Please review acids and bases in the General Chemistry 1 Study Guides before starting this section.
In a previous study guide, we mainly looked at the Arrhenius theory of acids and bases. An Arrhenius acid is defined as a substance that dissociates into water to produce hydrogen ions, H+. An Arrhenius base is a substance that dissolves in water to produce hydroxide ions, OH–. The substances HCl and HNO3 are acids while NaOH and KOH are bases. The Arrhenius theory does not explain why NH3 and CH3NH2 are bases, because there is no hydroxide in either formula. In addition, the Arrhenius theory is only valid for aqueous solutions.
Arrhenius base:
The Bronsted-Lowry theory of acids and bases includes non-aqueous systems and bases that do not have hydroxide ion in their chemical formulas. A Bronsted-Lowry acid is a proton donor and a Bronsted-Lowry base is a proton acceptor. The H+ ion is simply a proton. A base must have a lone pair of electrons in order to accept a H+ ion. In the reaction below, NH3 is a weak base and water is a weak acid. The water molecule donates a proton to NH3 and becomes the OH–. The NH3 accepts a proton to become NH4+ ion.
Notice, the products are also an acid and a base. The OH– ion is a strong base, and the NH4+ ion is a weak acid. These Bronsted-Lowry acid-base reactions are also called proton transfer reactions. The H2O molecule and OH– ion are a conjugate acid-base pair. The NH4+ ion and NH3 are also a conjugate acid-base pair. Their chemical formulas only differ by one hydrogen (proton). For example, the conjugate base of HPO42- is PO43-. The conjugate acid of HPO42- is H2PO4–. The conjugate base of HCl is Cl–. The conjugate acid of H2O is H3O+.
Species such as H2O and HPO42- are said to be amphoteric because they can act as an acid or a base depending on the reaction. In the following reaction water acts as a base.
In the following reaction water acts as an acid.
Recall, hydrogen ion is too reactive to exist in water, therefore, it will form a covalent bond with a water molecule resulting in hydronium ion, H3O+.
Please watch the following video
Exercises
Exercise 1. What is the conjugate base for each of the following?
b) HClO4
c) HSO4–
d) H2O
Exercise 2. What is the conjugate acid for each of the following?
b) H2O
c) NO2–
d) CN–
e) CH3NH2
Exercise 3. Write a balanced equation for the dissociation of HCO3– in water. Label the conjugate acid-base pairs.
Exercise 4. Write a balanced equation for the dissociation of HF in water. Label the conjugate acid-base pairs.
Exercise 5. Write a balanced chemical equation for the reaction of CH3NH2 with water. Label the conjugate acid-base pairs.
Back to Acids and Bases: Aqueous Equilibria
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