Lesson 14.2 Acid and Base Strengths
Suggested Reading- Zumdahl Chapter 14 Section 14.1, 14.2, 14.6, & 14.7
- What is the relationship between the strength of an acid (base) the the extent of dissociation?
- Distinguish between strong and weak acids and bases in terms of the extent of dissociation, reaction with water and electrical conductivity.
- State whether a given acid or base is strong or weak.
- Distinguish between strong and weak acids and bases, and determine the relative strengths of acids and bases using experimental data.
- Write the equilibrium expression for acid dissociation in water.
The Bronsted-Lowry concept considers an acid-base reaction as a proton-transfer reaction. It is useful to consider acid-base reactions as competitions between species for protons. From this point of view, you can order acids by their relative strengths as proton donors. The strong acids are those that lose their protons more easily than other acids. Similarly, the stronger bases are the ones that hold on to protons more strongly than other bases. Lets look at objective one from above.
Distinguish between strong and weak acids and bases in terms of the extent of dissociation, reaction with water and electrical conductivity:
Strong Acid
/ Weak Acid / Strong Base / Weak Base
Completely dissociates / Partly dissociates / Completely dissociates / Partly dissociates
Reactive with water (equilibrium position is far right) / Slightly reactive with water (equilibrium position is far left) / Reactive with water (equilibrium position is far right) / Slightly reactive with water (equilibrium position is far left)
High conductivity / Weak conductivity / High conductivity / Weak conductivity
Explaining the Term Strong Acid
When an acid dissolves in water, a proton (hydrogen ion) is transferred to a water molecule to produce a hydronium ion and a negative ion (anion).In general,
where HA is a strong acid, H3O+ is the hydronium ion, and A- is an anion.
These reactions are all reversible, but in some cases, the acid is so good at giving away hydrogen ions that we can think of the reaction as being one-way. The acid is virtually 100% ionized (completely dissociated).
For example, when hydrogen chloride (HCl in gaseous form is called hydrogen chloride. Watch out for this!) dissolves in water to make hydrochloric acid, so little of the reverse reaction happens that we can write:
At any one time, virtually 100% of the hydrogen chloride will have reacted to produce hydronium ions and chloride ions. Hydrogen chloride is described as astrong acid.A strong acid is one which is virtually 100% ionized in solution.Other common strong acids include sulphuric acid and nitric acid. Refer to your list of the 7 strong acids that you had to memorize for chapter 4.
You may find the equation for the ionisation written in a simplified form:
This shows the hydrogen chloride dissolved in the water splitting to give hydrogen ions and chloride ions in solution.This version is often used just to make things look easier. If you use it, remember that the water is actually involved, and that when you write H+(aq)what you really mean is a hydronium ion, H3O+.
Explaining the Term Weak Acid
A weak acid is one which doesn't ionize fully when it is dissolved in water.
Ethanoic acid is a typical weak acid. It reacts with water to produce hydroxonium ions and ethanoate ions, but the reverse reaction is more successful than the forward one. The ions react very easily to reform the acid and the water.
At any one time, only about 1% of the ethanoic acid molecules have converted into ions. The rest remain as ethanoic acid molecules.
Most organic acids are weak. Hydrogen fluoride (dissolving in water to produce hydrofluoric acid) is a weak inorganic acid that you should be familiar with.
Explaining the Term Strong Base
A strong base is something like sodium hydroxide or potassium hydroxide which, like a strong acid, fully ionizes. You can think of the compound as being 100% split up into metal ions and hydroxide ions in solution.
Each mole of sodium hydroxide dissolves to give a mole of hydroxide ions in solution.
Some strong bases like calcium hydroxide aren't very soluble in water. That doesn't matter - what does dissolve is still 100% ionized into calcium ions and hydroxide ions. Calcium hydroxide still counts as a strong base because of that 100% ionization.
Explaining the Term Weak Base
Ammonia is a typical weak base. Ammonia itself obviously doesn't contain hydroxide ions, but it reacts with water to produce ammonium ions and hydroxide ions.
However, the reaction is reversible, and at any one time about 99% of the ammonia is still present as ammonia molecules. Only about 1% has actually produced hydroxide ions.
A weak base is one which doesn't convert fully into hydroxide ions in solution.
Properties of Acidic and Basic Solution
Run the animation on the pHet web site to explore properties of acidic and basic solutions (you will need to click on the test apparatus and drag it into the solution to see the result. To find the simulation go to the following web site and click on Acid-Base solutions.
Relative Strengths of Acids and Bases
By comparing various acid-base reactions you can construct a table of relative strengths of acids and bases. In this context the terms stronger and weaker are used in a comparative sense.
An acid-base reaction normally goes in the direction of the weaker acid. You can use this fact to compare the relative strengths of any two acids, write acid-base equations, and to predict the direction of an acid-base reaction. Consider the following reaction.
In the reaction above, hydrogen chloride is the stronger acid, so the reaction tends to go from left to right (equilibrium position is on the right). HCl sits above the hydronium ion on the table to the right as a result.
Now consider the ionization (dissociation) of acetic acid, a weak acid.
Practice: What acid-base definitions can you apply to this reaction?
Experiment shows that in a 0.1 M solution of acetic acid, only about 1% of the acetic acid is ionized. This implies that acetic acid is a weaker acid than hydronium ion, so the reaction tends to go from right to left (equilibrium position is on the left). Furthermore, we see that hydronium ion sits above acetic acid in the table.
A definite relationship exists between acid and base strengths. The strongest acids have the weakest conjugate bases, and the strongest bases have the weakest conjugate acids. Notice how the chart shows that the strengths are inversely related.
Molecular Structure and Acid Strength
The strength of an acid depends on how easily the proton, H+, is lost from an H-A species. By understanding the factors that determine the ease of proton loss, you will be able to predict the relative strengths of similar acids.
Two factors are important in determining relative acid strengths. One is the polarity of the bond with the H atom. In a polar bond, the H atom has a partial positive charge, as shown in the bond between H and F. The more polarized the bond is, the more easily the proton is removed and the greater the acid strength. The second factor determining acid strength is the size of the atom, A. The larger the atom A, the weaker is the bond and the greater the acid strength.
Consider a series of binary acids, HA, formed from a given column of elements of the periodic table. As you go down the column of elements, each time adding a shell of electrons to the atom, the radius increases. The size of atom A is the dominant factor in determining acid strength. Thus, acid strength increases in going down a column of elements in the periodic table.
As you go across a row of elements, the atomic radium decreases and the effective nuclear charge increases. As the effective nuclear charge increases the polarity of the H-A bond becomes the dominant factor in determining acid strength. Thus, acid strength increases across a row.
Now consider the oxoacids. An oxoacid has the structure H-O-X. The acidic acid is always attached to the O, which is in turn attached to an atom X. Bond polarity appears to be the dominant factor in determining relative strengths of the oxoacids. This, in turn, depends on the electronegativity of X. If the electronegativity of X is large, the H-O bond is relatively polar and the acid strength is large. Thus, for a series of oxoacids of the same structure, differing only in the atom X, the acid strength increaseswith the electronegativity of X.
Consider, for example, the acids HClO, HBrO, and HIO. The electronegativity of Group VIIA elements decreases going down the column of elements, so the order of acid strength is
HIO < HBrOHClO
For a series of oxoacids, such as the oxoacids for chlorine or the oxacids of bromine, the acid strength increases with the number of oxygen.
In the series above perchloric acid is the strongest while hypochlorous acid is the weakest.
The last thing you need to know about acid strength and molecular structure is this. For polyprotic acids such as phosphoric acid, the acid strength decreases with each ionization.
The Equilibrium Expression for Acid Dissociation in Water
The simplest acid-base equilibria are those in which a single acid or base solute reacts with water. An acid reacts with water to produce hydronium ion and the conjugate base ion. The process is called acid ionization or acid dissociation. Consider the acid dissociation of acetic acid
The reaction involves the transfer of a proton from acetic acid to water. Often this reaction is just written as HC2H3O2(aq) ⇌ H+(aq) + C2H3O2-(aq). In this case the transfer of the proton is not as explicit, however, the production of H+ is expressed. Because acetic acid is a weak acid, it dissociates to a small extent (about 1% or less). For a strong acid (or base) such as HCl which dissociates completely, the concentrations of ions are determined by the stoichiometry of the reaction from the initial concentration. However, for a weak acid (or base), the concentration of the ions in solution are determined from the acid-ionization constant (or acid dissociation constant), which is the equilibrium constant for the ionization of a weak acid (or base) (Ka). Thus, Ka tells us something about the extent of dissociation. For the general reaction
or HA ⇌ H+ + A-
The equilibrium constant expression is
I can use either [H3O+] or [H+] in the expression above, they are equivalent.
For weak bases, the concentration of the ions are determined from thebase-ionization constant (Kb). For base dissociation with the general form B(aq) + H2O(l) ⇌ HB+(aq) + OH-(aq), the equilibirum constant is equal to
The electrons, illustrating the Lewis base concept, in the expression above are not necessary.
HOMEWORK: Practice exercises 16.4-16.5