4.16. Neutralization of Acids and Base: Acid-Base Titrations

Transcription

1 4.16. Neutralization of Acids and Base: Acid-Base Titrations A.Review Titration: used to measure exact amounts of acid added to base (or vice versa) Buret Moles of standard Moles of sample Standard Solution Sample Solution 5 factors of titration: 1. [acid] 2. vol of acid 3. [base] 4. vol of base 5. base/acid mole ratio Formulae c=n/v or M = mol/l eg) 25.0mL of 0.500M KOH soln. is used to titrate 19.8mL of HClO 4 soln. What is the [HClO 4 ] and the ph? Sample soln: Standard soln: HClO 4 + KOH KClO H 2 O conversion: vol. KOH #mol KOH #mol HClO 4 conc. HClO 4 moles HClO 4 = [HClO 4 ] = n = V ph = -log{h 3 O + } = 1

2 B. Equivalence Point (Stoichiometric Point) Equivalence Point: eg1) HCl + NaOH H 2 O + NaCl At equivalence point (eq. pt.): moles of NaOH = eg2) 2HCl + Ba(OH) 2 2H 2 O + BaCl 2 At eq. pt. : moles of Ba(OH) 2 C. Reading Data in the lab For increased accuracy titrations are repeated Titration accuracy should be ± 0.02 ml For beginners ± 0.1 ml Titration Volume of Standard Solution 1 st Titration ml 2 nd Titration ml 3 rd Titration ml throw out and volume used in your calculation should be the average of your accepted data average volume = Read p Hebden # Indicators Indicators: (p. 159) Solution colour depends on which form has higher [conc] if [acid form] = [base form], the intermediate colour is observed. eg) BromoThymol Blue = HBTB is acid form, and BTB - in base form HBTB H + + BTB - in basic solution(ph > 7.6), have BTB - (blue ) in acidic solution(ph < 6.0).have HBTB (yellow) Transition range = ph = range where colour changes Transition point = ph where [HBTB] exactly equals [BTB - ] 2

3 At transition point, [HBTB] = [BTB - ] we see yellow blue Ka of indicators: Ka is calculated from the transition point (ph = 6.8 for above example) Ka = [H + ][ BTB - ] [HBTB] also recall [HBTB] = [BTB - ] Ka expression simplifies to Ka = [H + ] Ka = [H + ] = HBTB H + + BTB - Q: What is Ka for ethyl orange? A: At transition point, pka = ph = 4.1 Ka = Q: What is Ka for phenol red? (Use indicator table p. 335) A: At transition point, pka = ph = Ka = TRANSITION POINT SUMMARY [HIndicator] = [Indicator - ] Ka (indicator) = [H 3 O + ] pka = Ph The colour is a 50/50 mixture of the acid and base colours o Yellow + Blue = Green Read p Hebden #

4 Standard Solutions & Titration Curves Standard solution: (p.164). Primary Standard: used to prepare standard solution (p.164) has known is and does not absorb or from the air Q: Why shouldn t NaOH be used as a Primary Standard? (p.164) Standardizing a Solution Note: NaOH can be made to an accuracy of 0.1 M (by standardizing method) and is used in the lab. Standardizing a Solution is done by titrating a solution with a in order to find its accurate concentration. Primary Standard Acid titrates NaOH Solution (or other non primary standard) titrates Acidic Sample Solution 1. Titrates and determines [NaOH] 2. Now [NaOH] is standardized 3. Standard solution of NaOH can determine [H3O + ] of acidic samples solutions Read p Hebden #

5 A Titration of a Strong Acid with a Strong Base E.g. NaOH + HCl Na + (aq) + Cl - (aq) + H 2 O ions produced no reaction [OH - ] = [H 3 O + ] ph = 7 E.g. A p. 166 Conc. of sample solution is known BUT The purpose of a titration is to determine the conc. of a sample solution. LET S PRETEND WE DON T KNOW THE CONC. OF THE ACID (the sample solution) Strong Base Strong Acid Data table from p. 166 could be obtained in the lab using a ph meter. The data can be used to draw a graph, which will tell us the ph of the sample solution. Your graph should have the following: A title Y axis labeled ph X axis labeled volume of NaOH Best fit line or curve Lets draw the graph. There are 4 stages in the above process: 1. Acid (sample) before any Base (standard) is added 2. Base added 3. Equivalence (Stoichiometric) Point reached 4. Base added in excess 5

6 Label the equivalence point (eq. pt) on the graph It is located in the middle of the steep portion of the graph At eq. pt. [OH - ] = [H 3 O + ] What is the ph at this point? Notice how quickly ph changes at even though not very much NaOH is added. Label V B on your axis V B = volume of base needed to reach eq. pt. What is V B? Note: the ph behaviour of a strong acid and base will resemble your graph Note 2: the eq. pt. can also be determined using an indicator instead of a ph metter. Hebden # 124 6

7 4.16. Neutralization of Acids and Base: Acid-Base Titrations A.Review Titration: used to measure exact amounts of acid added to base (or vice versa) Buret Moles of standard Moles of sample Standard Solution Sample Solution 5 factors of titration: 1. [acid] 2. vol of acid 3. [base] 4. vol of base 5. base/acid mole ratio Formulae c=n/v or M = mol/l eg) 25.0mL of 0.500M KOH soln. is used to titrate 19.8mL of HClO 4 soln. What is the [HClO 4 ] and the ph? Sample soln: (HClO 4 ) Standard soln: KOH) HClO 4 + KOH KClO H 2 O conversion: vol. KOH #mol KOH #mol HClO 4 conc. HClO 4 moles HClO 4 = (0.0250L KOH) x mol KOH x 1 mol HClO 4 = mol HClO 4 1 L KOH 1 mol KOH [HClO 4 ] = n = = M HClO 4 V L ph = -log[h 3 O + ] = -log[ ] =

8 B. Equivalence Point (Stoichiometric Point) Equivalence Point: point where the actual mole ratio of Sample-Standard mixture is same as the coefficient ratio in the balanced equation. eg1) HCl + NaOH H 2 O + NaCl At equivalence point (eq. pt.): moles of NaOH = moles of HCl eg2) 2HCl + Ba(OH) 2 2H 2 O + BaCl 2 At eq. pt. : moles of Ba(OH) 2 = moles of HCl x 2 C. Reading Data in the lab For increased accuracy titrations are repeated Titration accuracy should be ± 0.02 ml For beginners ± 0.1 ml Titration Volume of Standard Solution 1 st Titration ml 2 nd Titration ml 3 rd Titration ml throw out titration 1 and volume used in your calculation should be the average of your accepted data average volume = = Read p Hebden # Indicators Indicators: a weak acid or base that has different colours for its conjugate acid and base forms(p. 159) Solution colour depends on which form has higher [conc] if [acid form] = [base form], the intermediate colour is observed. eg) BromoThymol Blue = HBTB is acid form, and BTB - in base form HBTB H + + BTB - in basic solution(ph > 7.6), have BTB - (blue ) in acidic solution(ph < 6.0).have HBTB (yellow) Transition range = ph = range where colour changes Transition point = ph 6.8 where [HBTB] exactly equals [BTB - ] 8

9 At transition point, [HBTB] = [BTB - ] we see green yellow blue Ka of indicators: Ka is calculated from the transition point (ph = 6.8 for above example) Ka = [H + ][ BTB - ] [HBTB] also recall [HBTB] = [BTB - ] Ka expression simplifies to Ka = [H + ] Ka = [H + ] = = 1.6 x 10-7 HBTB H + + BTB - Q: What is Ka for ethyl orange? A: At transition point, pka = ph = 4.1 Ka = = 7.9 x 10-5 Q: What is Ka for phenol red? (Use indicator table p. 335) A: At transition point, pka = ph = 7.3 Ka = = 5.0 x 10-5 TRANSITION POINT SUMMARY [HIndicator] = [Indicator - ] Ka (indicator) = [H 3 O + ] pka = ph The colour is a 50/50 mixture of the acid and base colours o Yellow + Blue = Green Read p Hebden #

10 Standard Solutions & Titration Curves Standard solution: solution of accurately known conc used to titrate other another soln in order to determine their concs. (p.164) Primary Standard: used to prepare standard solution has known molar mass is pure and stable does not absorb water or CO 2 from the air (p.164) Q: Why shouldn t NaOH be used as a Primary Standard? (p.164) rapidly absorbs water and CO 2 from the air % pure Note: NaOH can be made to an accuracy of 0.1 M (by standardizing method) and is used in the lab. Standardizing a Solution Standardizing a Solution is done by titrating a solution with a primary standard in order to find its accurate concentration. Primary Standard Acid titrates NaOH Solution (or other non primary standard) titrates Acidic Sample Solution 1. Titrates and determines [NaOH] 2. Now [NaOH] is standardized 3. Standard solution of NaOH can determine [H3O + ] of acidic samples solutions Read p Hebden #

11 B Titration of a Strong Acid with a Strong Base E.g. NaOH + HCl Na + (aq) + Cl - (aq) + H 2 O Spectator ions produced no hydrolysis reaction [OH - ] = [H 3 O + ] ph = 7 E.g. A p. 166 Conc. of sample solution is known BUT The purpose of a titration is to determine the conc. of a sample solution. LET S PRETEND WE DON T KNOW THE CONC. OF THE ACID (the sample solution) Strong Base Strong Acid Data table from p. 166 could be obtained in the lab using a ph meter. The data can be used to draw a graph, which will tell us the ph of the sample solution. Your graph should have the following: A title Y axis labeled ph X axis labeled volume of NaOH Best fit line or curve Lets draw the graph. There are 4 stages in the above process: 1. Acid (sample) before any Base (standard) is added 2. Base added 3. Equivalence (Stoichiometric) Point reached 4. Base added in excess 11

12 Titration of HCl of an Unknown Concentration Using Standardized M NaOH Label the equivalence point (eq. pt) on the graph It is located in the middle of the steep portion of the graph At eq. pt. [OH - ] = [H 3 O + ] What is the ph at this point? 7.0 Notice how quickly ph changes at even though not very much NaOH is added. Label V B on your axis V B = volume of base needed to reach eq. pt. What is V B? 1.0 L Note: the ph behaviour of a strong acid and base will resemble your graph Note 2: the eq. pt. can also be determined using an indicator instead of a ph metter. Hebden #