Difference Between Strong and Weak Bases

Main Difference – Strong vs Weak Bases

A base is any compound that can neutralize an acid. Therefore, a base should have a hydroxyl group (-OH) that can be released as a hydroxyl ion. Since acids are capable of releasing protons (H+ ions), these protons can be neutralized by the hydroxyl ions released by the base. Bases can be categorized into two major types as strong bases and weak bases. Strong bases are highly reactive and are corrosive. But weak bases are not that much reactive. There are a number of strong bases and weak bases that are very useful in laboratory and industrial applications. The main difference between strong and weak base is that strong bases can completely dissociate to give all available hydroxyl ions to the system whereas weak bases are partially dissociated to give some of the hydroxyl ions it has.

Key Areas Covered

1. What is a Strong Base
      – Definition, Properties, Reactions with Examples
2. What is a Weak Base
      – Definition, Properties, Reactions with Examples
3. What is the Difference Between Strong and Weak Bases
      – Comparison of Key Differences

Key Terms: Acids, Dissociation, Dissociation Constant, Hydroxyl Ion, Strong Bases, Weak Bases

What is a Strong Base

A strong base is a compound that can completely dissociate into its cation and hydroxyl ion in an aqueous solution. The hydroxides of group 1 and group 2 metals are generally considered as strong bases since they are composed of cations that are very small when compared to the hydroxyl ion. Therefore, they have a strong ionic bond. In an aqueous solution, these two ions can be completely separated, providing the system with hydroxyl ions.

Some bases are not completely dissolved in water; for example, calcium hydroxide. But it is also considered as a strong base because it can completely split up into ions. The pH of a solution gives us an idea about the type of base present in there. If the pH value is very close to 14, then that solution is strongly basic. A high pH value means a very low concentration of H⁺ ions.

Figure 1: NaOH is a strong base and is corrosive due to high reactivity.

The molecular formula of the base will determine the amount of hydroxyl ions released by that base. A strong base having one hydroxyl group per one molecule will release one mole of hydroxyl ions per one mole of the base. Ex: NaOH is a strong base. It releases one hydroxyl ion per one NaOH molecule.

What is a Weak Base

A weak base is a compound that is partially dissociated into its hydroxyl ion and the cation creating an equilibrium condition. This is called a weak base because it cannot completely dissociate into its cation and the hydroxyl anion. The pH of a solution can be used to approximately identify the presence of a weak base. pH values that are higher than 7 are considered as weak bases.

Figure 2: Ammonium hydroxide is a weak base.

Weak bases can create equilibrium in an aqueous solution; since the base is partially dissociated into its ions, the remaining base molecules and dissociated ions are in equilibrium. At a given temperature, the dissociation of a weak base gives the same ratio between the ions and the remaining base. This constant value is called the equilibrium constant, Kb. For weak bases, the Kb value is very low. In other words, if the pKb value is very high it has characteristics of a weak base. But if the value of pKb is very low, it can be categorized as a strong base rather than a weak base.

Difference Between Strong and Weak Bases

Definition

Strong Bases: A strong base is a compound that can completely dissociate into its cation and hydroxyl ion in aqueous solution.

Weak Bases: A weak base is a compound that partially dissociates into its hydroxyl ion and the cation creating an equilibrium condition.

Dissociation

Strong Bases: Strong bases dissociate into ions almost 100%.

Weak Bases: Weak bases partially dissociate into ions.

pH

Strong Bases: A pH value closer to 14 indicates a strong base.

Weak Bases: A pH value closer to 7 but higher than 7 indicates a weak base.

Reactivity

Strong Bases: Strong bases are highly reactive.

Weak Bases: Weak bases are less reactive compared to strong bases.

pKb

Strong Bases: The pKb value for strong bases is almost zero or a very low value.

Weak Bases: The pKb value for weak bases is very high.

Conclusion

Strong bases and weak bases are basic compounds that can release hydroxyl ions (OH^–) to an aqueous solution. Although both of them are bases, there are several differences between them. The main difference between a strong base and a weak base is that strong bases can completely dissociate to give all available hydroxyl ions to the system whereas weak bases are partially dissociated to give some of their hydroxyl ions.

References:

1. “Strong Base: Definition & Examples.” Study.com, Available here. Accessed 14 Sept. 2017.
2. “Strong and weak bases.” Chemguide, Available here. Accessed 14 Sept. 2017.

Image Courtesy:

1. “Sodium Hydroxide 6mol Corrosive Lab Chemicals” by maticulous (CC BY 2.0) via Flickr
2. “NH4OH” by skycaptaintwo (CC BY 2.0) via Flickr