Unit 6 – Acids and Bases

 Indicators are dyes that change colour under varying conditions of acidity.  Although not as accurate as instruments such as pH meters in determining acidity, indicators can be used to give less precise measure of acidity.  Litmus is a very commonly used indicator which is red in acids and blue in bases.  Litmus is an indictor that changes colour from red to blue in the pH range of 5.5 to 8.0.  Other indicators and their colours are listed in the table of Acid - Base Indicators which you should keep available.Acid - Base Indicators

 Indicators are coloured compounds that exist in both acidic and basic forms. A general formula may be used for indictors - HIn - and a reaction written: HIn↔ H + + In - acidic formbasic form colour 1colour 2  Indicators may be in solution form or paper form. pH paper is prepared by treating the paper with the indictor solution. When the paper is then dipped into the solution you are testing, it will change colour depending on the acidity of the solution.

IndicatorpH rangeColour change methyl orange red to yellow litmus red to blue phenolphthalein colourless to pink  1. A given solution turns methyl orange yellow, litmus blue, and phenolphthalein red. What is the approximate pH of the solution? ◦ Methyl orange in yellow when pH is above 4.4 Litmus is blue when pH is above 8.0, and Phenolphthalein is red when pH is above ◦ Therefore the solution would have to have a pH above 10.0

IndicatorpH rangeColour change methyl orange red to yellow litmus red to blue phenolphthalein colourless to pink  2. What color would methyl orange, litmus, and phenolphthalein turn when testing:  a. vinegar (pH = 3) ◦ Methyl orange: red ◦ Litmus: red ◦ Phenolphthalein: colorless  b. sea water (pH = 8) ◦ Methyl orange: yellow ◦ Litmus: blue ◦ Phenolphthalein: colorless

 What happens when an acid such as HCl is mixed with a base such as NaOH: HCl (aq) + NaOH (aq) → NaCl (aq) + H 2 O (l)  When an acid and a base are combined, water and a salt are the products.  Salts are ionic compounds containing a positive ion other than H + and a negative ion other than the hydroxide ion, OH -.  Double displacement reactions of this type are called neutralization reactions.  We can write an expanded version of this equation, with aqueous substances written in their longer form: H + (aq) + Cl - (aq) + Na + (aq) + OH - (aq) → Na + (aq) + Cl - (aq) + H 2 O (l)  Removing the spectator ions we get the net ionic equation: H + (aq) + OH - (aq) → H 2 O (l)

 When a strong acid and a strong base are combined in the proper amounts - when [H + ] equals [OH - ] - a neutral solution results in which pH = 7. The acid and base have neutralized each other, and the acidic and basic properties are no longer present.  Salt solutions do not always have a pH of 7, however. Through a process known as hydrolysis, the ions produced when an acid and base combine may react with the water molecules to produce a solution that is slightly acidic or basic. We will not go into details here, but generally if a strong acid is mixed with a weak base there the resulting solution will be slightly acidic; if a strong base is mixed with a weak acid the solution will be slightly basic.

 A titration is a laboratory procedure used to determine the concentration of a solution.  During an acid-base titration an acid with a known concentration (standard solution) is added to a base with an unknown concentration (or vice-versa).  An indicator is also added to the solution which will signal (by a color change) when the base has been neutralized.

 Note that as soon as you see a color change the titration is complete and the [H + ] and [OH - ] are equal.  At this equivalency point the titration is stopped. By knowing: the initial volume of the base, the volume of acid added, and the initial concentration of the acid we can calculate the concentration of the base.

1) During a titration 75.8 mL of a M standard solution of HCl is titrated to end point with mL of a NaOH solution with an unknown concentration. What is the concentration of the NaOH solution? 2) A 20.0 mL solution of strontium hydroxide, Sr(OH) 2, is placed in a flask and a drop of indicator is added. The solution turns colour after 25.0 mL of a standard M HCl solution is added. What was the original concentration of the Sr(OH) 2 solution?