Acid and base theory
What is a Brønsted–Lowry acid?
A proton (H⁺) donor.
State two common properties of acids.
Acids have a pH below 7 and turn blue litmus red.
What does a pH of 7 indicate at 25°C?
The solution is neutral.
What is a neutralisation reaction?
A reaction between an acid and a base that produces a salt and, in typical aqueous neutralisation, water.
What does it mean if a neutralisation reaction is exothermic?
The reaction releases energy as heat to the surroundings.
What is a Brønsted–Lowry base?
A proton (H⁺) acceptor.
State two common properties of bases.
Bases have a pH above 7 and turn red litmus blue.
What is the difference between a strong acid and a weak acid?
A strong acid ionises essentially completely in water, whereas a weak acid only partially ionises.
Write the net ionic equation for the neutralisation of a strong acid by a strong base.
H+(aq)+OH−(aq)→H2O(l)
What is the enthalpy of neutralisation?
It is the enthalpy change when one mole of water is formed through the reaction of an acid and a base under specified conditions.
Identify the acid and base in the reaction and explain why:
HCl+H2O→H3O++Cl-
HCl is the acid because it donates H⁺.
H₂O is the base because it accepts H⁺.
What products are formed when an acid reacts with a metal carbonate?
A salt, water and carbon dioxide are formed.
Which solution contains the greater concentration of H⁺: pH 2 or pH 5?
The pH 2 solution contains a greater concentration of H⁺ ions.
What is the purpose of an indicator during an acid–base titration?
It indicates when the reaction has reached the endpoint by changing colour.
Why does the reaction between a strong acid and a strong base generally have a similar enthalpy of neutralisation regardless of which strong acid and strong base are used?
Strong acids and strong bases ionise essentially completely, so the net ionic reaction is primarily:
H++OH−→H2O
Identify the two conjugate acid–base pairs in:
NH3 + H2O ⇌ NH4+ + OH-
NH₃/NH₄⁺ is one conjugate pair.
H₂O/OH⁻ is the other conjugate pair.
Explain how you could experimentally distinguish between hydrochloric acid and sodium hydroxide.
An indicator could be added. Hydrochloric acid would show an acidic colour, while sodium hydroxide would show a basic colour. Alternatively, their different pH values could be measured.
Explain why pH alone cannot always be used to determine whether an acid is strong or weak.
pH depends on the concentration of H⁺ ions, which is affected by both the acid's strength and its concentration. A concentrated weak acid can therefore have a lower pH than a very dilute strong acid.
Explain why the choice of indicator is important in a titration.
The indicator should change colour over a pH range that is close to the equivalence point. If an inappropriate indicator is used, the observed endpoint may not accurately represent the equivalence point.
Identify two sources of error that could affect an experimental determination of the enthalpy of neutralisation.
Heat loss to the surroundings, which can cause the measured temperature change to be smaller than the actual change.
Heat absorbed by the equipment, such as the calorimeter or thermometer, which means not all released heat is measured as heating of the solution.
Explain why water is amphiprotic, using chemical equations to support your answer.
Water is amphiprotic because it can both donate and accept H⁺.
As an acid:
H2O+NH3→OH-+NH4+
As a base:
H2O+HCl→H3O++Cl-
A student reacts an unknown substance with dilute hydrochloric acid and observes vigorous bubbling. The gas produced turns limewater milky. What can the student conclude about the unknown substance?
The unknown substance likely contains a carbonate or hydrogencarbonate ion. The reaction with hydrochloric acid produces CO₂, which turns limewater milky.
Two solutions have the same concentration. Solution A is hydrochloric acid and Solution B is ethanoic acid. Explain why their pH values are different.
HCl is a strong acid and ionises essentially completely, producing a high concentration of H⁺. Ethanoic acid is a weak acid and only partially ionises, producing a lower H⁺ concentration. Therefore, HCl has the lower pH.
A student is titrating a weak acid with a strong base. Explain why the indicator used for this titration may be different from one used to titrate a strong acid with a strong base.
The equivalence point occurs at a different pH for a weak acid–strong base titration because the conjugate base of the weak acid affects the pH. Therefore, an indicator whose colour-change range occurs around this equivalence-point pH should be selected.
A student obtains an experimental enthalpy of neutralisation that is less exothermic than the accepted value. Explain why this result could occur.
Heat may have been lost to the surroundings or absorbed by the apparatus, meaning the measured temperature increase was smaller than the actual temperature increase. This would lead to an experimentally determined enthalpy change that appears less exothermic than the accepted value.