2. Acids, Alkalis and Titrations
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Synthetic vs Natural Plant Indicators
• Synthetic indicators are used to show the endpoint in titrations as they have a very sharp change of colour when an acid has been neutralised by an alkali and vice-versa • Litmus (plant) is not suitable for titrations as the colour change is not sharp and it goes through a purple transition colour in neutral solutions making it difficult to determine an endpoint
Red Litmus paper in acids
Stays red
Blue litmus paper in acids
Turns red
Blue Litmus paper in alkali
Stays Blue
Red litmus papers in alkali
Turns blue
phenolphthalein In acids
Stays Colourless
phenolphthalein In alkali
Turns from colourless to pink
methyl orange In acids
Orange to red
methyl orange In alkali
Orange to yellow
Is universal indicator suitable for acid base titrations?
No it doesn’t have a sharp colour change.
PH Scale description
• pH 0-3 = strong acid • Extremely acidic substances can have values of below 1 • pH 4-6 = weak acid • The higher the pH then the more alkaline the solution is • pH 8-10 = weak alkali • pH 11-14 = strong alkali • A solution of pH 7 is described as being neutral
Universal Indicator
• Universal indicator is a wide-range indicator giving an approximate pH value. • Made from a mixture of plant indicators covering a broad pH range. • Add a few drops to a solution, then match the colour to a pH chart. • Colours may vary by manufacturer, so always use the chart provided with the specific indicator
What do acids form when in water?
• When acids are added to water, they form positively charged hydrogen ions (H+) • The presence of H+ ions is what makes a solution acidic
What do alkali solutions form when in water?
• When alkalis are added to water, they form negative hydroxide ions (OH–) • The presence of the OH– ions is what makes the aqueous solution an alkali
Describe Neutralisation
An acid reacts with an alkali; H⁺ ions react with OH⁻ ions to form water.
What is the pH scale a measure of?
• The pH scale is a numerical scale which is used to show how acidic or alkaline a solution is • It is a measure of the amount of the hydrogen ions present in solution
Do all acid reactions count as neutralisations?
No — for example, when an acid reacts with a metal, a salt forms but no water is produced, so it isn’t a neutralisation.
The net ionic equation of all acid-base neutralisations and is what leads to a neutral solution, since water has a pH of 7. What is it?
H+ (aq) + OH– (aq)⟶ H2O (l)
What is a titration?
A method of analysing the concentration of solutions.
What do acid - base titrations do?
They determine exactly how much acid is needed to neutralise an alkali, and vice versa.
What tool is used to measure the volume of an alkali in a titration?
A 25cm3 pipette
What is the purpose of a white tile in a titration?
The purpose of the white tile is to make colour changes more obvious.
What piece of equipment is used to deliver an acid in a titration?
A burette
What colour does purple litmus paper turn in acids?
Red