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Acids and Bases
Quick revise

The amount of detail needed depends on exactly which part of the course you are on. Someone aiming for the higher level of the Further Chemistry course would need all of this. Check with you specification to find out exactly what you require.

Acids are hydrogen ion donors. When they react, they release H+ ion(s).

Bases neutralise acids. They are hydrogen ion acceptors. When they react, they accept H+ions.

Some bases are known as alkalis. They contain the OH-ion and are soluble in water.

Strong acids are totally dissociated in water. This means that for hydrochloric acid (HCl) all of the HCl molecules break down to give H+ and Cl- ions. There is no HCl left.

Weak acids are partly dissociated in water. This means that for ethanoic acid (CH3COOH) some of the CH3COOH molecules break down to give CH3COO- and H+ but some remain as CH3COOH. This is nothing to do with the CH3 bit not changing, because these hydrogen are never released. The important thing is that some of the molecules that release the hydrogen atoms that can be released as ions and some don't.

Strong and weak is different to concentrated and dilute. Concentrated means a lot of substance in only a little solvent; dilute means lots of solvent and only a little substance.

Ethanoic acid is only ever a weak acid (because that is how ethanoic acid molecules are) but you can have it in a dilute form (like vinegar) or in the very smelly concentrated form.

Be careful the next time someone asks you if you want coffee strong or weak. You can tell them that you cannot change how strong it is but you might want to make it more dilute or more concentrated.

Choosing an Indicator

We use an indicator to find the volume of an acid that just reacts with a certain volume of an alkali.

We are trying to find the volumes of each required to give equal numbers of moles of H+(aq) and OH-(aq) ions.

Strong acid with strong base

Assume we have the alkali in the flask and add acid to it. To start with, the pH is high (around 14) and stays there until just before the end-point when it falls steeply to around pH 1. Any indicator that changes colour between these pH values will do as an indicator for this reaction.

Weak acid with strong base

Assume we have the alkali in the flask and add acid to it. To start with, the pH is high (around 14) and stays there until just before the end-point when it falls steeply to around pH 8. After that, it slowly decreases to around pH 3. Any indicator that changes colour between 14 and 8 will do as an indicator for this reaction. A good example for this would be phenolphthalein (pink in pH > 10 and colourless in pH < 10).

Strong acid with weak base

Assume we have the alkali in the flask and add acid to it. To start with, the pH is fairly high (around 11) and decreases fairly quickly towards the end-point when it falls steeply from around pH 6 to around pH 1. After that, it stays low. Any indicator that changes colour between 6 and 1 will do as an indicator for this reaction. A good example for this would be methyl orange (red in pH > 4 and orange in pH < 4).

Weak acid with weak base

Sorry, there is no suitable indicator.

 

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