Strong & Weak Acids & Bases
- Strong and weak acids can be distinguished from each other by their:
- pH value (using a pH meter or universal indicator)
- Electrical conductivity
- Reactivity
pH
- An acid dissociates into H+ in solution according to:
HA → H+ + A-
- The stronger the acid, the greater the concentration of H+ and therefore the lower the pH
pH value of a strong & weak acid table
Acid | pH of 0.1 mol dm-3 solution |
HCl (strong) | 1 |
CH3COOH (weak) | 2.9 |
- The most accurate way to determine the pH is by reading it off a pH meter
- The pH meter is connected to the pH electrode which shows the pH value of the solution
Using a digital pH meter
The diagram shows a digital pH meter measures the pH of a solution using a pH electrode
- A less accurate method is to measure the pH using universal indicator paper
- The universal indicator paper is dipped into a solution of acid upon which the paper changes colour
- The colour is then compared to those on a chart which shows the colours corresponding to different pH values
How to use universal indicator paper
The diagram shows the change in colour of the universal indicator paper when dipped in a strong (HCl) and weak (CH3COOH) acid. The colour chart is used to read off the corresponding pH values which are between 1-2 for HCl and 3-4 for CH3COOH
Electrical conductivity
- Since a stronger acid has a higher concentration of H+ it conducts electricity better
- Stronger acids therefore have a greater electrical conductivity
- The electrical conductivity can be determined by using a conductivity meter
- Like the pH meter, the conductivity meter is connected to an electrode
- The conductivity of the solution can be read off the meter
Using a digital conductivity meter
The diagram shows a digital conductivity meter that measures the electrical conductivity of a solution using an electrode
Reactivity
- Strong and weak acids of the same concentrations react differently with reactive metals
- This is because the concentration of H+ is greater in strong acids compared to weak acids
- The greater H+ concentration means that more H2 gas is produced
The reaction of 0.1 mol dm-3 of a strong acid, HCl, with Mg
The reaction produces a lot of bubbles and hydrogen gas due to the high concentration of H+ present in the solution
The reaction of 0.1 mol dm-3 of a weak acid, CH3COOH, with Mg
The reaction produces fewer bubbles and hydrogen gas due to the lower concentration of H+ present in the solution
Examiner Tip
- The above-mentioned properties of strong and weak acids depend on their ability to dissociate and form H+ ions.
- Stronger acids dissociate more, producing a greater concentration of H+ ions and therefore showing lower pH values, greater electrical conductivity and more vigorous reactions with reactive metals.