Acid-Base Titration Curves Questions and Answers

Acid-Base Titration Curves Questions and Answers Acid-Base Titration Curves Titration curves show how pH varies during titration Strong Acid-Strong Base Titration Curve 1) Strong Acid-Strong Base Titration Curve. Due to the nature of the log scale i.e. a ten fold change in the [H+] is required to result in a change in pH of only one unit. 2) The pH then increases much more rapidly, and between a pH of about 3.5 and 10.5, the curve is virtually vertical This is due to the fact that the [H+] is low and adding NaOH has a larger effect on the pH. Equivalence point is 7 3) Equivalence point is 7, the strong acid and strong base have completely reacted with each other to form salt and water. At this point, there is no H+ ions from the SA in the solution. The [H+] in the solution is due to dissociation of water. 4) After the equivalence point, the pH changes only gradually again and as more as more NaOH is added, the pH gets closer to 13 All the acid has been neutralised and the OH-ions are in excess in this region Variations on Strong Acid-Strong Base Titrations Double the conc of base = eqivalance point at half original volume. Double conc base and acid = shape the same, lower start ph and higher end ph Adding acid to alkalai = flipping the curve Calculating the points on a Strong acid-Strong base titration curve Full dissociation, conc HCl = conc H+. pH at equivalance point = 7 Calculating the points on a Weak acid-Strong base titration curve Find initial pH using the existing Ka value. pH at the equivalence point is more than 7. Weak salt, conjugate base (strong), reacting with water to release HO- ions. With cotinual addition, it will reach the pH of the base. Why is H2O pH 7 Dissosiation of H+ and OH- ions SA-SB titration curve vs WA-SB titration curve SA-SB = Less steep, starts with a lower pH WA-SB = More steep, starts with higher pH, partially dissociated, conc ions is lower, adding the same vol of strong base will give a greater effect. Both end at the same pH pKa of acid= pH at half equivalence point Weak acid strong base WA-SB pka = pH at half equivalance point Strong Acid-Weak Base Titration Curve 1) pH initially changes very little, [H+] is reasonably high in this region and therefore adding NH3 does not affect the overall pH very much, small change in pH is also due to the nature of the log scale i.e. a ten fold change in the [H+] is required to result in a change in pH of only one unit 2) Weak base releases conjugate acid, This reaction increases the concentration of H+ ions in the solution and the pH at the equivalence point is therefore lower than 7 3. After the equivalence point, the pH changes only gradually again and as more as more NH3 is added, the pH gets closer to 9 All the acid has been neutralised and the OH-ions are in excess in this region As more and more NH3 is added, the solution resembles more and more closely to the original NH3 solution, whose pH is 9 pKb of base= pOH at half equivalence point Strong acid weak base pH + pOH = 14 at 25celcius all acid all base Initial pH can be calculated using the Ka value for weak acid WA-WB Ka= [H+][A-]/[HA] pH = -log [H+] Weak Acid-Weak Base Titration Curve here is no very steep/almost vertical part in the WA-WB titration curve, Change in pH throughout the titration is more gradual, pH at equivalence point may be lower than 7 or higher than 7, depending on the relative strength of acid and the base. Strong acid - less than 7, STorng base more than 7. pKa -> Ka 10^-pKa = Ka Monochrotic acid donates only 1 H+ H indicator (aq) <--> H+(aq) + indicator-(aq) Ionised (In-) and un-ionised (HIn) forms must have different colours for the substance to function as an indicator Indicators that are weak acids will dissociate according to the equation HIn(aq) <--> H^+ +In^-(aq) (colour 1 unionised) (colour 2 ionised) pH ranges of indicators and their suitability as indicators for different acid base titrations find table, memorise Equivalence point + indicators The equivalence point is the point where there are equal number of moles of acid and base. Half equivalence point refers to the point at which there are comparable/equal number of ionised and unionised forms of the acid in an acid dissociation. Therefore, pKa of the indicator = pH of the indicator at its half equivalence point. formula, pKa= pH. - step 1 know that equvalence point is 7 for SASB - step 2 find if you have SA SB WA WB, find their predicted equivalence point (above of below 7) - Step 3 find pH range of indicator (pKa +-1) - step 4 match