2.6 Acids, Alkalis and Titrations

REVISION NOTES

IGCSE Edexcel Chemistry

2.6 Acids, Alkalis and Titrations

2.6.1 Describe the use of litmus, phenolphthalein and methyl orange to distinguish between acidic and alkaline solutions

2.6.2 Understand how to use the pH scale, from 0–14, can be used to classify solutions as strongly acidic (0–3), weakly acidic (4–6), neutral (7), weakly alkaline (8–10) and strongly alkaline (11–14)

2.6.3 Describe the use of universal indicator to measure the approximate pH value of an aqueous solution

UNIVERSAL INDICATOR

Universal indicator is a wide range indicator
Can give only an approximate value for pH
A few drops are added to the solution and the colour is matched with a colour chart which indicates the pH which matches with specific colours

2.6.4 Know that acids in aqueous solution are a source of hydrogen ions and alkalis in a aqueous solution are a source of hydroxide ions

ACIDS

Source of hydrogen ions H⁺
The presence of H⁺ ions is what makes a solution acidic

ALKALIS

Source of hydroxide ions OH^–
The presence of OH^– ions is what makes the solution an alkali

2.6.5 Know that alkalis can neutralise acids

NEUTRALISATION

A neutralisation reaction occurs when an acid reacts with an alkali
H⁺ ions from acids and OH^– ions from alkalis react to produce water

2.6.6C Describe how to carry out an acid-alkali titration

ACID-ALKALI TITRATION

Titrations are a method of analysing the concentration of solutions
They can determine exactly how much alkali is needed to neutralise a quantity of acid – and vice versa
It relies on the colour change of an acid–alkali indicator
Calculations will be carried out for mole, volume and concentrations
Titrations can also be used to prepare salts

HOW TO CARRY OUT TITRATION

Use a measuring cylinder to measure a known volume of an acid, add it to a conical flask
Add a few drops of indicator to the same conical flask
Place the conical flask on a white tile (for easier observation of colour change)
Fill the burette with a known concentration of an alkali
Record the initial volume of the alkali
Slowly add the alkali to the flask in small portions, swirl the flask in between to ensure reaction fully takes place
Immediately close the tap when the colour changes (it means it reaches the end-point)
Record the final volume of the alkali in the burette