These notes were written for the old IB syllabus (2009). The new IB syllabus for first examinations 2016 can be accessed by clicking the link below.

IB syllabus for first examinations 2016

Acids and bases


9.2 Strong and weak acids and bases

Strong and weak acids are defined by their ease of losing (or donating) a proton.

A strong acid, when placed in water, will almost fully ionise/dissociate straight away, producing H3O+ ions from water.

A weak acid will, however, only partially dissociate into ions, leaving a high percentage of unreacted molecules in the solution. An equilibrium is established, and so when some of the H3O+ ions produced by a weak acid react, Le Chatelier's principle means that more of the acid will react to form H3O+ ions. This means that, given an equal number of moles of acid, they will be neutralized by the same amount of strong base, but their solutions will have different pH values.

A weak base is the same as this, only it accepts protons and so produces OH- ions from water rather than H3O+.

Conductivity

Any solution's ability to conduct electricity is conditioned by the concentration of ions it contains. A strong acid has more ions than a weak one, and so it's solution will be a better electrical conductor than a weak acid. The same goes for strong/weak bases.

Strong acids : HCl, HNO3, H2SO4.

Weak acids : CH3COOH, H2CO3.

Strong bases : group 1 hydroxides (ie NaOH etc), or lower group 2 hydroxides Ba(OH)2.

Weak bases : NH3, CH3CH2NH2.

The strength of an acid or base can be measured with an indicator (universal) or a pH meter. also the rate of reaction...hydrogen production with metals or CO2 with CaCO3 will reveal the strength of an acid.

Strong and weak acids


9.3 The pH scale

pH vales ranges from 0 to 14 (7 being the neutral value of pure water at 20c and 1 atm).

Lower pH value are acidic, higher values are basic.

pH can be measured with a pH meter, or with pH paper (paper containing a mixture of indicators to cause a continuous color change).

pH is a measure of the dissociation of an acid or base, and also of the concentration of that acid / base (actually its related to the concentration of H3O+ ions).

If we have two solutions with their pH values, the lower one will be more acidic and the higher one will be more basic (though they could both still be basic/acidic with respect to water -- pH 7).

Relationship between pH and acid concentration

A change of 1 in the pH scale represents a 10 times change in the acidity or basicity of the solution (because it's a log scale).

pH = - log [H+]

The pH scale


9.4 Buffer solutions

These are solutions that resist changes in pH when small amounts of acid or base are added

There are two types of buffer.

  1. Weak acid and the salt of the same weak acid, (for example a solution containing ethanoic acid and sodium ethanoate). This gives a buffer solution with a pH less than 7
  2. Weak base and the salt of a the same weak base (for example ammonia and ammonium chloride solution). This gives a buffer with a pH greater than 7

The first (acidic) buffer works in the following way.

If an acid is added it combines its free hydrogen ions with the ions from the salt of the weak acid making molecular weak acid that cannot affect the pH.

If a base is added the OH- ions from the base react with the H+ ions that are present from the weak acid dissociation. Having been removed from the solution this stimulates the weak acid to produce more H+ ions (Le Chatelier's Principle) and the original pH is re-established.

9.5 Acid - Base Titrations

A titration is where small quantities of one component is added a little at a time to a solution of the other component in the presence of an indicator until the indicator registers the neutral point.

A graphical plot of the volume of one component added against the pH, gives distinctive curves depending on the strength of the acid and base. Examples...