Acid Base Equilibria (a2 edexcel chem unit 4)

edexcel a2 unit 4 revision notes


1777, Lavoisier - acids have oxygen

***HCl has no oxygen!***

1810, Davy - acids have hydrogen

1838, Liebig - an acid is a compound containing hydrogen that can react with a metal to produce hydrogen gas

1884, Arrhenius - provided the first theory for the behavious of acid/base in solution


When acids/bases/salts dissolve in WATER ONLY they PARTIALLY/COMPLETELY dissociate into charged particles (ions)

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An electrolyte...

This is a substance that produces solutions of ions.

For example, acids are considered to be eletrolytes because the produce H+ ions in water;

HA <=> H+ (aq) +A- (aq)

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When equal volumes with the SAME molar concentrations of acid and base are mixed, no effect will be shown on litmus paper, i.e. the solution has been neutralised.

This occurs because the number of H+ is equal to the number of OH-.

H+ (aq) + OH- (aq) -> H2O (aq)


* note: this reaction is NOT equilibrium

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Oxonium ion = H3O+ ion

HCl and ethanoic acid do not show acidic properties when dissolved in an ORGANIC solvent for example, methylbenzene.

Evidence of the oxonium ion:

You can see evidence of this in an experiment, where the positive ions in an acid solution carrying current will move at the same rate as a water molecule.

However,  if H+ existed on its own, it would move much quicker as its mass is significantly smaller. Therefore, there is definately a oxonium ion present here.

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Bronsted and Lowry

These were chemists with a different definaition of acids and bases.

- An acid is a proton donater

- A base is a proton acceptor

Note: there is no water necessary to make these statements correct (Arrhenius refers to reactions IN WATER)

Conjugate pairs

The acid on one side of an equilibirum equation has a conjugate pair with the base on the other side etc.

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Examples of ACIDS/BASES


strong - HCl (hydrochloric acid)

weak - CH3COOH (ethanoic acid)


strong - NaOH (sodium hydroxide)

weak - NH4OH (ammonium hydroxide)

Equations to learn:

For a strong acid; pH = -log[H+]

For a weak acid; 10^√(KaC)        where Ka - acid constant C -

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Methyl orange

ACID - red

BASE - yellow


ACID - colourless

BASE - pink

Bromothymol blue                            Bromophenol blue

ACID - yellow                                   ACID - yellow

BASE - blue                                     BASE - purple

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On card 7 you have spelt phenolphthalein incorrectly but really good

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