Properties of Acids and Bases

Inquiry question: What is an acid and what is a base?

Chemical substances can be classified as acidic, basic or neutral. This classification is usually based on the concentration of hydrogen ions (H+) or hydronium ions (H3O+) produced when the substance is dissolved in water. Hydronium ions form when the hydrogen ion bonds to a water molecule by sharing one of the electron pairs of the oxygen atom. This type of bond is a type of covalent bond called a coordinate bond. A coordinate bond is an alternative covalent link produced when an electron pair from a single atom is shared. Coordinate bonds in structural formulae are shown using an arrow (Source: https://www.inspiritvr.com/general-chemistry/covalent-bonding/coordinate-covalent-bond-study-guide).

Using Brønsted–Lowry Theory

Inquiry question: What is the role of water in solutions of acids and bases?

The concentrations of acids can be compared using the pH scale. The pH scale is a logarithmic scale based on powers of ten. A change of 1 unit on the pH scale equals a tenfold change in hydrogen ion concentration [H+] or the hydronium ion concentration [H3O+]. 

The pH of a solution is defined as:

Quantitative Analysis

Inquiry question: How are solutions of acids and bases analysed?

Volumetric analysis is a quantitative laboratory technique used to determine the concentration of a solution by reacting it with a standard solution. For example, the concentration of an alkaline solution can be determined by reacting it with a primary standard solution of an acid.

To select the most appropriate indicator for the titration, the pH at the equivalence point must be matched to the pH range in which the indicator changes colour.

The end point of a titration occurs when the indicator just changes colour. The indicator must change colour when the graph of pH versus volume of titrant shows its greatest rate of change. This is the steep inflexion region of the graph, which coincides with the equivalence point of the titration. The following pH graphs show the inflexion region for three different titrations. The pH is measured using a calibrated pH glass electrode.

The equilibrium constant for the ionisation of a weak acid is given the symbol Ka and is called the acid dissociation constant or the acid ionisation constant.

Titrations can be used to determine the concentration of sodium hydroxide in a liquid drain cleaner. Industrial chemists perform such titrations to ensure quality control of this product.

Our bodily fluids must be maintained in a narrow pH range in order for the biochemical processes to occur at an optimal rate. Buffers are also used in a chemical laboratory to calibrate pH meters and also to provide a constant pH environment for a chemical process.

Buffer solutions contain either a weak Bronsted-Lowry acid and its conjugate base, or a weak Bronsted-Lowry base and its conjugate acid. The greatest buffering effect is obtained if the buffer solution contains equimolar concentrations of the weak acid and its conjugate base, or the weak Bronsted-Lowry base and its conjugate acid.

The graph below the text shows how adding different concentrations of the same buffer changes the amount of added sodium hydroxide required before a significant change in the pH of the solution.

The carbonic acid/hydrogen carbonate buffer system is an important buffer system in human blood. It helps to maintain the pH of blood within a narrow range between 7.35 and 7.45 (see the diagram below the text). The chemical equations for this buffer system are:

The forward reaction: CO2 + H2O ⇌ H2CO3 i.e. carbon dioxide (CO2) and water (H2O) react to form carbonic acid (H2CO3).

The reverse reaction: H2CO3 ⇌ HCO3- + H+  i.e. carbonic acid (H2CO3) dissociates into hydrogen carbonate (HCO3-) and a hydrogen ion (H+).

Together, these two reactions can be combined to form the overall equation for the carbonic acid/hydrogen carbonate buffer system: CO2 + H2O ⇌ H2CO3 ⇌ HCO3- + H+

Diagram reference: Brown et al. (2022), Chemistry - The Central Science, section 17.2, Buffered Solutions, online at https://chem.libretexts.org/, retrieved 7/3/23.