Acids and Bases  

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

8.2 - Properties of acids and bases


8.2.1: Outline the characteristic properties of acids and bases in aqueous solution.Bases that are not hydroxides, such as ammonia, soluble carbonates, and hydrogencarbonates should be included. Alkalis are bases that dissolve in water. Students should consider the effects on indicators and the reactions of acids with bases, metals and carbonates.


Acid and base characteristics

These are a group of compounds with similar chemical behaviour - i.e. they react in similar ways with other compounds. This is because all acids provide free H+ ions (hydrogen ions) in solution. It is these H+ ions that are actually reacting.

  1. Acids turn indicators characteristic colours (this depends on the indicator) representing.
  2. Acids react with bases to form salt and water (neutralisation). The base may be a metal oxide or a metal hydroxide.
  3. Acids react with carbonates forming a salt, carbon dioxide and water (neutralisation)
  4. Acids react with active metals to form salt and hydrogen (this is not actually a neutralisation reaction although the acid does get used up)

These properties are only expressed in aqueous solution (i.e. a solution in water). The reason for this is that acids can only release their hydrogen ions when they interact with water molecules.

hydrochloric acid + water hydrogen ions + chloride ions
HCl   (aq) H+(aq)   Cl-(aq)

Consequently, hydrochloric acid provides a solution that contains hydrogen ions and chloride ions - It is only the hydrogen ions that react with bases the chloride ions remain in the solution as spectator ions

Other common strong acids:

In all cases the acid provides a source of hydrogen ions


Bases

These are the chemical opposites of acids. The property of bases is caused by the presence of OH- (hydroxide) ions in solution.

The bases either produce these hydroxide ions directly by dissolving or they assist the water molecules in breaking up by removing H+ ions from the water. (The situation is a little more complicated than explained as the water is actually in equilibrium with its own ions)


8.2.2: State whether a given acid or base is strong or weak. Specified strong acids are hydrochloric acid, nitric acid and sulphuric acid. Specified weak acids are ethanoic acid and carbonic acid (aqueous carbon dioxide). Specified strong bases are all group 1 hydroxides and barium hydroxide. Specified weak bases are ammonia and ethylamine.


Strong and weak acids and bases

It is important to understand, and differentiate between, the terms 'concentration' and 'strength' when referring to acids (or bases)

Acidity is caused by the presence of hydrogen ions in the solution. If ANY acid has a high concentration then this will consequently increase the hydrogen ion concentration making the acid 'stronger'. The concentration of an acid (or any other solute ) is measured in moles/litre (mol dm-3).

When chemists refer to strong and weak acids they are referring to the degree with which the acid molecules break apart to give ions in aqueous solution (dissociation). A strong acid completely breaks apart to give ions in solution (100% dissociation) whereas a weak acid only slightly dissociates in solution (perhaps less than 1%)

Strong and weak acids are defined by the ease with which they lose (or donate) hydrogen ions (protons).

A strong acid, when placed in water, will almost fully ionise/dissociate straight away, producing H+ (aq) 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 dissociate to form more H+ 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 (H+ ions) and so produces OH- ions from water rather than H3O+ .

In this case the syllabus statement says it all. A student should recognise the strong acids and bases and be able to name a few weak acids and bases. It is taken as said that organic acids are weak.

Strong acids

Weak acids

Strong bases

Weak bases


8.2.3: Describe and explain data from experiments to distinguish between strong and weak acids and bases, and to determine the relative acidities and basicities of substances.


Methods for differentiation

If the concentration of the acid is known then measurement of the pH using a pH meter (or similar) will tell us whether or not an acid is strong or weak.

Example

A 0.1 M solution of hydrochloric acid has a pH value of 1.00

a 0.1 M solution of ethanoic acid has a pH value of 2.87

Comparison of the two pH values shows us that the hydrochloric acid is completely dissociated into ions whereas the ethanoic acid is only partially dissociated into ions

Neutralisation enthalpy

Any strong acid will release approximately - 57 kJ/mol of energy on neutralisation with strong base.

This energy release is due to the formation of water molecules from the H+ ions from the acid reacting with the OH- ions from the base.

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

When a weak acid is neutralised some of the energy is needed to dissociate the molecules to make the hydrogen ions available for neutralisation. The result is a value for neutralisation enthalpy lower than - 57 kJ/mol (numerically lower, not more negative!)

Note that as the weak acid is in equilibrium as the hydrogen ions are 'mopped up' by the base the equilibrium shifts to the right hand side to make more until eventually all of the acid is able to react. This does, however, absorb energy as bonds are being broken to dissociate the weak acid.

CH3COOH H+(aq) + CH3COO-(aq)
approx 99%                    approx 1%

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.

The conductivity may be measured using a power pack and two graphite electrodes connected to an ammeter. The apparatus is assembled and current values measured for a given voltage setting. The strong acids pass more current than the weak acids for the same voltage.

Strong acids : HCl, HNO3, H2SO4. - good conductors - large value for current passing

Weak acids : CH3COOH, H2CO3. - poor conductors - low value for current passing

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

Weak bases : NH3, CH3CH2NH2. - poor conductors

Reaction rates

Similarly, the rate of reaction will reveal the strength of an acid. The rate of a chemical reaction is usually proportional to the concentration of the reactants.

As it is the hydrogen ions that are reacting the concentration of these ions at any one time will be less for a weak acid of the same concentration than for a strong acid.

Example: The reaction between magnesium metal and 0.1 M acids (ethanoic acid and hydrochloric acid)

The hydrochloric acid had a hydrogen ion concentration of 0.1 mol/litre. These hydrogen ions can react directly with the magnesium producing hydrogen gas. There will be collisions between the magnesium atoms and the hydrogen ions.

The ethanoic acid has fewer free hydrogen ions in the solution. It has a hydrogen ion concentration of 0.0013 mol/litre consequently there will be fewer collisions between hydrogen ions and magnesium atoms and hence the reaction will be slower.

Experimentally both reaction rates can be determined by measuring the volume of gas released against time and plotting a graph. The slope of the graph will give the reaction rate at that time


Resources


Weak acid equilibrium

Measuring conductivity