Analysing Acids and Bases Chapter 4

Back to Basics – A little revision Many simple acid base reactions are neutralisation reactions represented by the general equation Acid + base → salt + water The reaction between aqueous solutions of HCl and NaOH is a neutralisation reaction.

Brønsted-Lowry Theory The acid – is a proton (H+) donor The base – is a proton (H+) acceptor An acid-base reaction involves two conjugate acid-base pairs. Lets look at the reaction of HCl and water!!

Brønsted-Lowry Theory Acids according to theory must contain hydrogen. However OH- contains a hydrogen and is almost always a base. Acids must be ready to donate a proton. Bases must clearly be capable of accepting protons (H+ ions). To do this they must have a significant region of negative charge, such as δ- on NH3 or H2O molecules or full negative charges as on OH- or CO32- ions.

Your Turn Some Revision Questions Page 38 Question 5

pH The concentration of H3O+ ions in a solution is referred to as the solutions acidity. The definition of pH is: pH = -log[H3O+] Where [H3O+] is the concentration in mol L-1 Neutral solutions pH = 7 at 25°C Acidic solutions pH < 7 Basic solutions pH > 7

Indicators An indicator is used during acid-base titration to identify the equivalence point of the reaction. An acid-base indicator is a substance whose colour depends on the concentration of H3O+ ions in a solution. Indicators are weak acids with their acid form being one colour and their conjugate base being another.

Common Indicators The colours of common indicators and the pH range over which they change colour are given

Indicators The indicator must be chosen carefully to ensure that the point during the titration where the indicator changes colour, the end point, closely matches the equivalence point.

Indicators a b Figure 4.4 pH curves showing change of pH during a titration of a a strong base with a strong acid, and b a weak base with a strong acid. Phenolphthalein, which changes colour in the pH range 8.2–10, gives a sharp end point in a but a broad end point in b. Methyl orange, which changes colour between pH 3.1 and 4.5, would be a more suitable indicator for the second titration.

Worked Examples Page 40 4.3a

Dilution Some analyses using titration involve dilution of the solution being analysed. The need to dilute is generally determined during the design of a particular chemical analysis. There are many reasons for deciding to dilute a sample prior to analysis. When the sample is not a colourless solution, dilution of the coloured solution may be necessary to enable effective identification of the endpoint of the titration. Also dilution may be necessary to ensure that the titre does not exceed the volume of the burette.

Worked Example A student analysing a household ‘cleaning agent’ for ammonia content followed the following procedure. A 20.0mL sample of the cleaning agent was weighed then added to a clean 250mL volumetric flask. The flask was then filled to the calibration mark with distilled water and the dilution thoroughly mixed. 20.0mL aliquots of this diluted cleaning agent solution were then titrated with 0.120 M HCl using methyl orange indicator. The following experimental data was recorded mass of cleaning agent used = 21.392 g average V(HCl) used = 25.25 ml

Determine n(HCl) used n(NH3) in 20.0 mL of diluted cleaning agent n(NH3) in 20.0 mL of cleaning agent The molarity of the cleaning agnet with respect to NH3 The percentage, by mass of NH3 in the cleaning agent

Your Turn Have a look at this worked example on page 41 Try page 42 Question 6 Question 8

Back Titration Some acids and bases are so weak that they do not produce a sharp colour change at the end point of a titration. A technique known as back titration is used to overcome this problem.

Back Titration The procedure for back titration has two parts If the substance two be analysed acts as a weak acid, it is mixed with an excess fo a strong base. The original amount of strong base is known. All of the weak acid reacts in stoichiometric proportions, leaving some of the strong base unused. The unused strong base is titrated as normal with a standard solution of a strong acid. Knowing the original amount of strong base and the amount of strong base left unused, it is possible to work back to find the amount of weak acid present.

Determining the concentration of ammonium ions in lawn fertiliser Worked Example 4.4 page 43

Your Turn Page 44 Question 9 Page 46 Question 26 and 28