What makes a bronsted acid strong

In the reverse reaction, an ammonium ion acts as an acid by donating a proton to a hydroxide ion, and the hydroxide ion acts as a base. Some common conjugate acid—base pairs are shown in Figure 2. Figure 1. The strongest acids are at the bottom left, and the strongest bases are at the top right. The conjugate base of a strong acid is a very weak base, and, conversely, the conjugate acid of a strong base is a very weak acid.

Skip to main content. Chapter 2: Acids and Bases. Search for:. Make certain that you can define, and use in context, the key terms below. Examples Questions.

Answers Show Answer 1. It is far more important that you understand that the strength of an acid or base and its conjugate base or acid is based on the equilibrium position for the dissociation ionisation reaction.

IUPAC prefers the term "hydron" rather than "proton" for the positively charged hydrogen ion. We use the term proton because most of the naturally occurring hydrogen is hydrogen-1, 1 H. When this isotope of hydrogen loses its electron, what is left is just a proton. However, there is a very small percentage of naturally occurring hydrogen-2, 2 H. When this hydrogen atom loses its electron, the result is a nucleus containing both a proton and a neutron. The equilibrium constant for an acid is called the acid-ionization constant, K a.

For the reaction of an acid HA:. Although water is a reactant in the reaction, it is the solvent as well, so we do not include [H 2 O] in the equation. Thus a stronger acid has a larger ionization constant than does a weaker acid.

The ionization constants increase as the strengths of the acids increase. A table of ionization constants of weak acids appears in Appendix H , with a partial listing in Table 2. Another measure of the strength of an acid is its percent ionization. The percent ionization of a weak acid is the ratio of the concentration of the ionized acid to the initial acid concentration, times Because the ratio includes the initial concentration, the percent ionization for a solution of a given weak acid varies depending on the original concentration of the acid, and actually decreases with increasing acid concentration.

Calculation of Percent Ionization from pH Calculate the percent ionization of a 0. Remember, the logarithm 2. Check Your Learning Calculate the percent ionization of a 0. We can rank the strengths of bases by their tendency to form hydroxide ions in aqueous solution. A weak base yields a small proportion of hydroxide ions. Soluble ionic hydroxides such as NaOH are considered strong bases because they dissociate completely when dissolved in water.

View the simulation of strong and weak acids and bases at the molecular level. As we did with acids, we can measure the relative strengths of bases by measuring their base-ionization constant K b in aqueous solutions. In solutions of the same concentration, stronger bases ionize to a greater extent, and so yield higher hydroxide ion concentrations than do weaker bases. A stronger base has a larger ionization constant than does a weaker base.

For the reaction of a base, B:. Again, we do not include [H 2 O] in the equation because water is the solvent. The chemical reactions and ionization constants of the three bases shown are:.

A table of ionization constants of weak bases appears in Appendix I with a partial list in Table 2. As with acids, percent ionization can be measured for basic solutions, but will vary depending on the base ionization constant and the initial concentration of the solution. Adding these two chemical equations yields the equation for the autoionization for water:.

Multiplying the mass-action expressions together and cancelling common terms, we see that:. The product of these two constants is indeed equal to K w :. Strong acids form very weak conjugate bases, and weak acids form stronger conjugate bases Figure 2. Figure 3 lists a series of acids and bases in order of the decreasing strengths of the acids and the corresponding increasing strengths of the bases. The acid and base in a given row are conjugate to each other.

The first six acids in Figure 3 are the most common strong acids. These acids are completely dissociated in aqueous solution. The conjugate bases of these acids are weaker bases than water. When one of these acids dissolves in water, their protons are completely transferred to water, the stronger base. Those acids that lie between the hydronium ion and water in Figure 3 form conjugate bases that can compete with water for possession of a proton. Both hydronium ions and nonionized acid molecules are present in equilibrium in a solution of one of these acids.

Compounds that are weaker acids than water those found below water in the column of acids in Figure 3 exhibit no observable acidic behavior when dissolved in water. Their conjugate bases are stronger than the hydroxide ion, and if any conjugate base were formed, it would react with water to re-form the acid.

The extent to which a base forms hydroxide ion in aqueous solution depends on the strength of the base relative to that of the hydroxide ion, as shown in the last column in Figure 3.

Those bases lying between water and hydroxide ion accept protons from water, but a mixture of the hydroxide ion and the base results. Bases that are weaker than water those that lie above water in the column of bases show no observable basic behavior in aqueous solution. Many acids and bases are weak; that is, they do not ionize fully in aqueous solution.

A solution of a weak acid in water is a mixture of the nonionized acid, hydronium ion, and the conjugate base of the acid, with the nonionized acid present in the greatest concentration. Thus, a weak acid increases the hydronium ion concentration in an aqueous solution but not as much as the same amount of a strong acid. When we add acetic acid to water, it ionizes to a small extent according to the equation:.

This equilibrium, like other equilibria, is dynamic; acetic acid molecules donate hydrogen ions to water molecules and form hydronium ions and acetate ions at the same rate that hydronium ions donate hydrogen ions to acetate ions to reform acetic acid molecules and water molecules. If the number of hydrogens has decreased that substance is the acid donates hydrogen ions. If the number of hydrogens has increased that substance is the base accepts hydrogen ions.

These definitions are normally applied to the reactants on the left. If the reaction is viewed in reverse a new acid and base can be identified.