18.5 INDICATORS Watch the videos on natural indicators and acid base titration and state three factual pieces of information you learned about indicators. http://wps.pearsoned.com.au/ibchl/107/27555/7054172.cw/index.html Read the information about indicators online (because it is in color) at: http://www.chemguide.co.uk/physical/acidbaseeqia/indicators.html Before you move onto the questions make sure that you can: 1. Qualitatively describe the action of an acid base indicator 2. State and explain how the pH range of an acid‐base indicator relates to its pKa value 3. Identify an appropriate indicator for a titration, given the equivalence point of the titration and the pH range of the indicator. Examples of indicators are listed on page 17 of the IB Chemistry data booklet. Questions 1. (M06/H) a) Describe how an indicator, HIn works. [4] b) Name a suitable indicator for the reaction between ethanoic acid and sodium hydroxide. Use the information on page 17 of the Chemistry data booklet. Explain your choice. [2] 2. A.
B.
C.
D.
(M05/H) Which statement about indicators is correct? The mid point of an indicator’s color change is at pH = 7 The pH range is greater for indicators with higher pKa values. The color red indicates an acidic solution. The pKa value of the indicator is within its pH range. 3. (M05/H) An experiment was carried out to determine the concentration of an aqueous solution of ammonia by titrating it with a solution of sulfuric acid of concentration 0.150 moldm‐3. It was found that 25.0 cm3 of the ammonia solution required 20.1 cm3 of the sulfuric acid solution for neutralization. a) Write the equation for the reaction and calculate the concentration in mol dm‐3 of the ammonia solution. [4] 1
b) Several acid‐base indicators are listed in Table 17 of the Data Booklet. State and explain which one of the following indicators should be used for this experiment: bromocresol green, phenol red or phenolphthalein. [3] 4. 5. 6. 7. (N05/H) The pH of a solution is 4.8. Using the information on Table 17 of the Data Booklet, deduce and explain the colours of the indicators bromophenol blue and phenol red in this solution. [3] (N00/5) Write the equilibrium expression for HIn and show how the pKa value of the indicator relates to the pH value at which is changes colour. [4] (N005/H) Explain which indicator will be the most suitable for a reaction between a monoprotic acid HA with a base MOH with an equivalence point pH = 8? [2] (M98/H) What color would be observed when two drops of phenolphthalein is added to 10cm3 of the CH3COONa solution? [2] 8. (N06/H/1) Which neutralization reaction could use phenolphthalein (pka = 9.3) and not methyl orange (pka = 3.7) as an indicator? A.
sodium hydroxide and nitric acid B.
ammonia and ethanoic acid C.
sodium hydroxide and ethanoic aicd D.
ammonia and nitric acid 9. Distinguish between the end point and equivalence point in a titation. 10. The best indicator will be when the pKa(indicator) is is close to the pH(equiv point). For the following titrations: I. Identify with a reason the acid‐basic nature of the salt II. Suggest an equivalence point pH for the titration III. Suggest with a reason a suitable indicator for detecting the end point. a) strong acid and strong base b) strong acid and weak base c) strong base and weak acid 2
18.6 INDICATORS 1. (M06/H) a) HIn is a weak acid ; (1) ⇔ HIn(aq) Weak acid b) H+(aq) + In‐(aq) ; (1) Conjugate base Answers In acid equilibrium moves left / to HIn ; (1) In base equilibrium shifts to the right / to In‐ ; (1) phenolphthalein / penol red / bromothymol blue ; (1) color change of indicator occurs within the range of pH at equivalence point / on vertical part of the graph. ; (1) salt produced will be basic so pH range at equivalence point 7 ‐ 10 pKa(indicator) changes color in the range of pH(equiv point) 2. (M05/H) D ; (1) 3. (M05/H) a) 2 NH3 + H2SO4 → (NH4)2SO4 ; (1) n(H2SO4) = c x v = 0.150 x (20.1 ÷1000) = 0.003015 mol ; (1) from mole ratio in the equation NH3 + H2SO4 2 : 1 n(H2SO4) = 2 x n(NH3) n(NH3) = 2 x 0.003015 = 0.00603 mol ; (1) [NH3] = n ÷ v = 0.00603 ÷ (25.0 ÷ 1000) = 0.241 (moldm‐3) (3SF) ; (1) b) bromocresol green ; (1) reaction of a weak base and a strong acid ; (1) pH range bromocresol green is 3.8 to 5.4 / occurs at pH < 7 ; (1) 3
4. (N05/H) bromophenol blue is blue and phenol red is yellow in pH of 4.8 ; (1) pH of 4.8 is above the range for bromophenol blue / bromophenol blue shows its alkaline (blue) colour ; (1) pH of 4.8 is below the range of phenol red / phenol red shows its acidic (yellow) color / OWTTE ; (1) 5. (N00/H) +
⇔ HIn(aq) H (aq) + In‐(aq) ; (1) Weak acid Conjugate base The dissociation constant equation for the indicator is: [H+] Ka(indicator) = x [In‐] [HIn] Since [HIn] = [In‐] at the equivalence point ; (1) Therefore [H+] Ka = pKa(indicator) ; (1) pH ; (1) 6. (N00/H) at equivalence point the pKa(indicator) should be close to the pH(equiv point) since pH(equiv point) is 8 then the best indicator would have a pKa close to 8 ; (1) phenol red has a pKa = 7.9 ; (1) 7. (M98/H) ↔ CH3COO‐(aq) + H2O CH3COOH(aq) + OH‐(aq) ; (1) Since OH‐ ions are produced, the solution is alkaline so it will turn red ; (1) Or CH3COONa is a basic salt formed from a strong base and weak acid ; (1) so phenolphthalein will turn red ; (1) 8. (N06/H/1) C ; (1) 4
9. d) strong acid and strong base – the salt formed is neutral. The pH at equivalence point = 7 so the pKa of the indicator should be about 7. Bromothymol blue would be a good choice. e) strong acid and weak base – the salt is acidic. The pH at the equivalence point < 7 so the pKa of the indicator should be < 7. Methyl orange (pKa3.7), methyl red (pKa5.1), bromocresol green (pKa4.7), or bromophenol blue (pKa4.0) could be used depending on the exact equivalence point pH. f) strong base and weak acid – the salt is basic. The pH at equivalence point > 7 so the pKa of the indicator should be > 7. Depending on the actual pH range at the equivalence point phenol red (pKa7.9), or phenolphthalein (pKa9.3) could be used. 5