Chemistry C3 - Acids and Alkalis

• acid + alkali → salt + water
• acid + metal oxide → salt + water
• acid + metal hydroxide → salt + water
• acid + metal carbonate → salt + water+ carbon dioxide
• acid + metal → hydrogen gas + salt

When any acids and alkalis react together, the neutralisation can be summarised as
H⁺_(aq) + OH^- _(aq) → H₂O _(l)

Robert Boyle - acids had particles that squeeze in between spaces and break materials apart.

Lavoisier - gave oxygen its name meaning 'acid maker' because he thought all acids contained oxygen

Laurent - showed that acids contain hydrogen, but didn't explain how it worked

Arrhenius suggested that in water, substances could be split into positive and negative ions.

He called this 'electrolytic dissociation'

This gave rise to the Arrhenius definition for acids and bases:

• an acid is any substance that produces positive hydrogen ions (H⁺) in water
• an alkali is anything that produces negative hydroxide ions (OH^-) in water.

Arrhenius' definition was not widely accepted because -

• People did not believe that molecules can be split
• Ahead of its time as subatomic particles have not yet been discovered
• Not enough infomation/proof
• His scientific reputatation is not regarded highly

Suggested that:

• Acids are PROTON DONORS
• Bases/Alkalis are PROTON ACCEPTORS

Brønsted and Lowry realised that in water, the H+ ions bond to water molecules (hydrated) forming H₃O _(aq) ions - represented as H

By this point, protons and electrons were well understood and accepted, so scientists had little difficulty accepting Brønsted and Lowry's definition

Concentration - a measure of how much acid or alkali is dissolved in a known volume of liquid, eg water

Molarity - a measure of concentration: the number of moles dissolved in 1000cm³ of water

pH scale - a measure of the concentration of hydrogen ions

Strong Acids have a large number of H+ ions. They are completly (100%) ionised

Examples -

• Hydrochloric Acid - HCl
• Nitric Acid - HNO₃
• Sulphuric Acid - H₂SO₄

Weak Acids have few H+ ions. They are only partly ionised.

Examples -

• Vinegar
• Citric Acid
• Carbonic Acid

Strong Alkalis have a large number of OH- ions. They are completly (100%) ionised

Examples

• Sodium Hydroxide - NaOH
• Postassium Hydroxide - KOH

Weak Alkalis have few OH- ions. They are only partly ionised.

Examples -

• Ammonium

Titrations are used to find concentrations

Titrations Steps

• Add Alkali to conical flask using pipette
• Add Acid to burette
• Add Indicator to flask
• Open tap and gently release acid
• Swirl the conical flask to ensure the acid and alkali are reacting
• Continue until indicator changes colour
• Record volume of Acid in burette

PIPETTE - used to measure out a fixed solution

BURETTE used to measure the volume of solution added

MOLES = CONCENTRATION x VOLUME

MASS = MOLES x RELATIVE FORMULA MASS

For Mol/dm3 - (or mol dm -3)

1) Calculate Moles

2.) Balanced Equation & Ratio

3.) Concentration - mol/dm3

For grams/dm3

4.) RFM - relative formula mass of acid

5.) Convert Moles to grams

STRONG acid + STRONG alkali = ANY INDICATOR

WEAK acid + STRONG alkali = PHENOLPHTHALEIN

Colour change -

In Alkalis - Colourless --> Pink

STRONG acid + WEAK alkali = MEYTHL ORANGE

Colour change -

In Acids - Orange --> Yellow

In Alkalis - Orange --> Pink