Question

You are given two beakers, one containing an aqueous solution of strong acid (HA) and the other an aqueous solution of weak acid (HB) of the same concentration. Describe how you would compare the strengths of these two acids by (a) measuring the \(\mathrm{pH},\) (b) measuring electrical conductance, and (c) studying the rate of hydrogen gas evolution when these solutions are combined with an active metal such as \(\mathrm{Mg}\) or \(\mathrm{Zn}\).

Short answer

Strong acids have lower pH, higher conductivity, and faster hydrogen gas evolution than weak acids.

Step-by-step solution

Understanding pH Measurement

To compare the strengths of the strong acid (HA) with the weak acid (HB) using pH measurement, use a pH meter for each solution. The pH is a logarithmic measure of the hydrogen ion concentration in the solution: \[\text{pH} = -\log_{10} [H^+]\].For a strong acid, HA, which fully dissociates, the pH will be significantly lower than that of a weak acid, HB, which only partially dissociates. Measure the pH of both solutions and note that the strong acid will have a lower pH value indicating higher acidity.

Measuring Electrical Conductance

Use a conductivity meter to measure the electrical conductance of both acid solutions. The strong acid will dissociate completely in water, leading to a higher concentration of ions that results in higher conductance. The weak acid will dissociate partially, leading to lower ion concentration in solution and therefore lower conductance. Compare the conductance values; the strong acid will exhibit greater electrical conductance.

Observing Rate of Hydrogen Gas Evolution

Combine each acid solution with an active metal such as Mg or Zn and observe the rate of hydrogen gas evolution. Strong acids, due to complete ionization, react faster and produce hydrogen gas more quickly than weak acids, which only partially ionize and thus react slower. Measure and compare the volumes of hydrogen gas evolved over time for the two acids; the strong acid will evolve hydrogen gas at a faster rate.

Key concepts

pH measurement

Measuring the pH of an acid solution helps in gauging its acidity. pH is a scale used to specify the acidity or basicity of an aqueous solution.
It is calculated as the negative logarithm (base 10) of the hydrogen ion concentration: \( \text{pH} = -\log_{10} [H^+] \).
When comparing strong and weak acids, a strong acid, like HA, will fully dissociate in solution, leading to more hydrogen ions and hence a lower pH. In contrast, a weak acid, such as HB, only partially dissociates, resulting in fewer hydrogen ions and a higher pH.
You can use a pH meter to measure accurately the pH of each solution. The meter will provide a digital readout, which makes it easy to compare the acidity levels. For identical concentrations, the strong acid will always show a lower pH value than the weak acid.

electrical conductance

Electrical conductance in solutions measures how well an electrolyte can conduct electricity. When acids dissolve in water, they either completely or partially dissociate into ions.
A strong acid, like HA, dissociates completely in water, producing more ions. These ions are charge carriers, enhancing the solution’s ability to conduct electricity. A conductivity meter can precisely measure these differences.
For weak acids like HB, fewer ions are produced due to partial dissociation, resulting in lower conductance.
Comparing the conductance values of both solutions, you'll find that the strong acid exhibits greater electrical conductance, a clear indicator of higher ion concentration and stronger acidity.

hydrogen gas evolution

Combining acids with active metals like magnesium (Mg) or zinc (Zn) results in a chemical reaction that releases hydrogen gas. This reaction occurs when acids donate protons to the metal atoms.
Strong acids provide more protons due to full ionization, leading to quicker and more vigorous hydrogen gas evolution compared to weak acids.
During the experiment, observe which solution produces gas bubbles more rapidly; a stronger acid will evolve hydrogen gas faster.
By measuring the volume of hydrogen gas evolved over a certain period and comparing it for the strong and weak acids, you will discover that the strong acid results in more hydrogen gas due to its complete dissociation.

strong vs weak acids

The primary difference between strong and weak acids is the extent of their ionization in water.
Strong acids, like HA, undergo complete ionization, which means they dissociate into their constituent ions entirely, releasing more hydrogen ions into the solution.
This results in lower pH values, higher electrical conductance, and more vigorous hydrogen gas evolution in reactions.
In contrast, weak acids, like HB, only partially ionize, indicating that only a fraction of the acid molecules dissociate into ions. Hence, they have higher pH values, lower conductivity, and slower hydrogen gas evolution when reacting with metals.
These differences are critical when comparing acid strengths and their behavior in chemical reactions.

acid-metal reaction

When acids react with metals, an acid-metal reaction takes place, resulting in the liberation of hydrogen gas and the formation of a salt.
This reaction is a straightforward demonstration of how the strength of acids affects their chemical behavior.
Active metals like magnesium and zinc donate electrons readily, which interact with the hydrogen ions from acids.

• For strong acids, which fully ionize, this electron donation and proton reception happen rapidly, resulting in a fast and vigorous release of hydrogen gas and more substantial reactions.
• Weak acids have fewer dissociated ions available, causing slower reactions with fewer hydrogen gas bubbles appearing over a given time.

These reactions not only highlight acidity but also provide insight into how different acids participate in chemical transformations.