots C x x +x +x [c] 0.100x Lots x x Using the HendersonHasselbach equation: ph = log ( ) + log (0.100 / 0.100) ph = log 1 ph = ph = 4.74 Note: When a common ion is present in the same solution, the strategy to solve the problem requires a Buffer Type of calculation. 5 Common Ion Effect

6 HendersonHasselbach Equation and Buffer Problems (srf) A buffer M acetate and M acetic acid is prepared (K a = ). i) What is the ph of the buffer? ii) Calculate the initial ph, final ph, and change in ph that result when 1.00 ml of M HCl is added to ml of the buffer. iii) Calculate the initial ph, final ph, and change in ph that result when 1.00 ml of M HCl is added to ml of water. Note: HCl = M 1.00mL = 0.1 mmol. C 2 H 3 O 2 =0.100 M 100mL = 10 mmol and HC 2 H 3 O 2 =0.200 M 100mL = 20 mmol i) ph = pka + log Cb/Ca = log( ) + log ( 0.10 / 0.20) g ph = 4.44 ii) C 2 H 3 O 2 + H 3 O + D HC 2 H 2 O 2 + H 2 O s 10mmol 0.1 mmol 20 mmol Lots R f Lots [c] 9.9/ /101 V T = 101 ml ph = log ( )+log [(9.9/101) / [20.1/101)] = ph = 4.43 ph (initial) = 4.44, ph (final) 4.43, DpH (change) = Common Ion Effect

7 ...continue...continue: HendersonHasselbach Equation and Buffer Problems A buffer M acetate and M acetic acid is prepared (K a = ). Reger iii) Calculate the initial ph, final ph, and change in ph that result when 1.00 ml of M HCl is added to ml of water. Note: HCl = M 1.00mL = mmol. iii) HCl + H 2 O D H 3 O + + Cls mmol M R 0.100mmol mmol f mmol [c] mmol / 101 ml [H 3 O + ] = M g ph = 3.00 ph (initial) = 7.00, ph(final) 3.00, DpH(change) = Common Ion Effect

8 Essential Feature of Buffer Systems A buffer solution exhibits very small change in ph changes when H 3 O+ and OH is added. A buffer solution consists of relatively high concentration of the components of a conjugate weak acidbase pair. The buffercomponents concentration ratio determines the ph, and the ratio and ph are related by the HendersonHasselbalch equation. A buffer has an effective range of pka + 1 ph unit. 8 Common Ion Effect

9 Blood Buffer System Buffer A solution whose ph is resistant to change Your body uses buffers to maintain the ph of your blood Blood ph Buffer system in body 1. Proteins 2. Phosphates HPO 2 4 / H 2 PO 4 : 1.6 / 1 3. Carbonates H 2 CO 3 / HCO 3 : 10 / 1 Reaction: H 3 O + + HCO 3 D H 2 CO 3 + H 2 O H 2 CO 3 g H 2 O + CO 2 (exhale) 9 Common Ion Effect

10 Acidosis Blood ph i 7.35 (ACIDOSIS) Depression of the acute nervous symptom. Or respiratory center in the medulla of the brain is affected by an accident or by depressive drugs. Symptoms: Depression of the acute nervous system Fainting spells Coma RIP Causes: 1. Respiratory Acidosis Difficulty Breathing (Hypoventilation) Pneumonia, Asthma anything which diminish CO 2 from leaving lungs. 2. Metabolic Acidosis Starvation or fasting Heavy exercise Mechanism: 1. Respiratory Acidosis CO 2 doesn t leave lungs which result in the build up of H 2 CO 3 in the blood 2. Metabolic Acidosis If body doesn t have enough food then Fatty acids (Fat) are used. Fatty Acids g Acidic. Furthermore, exercise leads muscle to produce lactic acid. 10 Common Ion Effect

11 Alkalosis Blood ph h 7.45 (ALKALOSIS) Hyperventilation during extreme fevers or hysteria. Excessive ingestion of basic antacids and severe vomiting Symptoms: Over simulation of the nervous system Muscle cramps Convulsion Death Causes: 1. Respiratory Alkalosis Heavy rapid breathing (hyperventilation). Results from fear, hysteria, fever, infection or reaction with drugs. 2. Metabolic Alkalosis Metabolic irregularities or by excess vomiting Mechanism: 1. Respiratory Alkalosis Excessive loss of CO 2 lowers H 2 CO 3 and raise HCO 3 level (Can be remedied by breathing in a bag) 2. Metabolic Alkalosis Vomiting removes excess acidic material from stomach. (ph of stomach equals one). 11 Common Ion Effect

12 Buffer System at Work Buffer System that resists change in ph when H 3 O + or OH is added. Buffer solution may be prepared by a weak acid and its conjugate base. How it Works: A g HA H 3 O + Buffer H 2 O Remember ph = Conc. of H 3 O + Your blood Rxn: HCO 3 D H 2 CO 3 Acidosis Excess H 3 O + + HCO 3 g H 2 CO 3 + H 2 O H 3 O + CO 2 + H 2 O Alkalosis Excess OH + H 2 CO 3 D HCO 3 + H 2 O OH 12 Common Ion Effect

13 Summary The following summary lists the important tools needed to solve problems dealing with acidbase equilibria. Equation / Concept Function 1 [H + ] [OH ] = K w Permits the calculation of [H + ] or [OH ] when the other is known. 2 p X = log X This equation is the basis of the pscale. 3 ph + poh = This equation shows the relationship between the ph and the poh 4 HA! H + + A K a = [H+ ] [A ] [HA] 5 B + H 2 O! HB + OH K b = [HB] [OH ] [B] 6 Percent ionization (α) amount ionized α = 100% initial amount This is the Mass Action Equation for the ionization of a weak acid in water. This equation yields the k a given the equilibrium concentration of all specie. The equation also yields the [H 3 O+] given the initial concentration of the weak base [HA] and the k a. This is the Mass Action Equation for the ionization of a weak base in water. This equation yields the k b given the equilibrium concentration of all specie. The equation also yields the [OH] given the initial concentration of the weak base [B] and the k b. The percent ionization can be calculated from the initial concentration of the acid (or base) and the change in the concentration of the ions. Given the percent ionization (α) and the ph, the k a (or k b ) can be determined. 7 K a K b = K w This equation relates K a and K b for conjugate pairs in aqueous solution, 8 ID of the solute as : i) only a weak acid ii) only a weak base iii) a mixture of a weak acid and its conjugate base Identification of the function of the solute leads to the correct Mass Action expression and thereby leading to the correct equilibrium law. This is a critical first step to solve any acidbase equilibria 9 Identification of acidic cations and basic anions 10 Assumption which simplifies Mass Action 11 Reactions when H+ or OHare added to a buffer solution. Identification of function of cation and anion of a salt lead to ph of the salt solution. Given the k a or k b of the conjugates of these ions leads to the calculation of the ph or poh In order to simplify the math calculation of a Mass Action expression, assumption can be made base on the k a or k b value. Understanding the buffer reaction permits the determination of the effect of a strong acid or strong base on the ph of the solution. Adding H+ lowers the [A] and raises [HA], adding OH lowers [HA] and raises [A]. 13 Common Ion Effect