Neutralization, Indicators, and Titrations

Neutralization Reactions So far we have only looked at acid and base reactions with water Ka and Kb reactions are only looking at what happens when we put an acid or base in water. Nothing else.

Neutralization Reactions When an acid and base are both placed in the same aqueous system, they react. These are a special type of double replacement reaction called a neutralization reaction

Neutralization Reactions In a neutralization reaction, an acid and base react to make water and salt Salt is just a name for an ionic compound Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Neutralization Reactions HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) Let’s look at the ionic equation for this reaction. H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) → Na+(aq) + Cl-(aq) + H2O(l) Now if we look at the net ionic equation H+(aq) + OH-(aq) → H2O(l)

Neutralization Reactions We will ONLY look at STRONG ACID AND STRONG BASES reacting This is because when we have equal moles of acid and base, we will have a neutral solution A pH of 7 If you did not have a BOTH strong acids and bases, the pH when the moles are equal would not be exactly 7. It would depend on the Ka and Kb of the acids and bases

Titrations Neutralization reactions are used in a lab procedure called a Titration In a titration, an acid and base are mixed until their moles are equal. When they are neutralized This can be used to find the concentration of either substance When the moles are equal this is called the equivalency point

Titrations Typically the substance with the known concentration (standard) is added to the substance with the unknown concentration The substance being added from the burette is called the Titrant The substance in the flask being added to is called the Analyte

Indicators During a titration, the equivalency point is shown using an indicator Indicators change colour in acidic or basic conditions Every indicator has it’s own equilibrium that allows it to change colour

Indicators Indicators change colour based on their own equilibrium In-(aq) + H+(aq) ⇌ HIn(aq) Basic Form Acidic Form Colour 1 Colour 2 Phenophthalein

Indicators Because indicators work using equilibrium, they have a range that they will change colours over. This means that when an indicator changes colour, it is not an EXACT pH that they do this at. Because of this, different titrations may need different indicators. Indicator pH range Colour change methyl orange 3.2 - 4.4 red to yellow litmus 5.8 - 8.0 red to blue phenolphthalein 8.2 - 10.0 colourless to pink

Indicator Examples Indicator pH range Colour change methyl orange 3.2 - 4.4 red to yellow litmus 5.8 - 8.0 red to blue phenolphthalein 8.2 - 10.0 colourless to pink Example - A given solution turns methyl orange yellow, litmus blue, and phenolphthalein pink. What is the lowest possible pH of the solution? Methyl orange in yellow when pH is above 3.2 Litmus is blue when pH is above 5.8, and Phenolphthalein is pink when pH is above 8.2. Therefore the solution would have to have a pH of 8.2

Indicators The moment the colour changes in a titration is called the Endpoint The moles may not be equal at the endpoint, but the colour has changed indicating that you should stop adding your substance.

Titration Examples During a titration 75.8 mL of a 0.100 M standard solution of HCl is titrated to end point with 100.0 mL of a NaOH solution with an unknown concentration. What is the concentration of the NaOH solution? 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 0.0500 M HCl solution is added. What was the original concentration of the Sr(OH)2 solution?