Ionic Equilibrium in Solutions Chapter 12.2

Electrolytes What is an electrolytic solution? What is the difference between a strong and weak electrolyte? Explain electrolytic dissociation. The dissolution of a weak electrolyte will lead to an ionic equilibrium in the aqueous solution.

Theories of Acids & Bases The ions present in a solution will determine whether the solution is acidic, neutral, or basic. We will learn about two different acid-base theories: Arrhenius theory Bronsted-Lowry

Arrhenius Theory This description of acids and bases is still used as a simplified explanation. An acid dissociates in an aqueous solution to produce hydrogen ions. AH(aq)  H+(aq) + A-(aq) A base dissociates in an aqueous solution to produce hydroxide ions. BOH(aq) B+(aq) + OH-(aq)

Arrhenius Theory These definitions of acids and bases are limiting; they state that acids must contain at least one hydrogen atom and bases must contain at least one oxygen atom. This definition permits us to see what happens during dissolution and neutralization (where an acid and a base react to form water and salt).

Arrhenius Theory Neutralization HCl (aq) + NaOH(aq)  NaCl(aq) + H2O(l) H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq)  Na+(aq) + Cl-(aq) + H2O(l) Net-ionic equation H+(aq) + OH-(aq)  H2O(l) The Cl-(aq) and Na+(aq) are spectator ions because they do not react. This definition does not make much sense because it is highly unlikely that the H+ ion would be present in water.

The Hydronium Ion In water, the electronegativity of oxygen means that it is pulling the electrons from the hydrogen atoms towards it; this makes the oxygen partially negative. When an acid dissociates it produces H+ ions, which will interact with the water molecules. The H+ ions will be attracted by the oxygen atoms (negatively charged) of the water molecules; this results in the formation of the hydronium ion H3O+.

More Limitations The Arrhenius definition of an acid and a base does not explain the basic properties of ammonia, NH3, or calcium carbonate, CaCO3.

A New Definition Bronsted-Lowry What is the role of acids and bases in a chemical reaction? An acid is a proton donor, H+ A base is a proton acceptor (includes any negative ion, not only OH-)

Bronsted-Lowry Acid-base reactions can occur in any environment, not only in water; the only condition that is required is that there must be a substance willing to donate a proton and a substance to receive that proton. In other words, any substance can behave like an acid as long as there is, at the same time, another substance behaving like a base.

Conjugates A conjugate acid-base pair is a pair of substances that differ by a single proton. The conjugate base of an acid is what is left over after a substance has lost a proton. The conjugate acid of a base is the result of a proton being added to that base. In other words, if the substance has had a proton added to it it is a conjugate acid and if has had a proton removed from it it is a conjugate base.

Conjugate Acid-Base Pair Since H3O+ is the result of H2O having had a proton added to it it is the conjugate acid. Since Cl- is the resultant from HCl having had a proton donated it is the conjugate acid.

Another example

Both? There are substances, such as water, that can behave as either an acid or a base depending on the conditions. Substances that behave this way are amphoteric. While the Bronsted-Lowry theory does not explain why substance donate or accept protons, it does describe, explain and predict reactions.