STRONG AND WEAK ACIDS AND BASES
Introduction
When acids or bases dissolve in water, they dissociate (break apart) into their consituent ions as follows:
Acids: HA(aq)  H+(aq) + A-(aq)
Bases: BOH(aq)  B+(aq) + OH-(aq)
However, this is not the full story as only some acids (like HCl) and bases (like NaOH) dissociate fully as
shown in the equation. These are called strong acids and bases.
Some acids and bases rather than dissolving as above dissolve more like this:
Acids: HA(aq)  H+(aq) + A-(aq)
Bases: BOH(aq)  B+(aq) + OH-(aq)
You will (hopefully) notice that the arrow has been replaced with an equilibrium symbol. This means that in
a solution of these, you will have a mixture of dissolved but un-dissociated molecules and dissolved and
dissociated ions. We call these weak acids and bases and they include acids such as carbonic acid and the
carboxylic acids and bases such as ammonia and the amines.
The distinction between strong and weak acids and bases is important because it means that, for example
1.0 M solutions of a strong and weak acid will have different concentrations of H+ and thus different pH. It
also means that weak acids and bases are affected by equilibrium effects (Le Chateliers principle etc) in a
way that strong acids are not, which leads to quite different behaviour.
In this activity you will conduct a few short experiments to compare the similarities and differences of
strong and weak acids and bases.
Experiment 1: Reactions of Acids
Intro:
In this experiment we will compare similar reactions involving strong and weak acids.
Method: All observations should be carefully recorded.
Place 5-10 cm3 of 1.0 M hyrochloric acid in one boiling tube and of 1.0 M ethanoic (acetic)
acid in another. Add a 1 cm length of magnesium ribbon to each and compare. Do similar
but instead add a pipette-full of sodium carbonate solution.
Analysis: 1. Write balanced equations equations for all reactions.
2. How do the observations for the strong acid compare with those for the weak? Explain.
Experiment 2: Conductivity
Intro:
Observe the difference in the conductivity of a strong and weak acid
Method: Place 25 cm3 of 1.00 M hydrochloric acid in a small beaker. Connect a wire and croc-clip to
the positive terminal of a power-supply and wire/ammeter/wire/croc-clip to the negative
terminal. Set the power supply to 6V and switch it on. Place the crocodile clips into the acid,
holding them approx 1 cm apart (do not touch them) and record the current. Repeat with
1.00 M ethanoic acid.
Analysis: 3. Use your understanding of strong/weak acids and your knowledge of electricity to
account for the difference.
Experiment 3: Reactions of Bases
Intro:
In this experiment we will compare the nucleophilic substitution of bromobutane. This is
something you will meet in detail in the organic chemistry unit. The reaction produces
bromide ions which, in the presence of Ag+ ions will form an observable precipitate:
C4H9Br(aq) + OH- (aq)  C4H9OH(aq) +Br-(aq)
Br-(aq) + Ag+(aq)  AgBr(s)
Method:
In each of two test-tubes, place about 1 cm3 of ethanol and 2-3 drops of 1-bromobutane.
Simultaneously add about 1 cm3 of either 1.0M NaOH or 1.0M NH3. Start a stopwatch and
observe closely. If the reaction is going too slowly, you may want to speed things up with
the aid of a warm water bath.
Analysis: 4. How did the rate of reaction with the strong base compare to that for the weak base?
Experiment 4: Neutralisation
Intro:
In this experiment you will be measuring the change in pH as you add portions of strong base
to strong/weak acid and strong acid to strong/weak base.
Method:
Use this general method with each of your strong/weak acids/bases.
Use a bulb pipette to add 25 cm3 of a 1.0M solution of either HCl/CH3COOH/NaOH/NH3 to a
100 cm3 beaker. Set up a burette with 50 cm3 of either 1.0M HCl or 1.0M NaoH (depending
on what is in your beaker. Use a pH probe to measure the pH in the beaker and record this
(you may need to tilt the beaker to get it deep enough). Gradually add liquid from the
burette until you have neutral solution and record the total amount added.
Analysis:
5. How does the amount of base required to neutralise the strong acid compare to the
weak acid? Why does this happen?
6. How does the amount of acid required to neutralise the strong base compare to the
weak base? Why does this happen?
Experiment 5: Dilutions
Intro:
In this experiment you will look at the change in pH as you dilute strong and weak acids.
Method:
Using a measuring cylinder measure out 50 cm3 of 1.0 M hydrochloric acid, place it in a 100
cm3 conical flask and measure the pH with a pH probe. Use a graduated pipette to transfer 5.0
cm3 of your acid to a new conical flask, dilute it by adding 45 cm3 distilled water from a
measuring cylinder, mix well and measure the pH. Repeat using your newly diluted acid until
you have 7 conical flasks of increasingly dilute acid. Place a splash of universal indicator in each
one. Repeat using 1.0 M ethanoic acid.
Analysis:
7. How does the change in pH on dilution for the strong acid compare to the weak acid?
8. Fully explain why using ideas about equilibrium.