Strong acids ionize completely in aqueous solutions, but weak acids ionize only partially. Introduction to K a for Weak Acids

We’ll consider a 0.10 M solution of the weak acid, CH 3 COOH. This is called ethanoic acid or more commonly, acetic acid. We’ll consider the weak acid CH 3 COOH. It is called ethanoic or acetic acid.

H Here is a diagram showing models of a CH3COOH molecule in a water solution. O H H H C O H C O H O H H O H H O H H H O H H O H H O H H O H H O H

In this diagram, we’ll show just one of the water molecules. H H C O H C O H H O H

Almost all of the collisions between CH3COOH and water molecules (click) result in no reaction. They just bounce off of each other. H H C O H C O H H O H

So almost all of the acetic acid molecules in 0.10 M acetic acid remain as neutral CH 3 COOH molecules. H H C O H C O H H O H Almost all of the CH 3 COOH molecules in 0.10 M CH 3 COOH remain as neutral molecules

However, in rare cases, the collision will be successful. H H C O H C O H H O H

A water molecule will collide with the hydrogen atom (click) attached to the oxygen atom in CH3COOH. H H C O H C O H H O H

And it will remove that hydrogen atom, forming a hydronium ion and a CH3COO minus ion. H H C O H C O H H O H + –

We show the formulas here, H H C O H C O H H O H + – H 3 O + ion CH 3 COO – ion

Hydronium Ion Of course, H3O+ is called the hydronium ion H H C O H C O H H O H + – H 3 O + ion CH 3 COO – ion

Ethanoate Ion CH3COO minus is called the ethanoate ion, H H C O H C O H H O H + – H 3 O + ion CH 3 COO – ion Hydronium Ion

Ethanoate Ion Or more commonly, the acetate ion. H H C O H C O H H O H + – H 3 O + ion CH 3 COO – ion Acetate Ion Hydronium Ion

Because only a very small percentage of the acetic acid molecules form hydronium and acetate ions, we can say that the weak acid, acetic acid is much less than 100% ionized. The Weak Acid CH 3 COOH is much less than 100% ionized in aqueous solution.

Here’s the equation for the ionization of acetic acid. The Weak Acid CH 3 COOH is much less than 100% ionized in aqueous solution. Acetic Acid Acetate ion Hydronium ion

This does not go to completion. An equilibrium is established between the neutral acetic acid molecule and the ions it forms. The Weak Acid CH 3 COOH is much less than 100% ionized in aqueous solution. An Equilibrium

And in this equilibrium (click), the molecular form of CH3COOH is highly favoured. The molecular form is favoured

And the concentrations of the ions are very low compared to that of the molecules. The concentrations of the ions are very low

So a solution of acetic acid consists mostly of neutral CH 3 COOH molecules. A solution of acetic acid is mostly neutral CH 3 COOH molecules

And the concentrations of the ions in this solution are very low. The concentrations of the ions are very low A solution of acetic acid is mostly neutral CH 3 COOH molecules

If we add enough acetic acid (click) to water to form a 0.10 M solution…

A small number of the acetic acid molecules ionize, and the bulb glows dimly. Low conductivity is a property of weak acid solutions.

In the equilibrium constant or Keq expression for this reaction, concentrations of the products, hydronium and acetate ions (click) go on top and the concentration of the reactant acetic acid (click) goes on the bottom.

Because water is a liquid in this equation, it is Not included in the Keq expression. Because H 2 O is a liquid, it is NOT included in the K eq expression.

Because CH3COOH is a weak acid, Weak acid

The equilibrium constant is called (click) K a Weak acid a

Because weak acids have a low degree of ionization, the numerator of the Ka expression is small. a The numerator is small

And the denominator is large a The numerator is small The denominator is large

So the values of Ka for weak acids are typically quite small. a The numerator is small The denominator is large The Value of K a is small

The extent of any ionization depends on the temperature a The extent of ionization depends on the temperature

Therefore, the value of Ka depends on temperature. a The extent of ionization depends on the temperature The value of Ka depends on the temperature

The values of K a on the Acid Table

Are all measured at room temperature or 25°C The values of K a on the Acid Table

Looking on the table, we see that the value of Ka for CH3COOH, which is called ethanoic or acetic acid, at 25°C is 1.8 × a = 1.8 × 10 –5

We see that as we move down the table, the values of Ka progressively get smaller from 1.7 × for iodic acid (click) to 1.0 × for water.

The relative strengths of different acids can be compared using their Ka values. For example, benzoic acid, with a Ka of 6.5 × 10 -5

Is stronger than boric acid, with a Ka of 7.3 × stronger weaker

Notice that the Ka values for the 6 strong acids on the top of the table are all listed as “very large” Strong Acids

For example, if we look at perchloric acid HClO4,

The single arrow pointing to the right means that perchloric acid is essentially 100% ionized in aqueous solution at 25°C. 100% ionization

So the concentrations of hydronium and perchlorate ions are high 100% ionization High

And the concentration of neutral HClO4 molecules is close to zero 100% ionization Close to zero

This means that the ratio of the concentrations of ions to the concentration of neutral HClO4 molecules is very large. 100% ionization Large Close to zero

All Strong Acids are 100% ionized, so their K a values are listed as “Very Large”

In Chemistry 12, we use actual Ka values, only when dealing with Weak Acids. K a values for Weak Acids Weak Acids