Acids and Bases: pH curves, titrations and indicators and Buffer action

Equivalence point

Point at which the acid and base are in the correct molar ratio (the correct stoichiometric amounts)

What pH range is indicator Methyl orange used for and what is its colour in acid and in alkali?

• 3.2 - 4.4

• Colour in acid: Red

• Colour in alkali: Yellow

What pH range is indicator Phenolphthalein used for and what is its colour in acid and in alkali?

• 8.2 - 10.0

• Colour in acid: Colourless

• Colour in alkali: Pink

What is half equivalence point of a titration and what can it be used to measure?

• When the volume of base added is equal to half the volume needed for equivalence

• Ka

• [H+] = Ka as HA = A^-

Buffer

A solution that maintains an approximately constant pH on addition of small amounts of acid or base

What are 2 household applications of buffers?

• Shampoos

• Biological washing powder

What is an acidic buffer solution?

A mixture of a weak acid, HA, and one of its salts, A^-. It maintains a pH below 7.

Outline 2 ways you can make an acidic buffer solution

• Add together a weak acid and one of its salts. HA + A^-

• Partly neutralise a weak acid to get some HA and some A^-

What is a basic buffer solution?

A mixture of a weak base, B, and one of its salts, BH⁺. It maintains a pH above 7.

Outline 2 ways you can make a basic buffer solution

• Add together a weak base, B, and some of its salt, BH⁺

• Partly neutralise a weak base, B, to leave some B and some of its salt, BH⁺

Equation to calculate concentration of H⁺ in acidic buffer solution

[H⁺] = ( Ka [HA] ) / ( [A^-] )

Enthalpy of neutralisation

The energy change that accompanies the neutralisation of an aqueous acid by an aqueous base to give one mole of water under standard conditions