Learning Objectives

Acid-Base Theories

Acids are characterized by a sharp sour taste, which we experience when we taste vinegar, which contains acetic acid. Acids react with a blue vegetable dye called litmus. Litmus will turn pink in the presence of an acid. This phenomenon is known as the "litmus test".  This test for acids is so commonly used in chemistry that the term litmus test is used as an expression in everyday speech by a lot of folks who probably do not have a clue what it really means.

Bases are characterized by a bitter taste and are slippery to the touch. Soap is an example of a basic compound.

The first acid-base theory was the Arrhenius Theory. According to this theory, acids are defined as compounds that donate protons, H⁺.  Bases are compounds that donate hydroxide ions, HO^-.

Some of the most common Arrhenius acids are...

HF (aqueous) hydrofluoric acid...remember the (aq) is needed to differentiate this compound from the gas phase.

HCl (aq) hydrochloric acid

HBr (aq) hydrobromic acid

HI (aq) hydroiodic acid

HNO₃ nitric acid...the (aq) is not necessary...this is always nitric acid whether in solution or not

H₂SO₄ sulfuric acid

H₃PO₄ phosphoric acid

HC₂H₃O₂ acetic acid

Some of the most common Arrhenius bases are...these are still named as ionic compounds

NaOH  sodium hydroxide

KOH  potassium hydroxide

Mg(OH)₂  magnesium hydroxide

Ca(OH)₂  calcium hydroxide

One of the problems with the Arrhenius theory was that it did not cover many of the common acid-base reactions such as the reaction of an acid with ammonia, NH₃. Ammonia clearly does not contain a hydroxyl group but yet it reacts readily with acids.

Brønsted proposed a theory that alleviated this problem. The Brønsted Theory still defined an acid as a proton donor but it went further to define a base as anything that would react with a proton, namely a proton acceptor. The acids listed as Arrhenius acids are thus Brønsted acids but now there are other bases.

Some Brønsted bases are...

HO^- from the above bases

NH₃ (ammonia)

NaHCO₃

Na₂CO₃

CaCO₃

Amphoteric Compounds

Water can react with most Brønsted acids and some Brønsted bases.  This dual nature is called amphoteric. An amphoteric compound can act either as an acid or a base.

Most acid-base reactions take place in water solutions so the amphoteric nature of water is very important. Hydrogen chloride is a gas. When HCl is dissolved in water, it reacts essentially completely to form the hydronium ion, H₃O⁺, and chloride ion.

HCl   +   H₂O   D   H₃O⁺   +   Cl^-

When the gas ammonia is dissolved in water, some of the ammonia reacts with water to form the ammonium ion, NH₄⁺, and hydroxide ion, HO^-.

NH₃   +   H₂O   D   NH₄⁺   +   HO^-

Concept of pH

The concept of pH was invented by a chemist named Sørenson in order to simplify describing the acidity of weak acids or dilute solutions of strong acids.

A pH value of 7 denotes a neutral solution, pH values that are less than 7 denote an acidic solution and pH values greater than 7 denote a basic solution (one that has excess hydroxide ions).

The pH values of some common solutions are given in the following table. You can arrive at the hydronium ion, concentration in moles per liter by simply making the pH the negative exponent of 10. The hydronium ion concentration in the stomach would be about 10^-1 or 0.1 molar.

Acid Rain

Pure unpolluted rain water has a pH of 5.6 which means that it is slightly acidic.  Some carbon dioxide in the atmosphere dissolves in water and reacts to form a small amount of H₂CO₃, carbonic acid.

CO₂   +   H₂O   D   H₂CO₃

Carbonic acid is a very weak acid and dissociates to give a hydronium ion concentration of 2.5 x 10^-6. 

H₂CO₃   +   H₂O   D   H₃O⁺   +   HCO₃^-

The pH of rainwater in most parts of the world is less than 5.6 which means extra acidity must be coming from somewhere other than carbon dioxide. The most acidic rain ever recorded had a pH of 2.4 and that acidity is great enough to cause serious problems in our environment.

The source of the extra acidity in rain comes from anthropogenic sources. This includes the combustion of fossil fuels, burning of biomass and industry. The problem with acid-rain is centered over highly populated industrialized areas. However, the problem is regional and acid rain can impact people who do not even benefit from the industrialization.

The compounds responsible for the extra acidity are nitrogen oxides and sulfur dioxide.

Conjugate Acids and Bases

If you remove a proton from any of the acids in our previous examples, you would be left with an anion that is called the conjugate base of that acid. If you add this proton to any of the bases given as examples, you would produce a compound that is called the conjugate acid of that base. This dynamic process is a "tug of war" for the proton.

In the following example, the acid HBr reacts with the base Mg(OH)₂ to form MgBr₂ (a salt) and water.  Bromide, Br^-, is the conjugate base of the acid HBr. Water is the conjugate acid of the base HO^-. The Mg⁺² acts as a counter ion, first to the hydroxide ion and then to the bromide ion.

2 HBr   +   Mg(OH)₂   ®   MgBr₂   +   2 H₂O

Buffer Solutions

A buffer solution is a solution that resists a change in pH when small amounts of an acid or base are added to the solution. A buffer is the solute present in a buffer solution that causes the solution to be pH-change-resistant.

A buffer contains a substance that reacts with and removes added base and a substance that reacts with and removes added acid.

Buffer solutions are very important in the functioning of life processes. Body fluids have definite pH values that must be maintained over narrow pH ranges. Buffers in these fluids provide protection against pH change.

Relative Acidities and Basicities

The relative strengths of some common acids and bases are shown in the following table. The acidity constants are given as K_a values...a large number denotes a strong acid.

All anions are assumed to have a positive counter-ion such as a metal cation, which is merely a "spectator ion" and does not participate in the reaction. It merely "hangs out" with whatever atom has the negative charge.

All cations are assumed to have a negative counter-ion such as a non-metal anion, which, again, is a "spectator ion".

Any conjugate base will react with any acid above it on the list.

You will be able to work out most of the acid-base reactions in question 12 of the problem set by this method. Acids like sulfuric acid and phosphoric acid have more than one acidic hydrogen atom and the product should show all acidic hydrogen atoms reacting. Helpful Hints!

Reasons for Relative Acidities and Basicities

Not all of the acids and bases can be compared with each other using the same theoretical principles because it is analogous to comparing apples and oranges. However, there are some theoretical principles that can be used to make decisions about the differences in acidity or basicity among similar acids and bases.

Remember that a strong Brønsted acid must have a weak conjugate base because if the conjugate base was strongly attracted to the proton, the proton could not be lost easily and you would have a weak Brønsted acid. The reverse statement is also true. A weak Brønsted acid will have a strong conjugate base. The Brønsted acids in the chart all have one thing in common. They all have a hydrogen atom attached to an element that has a greater electronegativity.  This is important because the hydrogen atom must have some positive character in order to react with a base In other words, the hydrogen atom must have some of the characteristics of a proton.

The Brønsted bases in the chart all have electron pairs that are readily available for sharing.

Let's look at the relative acidity of the related compounds H–F, H–OH, H–NH₂ and H–CH₃. Why is hydrofluoric acid the strongest acid in this group?

Hydroiodic acid is one of the strongest acids in our chart and hydrofluoric acid is a fairly weak acid. How can we explain this relative acidity?

Sulfuric acid is a stronger acid than nitric acid, which is a stronger acid than acetic acid. All of the hydrogen atoms are attached to oxygen atoms in these acids. We need to draw out the electron-dot formulas for these compounds and calculate formal charges to see the logic in this order.

How can a comparison be made between sulfuric acid and hydroiodic acid?

Why is hydroxide ion a much stronger base than fluoride ion?

Why is hydroxide ion a much stronger base than water?

Why is ammonia a stronger base than water?

Problems

1. Name the following compounds...

H₂SO₄

H₃PO₄

HNO₃

2. Write chemical formulas for the following compounds...

hydrofluoric acid

hydrobromic acid

acetic acid  

3. What is the pH of a solution, which has a hydrogen ion concentration of 0.000001 mole/liter?

4. What is the hydrogen ion concentration in moles/liter for a solution having a pH equal to 3?

5. A high pH (number larger than 7) refers to a solution that is…

6. Which of the following compounds is the strongest acid?

H₂SO₄

HC₂H₃O₂

HNO₃

7. Which of the following compounds is the strongest acid?

HCl

HI

HBr

HF

8. Which of the following compounds is the strongest acid?

CH₄

NH₃

H₂O

HF

9. Which of the following compounds is the strongest base?

H₃N

H₂O

HF

CH₄

10. Which of the following compounds is the strongest base?

NaOH

NaNH₂

NaF

11. Which of the following compounds is the strongest base?

HSO₄^-

C₂H₃O₂^-

NO₃^-  

12. Complete and balance the following acid-base reactions. If you have trouble doing these problems, check out this link.

HCl   +   NaOH   ®   answer

HF   +   NH₃   ®   answer

NaOH   +   HNO₃   ®   answer

H₂SO₄  +   KOH   ®   answer

H₃PO₄   +   NaOH   ®   answer

NaHCO₃   +   HCl   ®   answer

CaCO₃   +   H₂SO₄   ®   answer

HCl   +   NH₃   ®   answer

NH₃   +   H₂SO₄   ®   answer

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Copyright © January 2001 by Richard C. Banks...all rights reserved.