C3aL6 Acids and Bases

Key Words

Acid - a compound that produces hydrogen ions in aqueous solution

Alkali  - a soluble base.

Aqueous - state of being dissolved in water.

Base - a substance that is a hydroxide, carbonate, or oxide.

Dissociation - splitting apart into ions.

Hydrated - water forms part of the structure.

Hydrogen ion - a proton.

Hydroxide ion - OH-.

Neutralisation - a reaction that produces between an acid and a base to produce a solution of pH 7

Proton acceptor - a base

Proton donor - an acid give up H+ ions when it's in aqueous solution.

Titration - a neutralisation reaction in which an acid is added to a known quantity of base, and the acid is measured when the pH7 point is reached

Test Yourself

Homework
Physics GCSE
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Grade E

Hydrogen reacts with other non-metals to form molecules, which in turn dissolve in water to form acids, for example, hydrochloric acid from hydrogen chloride.  When the hydrogen chloride dissolves in the water, the molecules dissociate:

  • The hydrogen ends up as an H+ ion, which is a proton;
  • The chloride ends up as a chloride ion, Cl-.  It is a negative ion because it has the electron from the hydrogen atom.

Compare the two pictures:

 

The hydrogen that was in the molecule has split away, or dissociated.  Since it has lost its electron, it is an H+ ion, which is a single proton.  The hydrogen chloride has given up a proton, so it's called a proton donor.

All acids are proton donors.

 

Bases are oxides, hydroxides, and carbonates.   All bases are proton acceptors. Bases that dissolve in water form alkalis, because they produce hydroxide (OH-) ions.  Here are some typical acids and bases on the pH scale.

 

Grade C

Strong acids dissociate almost completely into ions.  In weak acids there is a high proportion of the molecule still existing as molecules, with only limited dissociation.  With weak alkalis the proportion of  OH- ions is low compared to the molecules.  In strong alkalis, there is almost complete dissociation.  For example:

HNO3(aq) ® H+(aq) + NO3-(aq)

NaOH(aq) ® Na+(aq) + OH-(aq)

Strong and concentrated acids are not the same thing.  You can have a concentrated weak acid, or a dilute strong acid.

 

Acid Reactions:

metal + acid ® salt + hydrogen

Ca(s) + H2SO4(aq) ® CaSO4(aq) + H2(g)

Acid + base ® salt + water

CuO(s) + H2SO4(aq) ® CuSO4(aq) + H2O(l)

2KOH(aq) + H2SO4(aq) ® K2SO4(aq) + 2H2O(l)

All of these result in neutral salts, so they are called neutralisation reactions. 

Grade A

You will have done titrations as an experiment.  Revise titrations by clicking on the link.  You will have plotted your data onto a graph like this.  (You might have cheated and used a data logger.)

There are indicators that change colour when you get to the endpoint:

  • Phenolphthalein changes from pink to clear when the solution goes neutral from alkaline.  It remains clear if the solution is acid.

  • Methyl orange changes from orange to clear when the solution goes neutral from acid.  It remains clear if the solution is alkaline.

  • Bromothymol blue changes to bluish green as the solution goes neutral.  If the solution is acid, it is yellow. If alkali, it is blue.

The neutralisation point comes very suddenly.  If there is an excess of acid, the pH drops rapidly to 1.

You can add alkali to acid (it's not good for the burette) and the curve is the opposite way round.