L5 - Strong and Weak Acids & Bases

objectives

Understandings:

• Strong and weak acids and bases differ in the extent of ionization.

• Strong acids and bases of equal concentrations have higher conductivities than weak acids and bases.

• A strong acid is a good proton donor and has a weak conjugate base.

• A strong base is a good proton acceptor and has a weak conjugate acid.

Applications and skills:

• Distinction between strong and weak acids and bases in terms of the rates of their reactions with metals, metal oxides, metal hydroxides, metal hydrogen carbonates and metal carbonates and their electrical conductivities for solutions of equal concentrations.

Guidance:

• The terms ionization and dissociation can be used interchangeably.

• See section 21 in the data booklet for a list of weak acids and bases.

Strength of Acids and Bases

• Strength of an acid or base depends on the degree to which it ionizes (dissociates) in water
• In the following reactions, an acid or a base reacts with water to produce conjugates
  • Strong acid is an effective proton donor that is assumed to completely dissociate in water.
    • Examples: HCl, H₂SO₄ and HNO₃
    • Conjugate base of strong acid is weak base
  • Weak acid only partially dissociates in water; it is a poor proton donor.
    • Examples: CH₃COOH and H₂CO₃
    • The dissociation of a weak acid is a reversible reaction that reaches equilibrium
    • Only small portion of acid molecules have dissociated at equilibrium
    • Conjugate base of weak acid is strong base
  • Strong base completely dissociates in water
    • Examples: KOH, NaOH (Any group 1 metal hydroxides)
    • Note: Metal Oxides don’t act as Bronsted-Lowry bases, however in solution the hydroxide ion OH^– acts as a base
  • Weak base partially dissociates in water
    • Example: NH₃
• Words strong and concentrated, and weak and diluted have very different meanings.
  • Strong: completely dissociated into ions
  • Concentrated: a high number of moles of solute per litre of solution
  • Weak: slightly dissociated
  • Diluted: a low number of moles of solute per litre of solution

Experimental determination of the strength of acids and bases

• Techniques to compare strength of acids and bases of equal concentration
• Conductivity
• All acids and bases dissociate in water to form ions. The conductivity of an aqueous solution depends on the concentration of ions present. Voltage is applied to different solutions (of equal concentration), and the voltage reflects on concentration of ions, and thus strength as well.
• Strong acids and bases have higher conductivities
• Weak acids and bases have lower conductivities

Energy changes on neutralization

• Neutralization: occurs when an acid and base react together
• This reaction is exothermic, ΔH < 0
• The enthalpy change of neutralization for a strong acid is almost identical to that of weak acid
• For strong acid or base:
• They are completely dissociated in water, thus only enthalpy change to consider is exothermic formation of water from H⁺ and OH^– ions
• For weak acid or base:
• Dissociation of weak acid or base is mildly endothermic
• Enthalpy change for strong base-weak acid is less than strong base-strong acid
• The weaker the acid, the more endothermic, and thus lower enthalpy change of neutralization
• 
  

experimentally determining strength of acids and bases

Comparison by pH

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. Stronger acids have lower pH values for the same concentration.

The concentration of any acid, strong or weak, may be found by titration against a standardised base solution. Suitable concentrations for titration are in the range 0.01 - 0.2 mol dm-3.

If we wish to investigate the strength of an acid using pH measurements than the concentration of each acid must be controlled.

Using a pH Meter

These are instruments that rely of the small differences of potential set up between an internal reference (normally a glass electrode) and a solution of hydrogen ions. Before being used the pH meter should be calibrated for the range of pH that it is to be used to measure.

The pH meter can only be used to differentiate between weak and strong acids providing that the concentrations are known. However, it is convenient to determine the concentration of an acid (or base) by titration.

If two samples of acid, weak and strong, have the same concentration then the solution with the lower pH is the stronger acid..

Enthalpy of Neutralisation

All strong acids 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, making the hydrogen ions available for neutralisation. The result is a value for neutralisation enthalpy lower than - 57 kJ/mol (numerically lower, not more negative!).

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

In reality, the difference in enthalpy of neutralisation is not large. The value for ethanoic acid is about -55 kJ/mol.

Electrical Conductivity

A 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 (or something related to it) 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.

Strong acids pass more current than the weak acids for the same voltage and concentration.

The actual conductance of the ionic solution is more difficult to calculate, as it depends on the distance between the electrodes and the overall resistance of the circuit.

• 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

The reason for this is that strong acids (and bases) are 100% dissociated into ions. This means that they provide more ions for conductivity than weak acids (and bases) of comparable concentrations.

Reaction Rate

Similarly, the rate of reaction will reveal the strength of an acid. The rate of a chemical reaction is proportional to the concentration of the reactants. If a suitable reaction of acids is chosen, then the reaction rate should demonstrate which of any pair is a strong and which is a weak acid.

The rate of reaction between an acid and a metal carbonate could be investigated.

Two acids of equal concentration are added to samples of marble chips of equal mass. The rate of the reaction is studied by plotting the volume of gas evolved against the time taken.

The rate at any time is obtained from a graph of volume against time by measuring the gradient to the curve.

The ionic equation for this reaction is:

CO32-(aq) + 2H+(aq) --> CO2(g) + H2O(l)

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.