Objective

In this lesson we will answer the following questions:

• What are acids and bases?
• How do acids and bases participate in chemical reactions?
• How are acids and bases measured?
  

 

 

Reading Assignment

In addition to the online lecture, read chapter 6 in Basic Chemistry for Water and Wastewater Operators.




Lecture

Acids and Bases

Introduction

Acid-base chemistry is based on one simple reaction - the ionization of water:


As we saw in the last lesson, this equation means that water can break apart into a hydrogen ion and a hydroxide ion.  A hydrogen and a hydroxide ion can also join together to form a water molecule. 

In pure water, hydrogen and hydroxide ions are present in a 1:1 ratio since the only source of these ions is the ionization of water.  Acids and bases are substances which change the balance of hydrogen and hydroxide ions in water.  In this lesson, we will first explore the chemistry of acids and bases, then we will show how to calculate their concentration mathematically.  In the next lesson, we will find out how to calculate the concentration of acids and bases in the lab.



Acids

What do you think of when you think of acids?  You might think of sour-tasting acidic foods such as lemons.  Or you might think of strong acids, such as battery acid, which can burn your skin or corrode metal.  In this lesson, we will be concerned with the less visible properties of acids - their chemical properties. 

Chemically, acids are substances which increase the concentration of hydrogen ions when they are placed in water.  In this section we will be concerned only with strong acids.  Weak acids act slightly differently and will be discussed in a later section. 

You can recognize strong acids because they are ionic compounds which contain hydrogen ions.  For example, consider the strong acids listed in the table below.  Notice that each compound consists of a hydrogen ion bonded to some sort of anion. 

Name	Formula	Cation	Anion
Nitric acid	HNO3	H+	NO3-
Perchloric acid	HClO4	H+	ClO4-
Sulfuric acid	H2SO4	H+	SO42-
Hydrochloric acid	HCl	H+	Cl-
Hydrobromic acid	HBr	H+	Br-
Hydriodic acid	HI	H+	I-

When a strong acid is placed in water, it will ionize completely, breaking down into its constituent ions.  For example, hydrochloric acid reacts as shown below:


The ionization of a molecule of hydrochloric acid introduces a hydrogen ion and a chloride ion to the solution.  The chloride ion has no effect on the acidity of the water, but the hydrogen ion makes the solution more acidic. 




Bases


You probably have less familiarity with bases than with acids, so you may be surprised to learn how many bases you deal with in your everyday life.  Soap, baking soda, milk of magnesia, and ammonia all contain bases.  These substances exhibit some of the physical properties of bases, such as feeling slippery, tasting bitter, and dissolving greases.  Bases also irritate the skin and eyes just as acids do. 

Chemically, a base is a substance which decreases the concentration of hydrogen ions when it is placed in water.  Strong bases decrease the hydrogen ion concentration by increasing the hydroxide ion concentration. 

Let's consider what happens when we put sodium hydroxide (a strong base) in water.  Like strong acids, strong bases ionize completely, so the sodium hydroxide breaks down into a sodium ion and a hydroxide ion:


The hydroxide ion released by the sodium hydroxide then reacts with a hydrogen ion from the water, forming a water molecule:


The reaction between the hydroxide ion and the hydrogen ion removes the hydrogen ion from the solution, making the solution less acidic and more basic.  The two diagrams below show the net reaction of sodium hydroxide with water schematically:

Notice that in the top diagram, three water molecules have ionized to produce hydrogen and hydroxide ions.  When three sodium hydroxide molecules are added (in the bottom diagram) the hydroxide ions from the base's dissociation combine with the three hydrogen ions in the solution, forming water.  So the net result of the addition of sodium hydroxide to the water is that the concentration of hydrogen ions becomes lower and the acidity of the solution is decreased. 

In the table below, I have listed some of the strong bases which you may run into in the lab:

Name	Formula
Sodium hydroxide	NaOH
Lithium hydroxide	LiOH
Potassium hydroxide	KOH
Rubidium hydroxide	RbOH
Cesium hydroxide	CsOH
Calcium hydroxide	Ca(OH)2
Strontium hydroxide	Sr(OH)2
Barium hydroxide	Ba(OH)2

Weak Acids and Bases

So far we have talked about strong acids which contain a hydrogen ion and strong bases which contain a hydroxide ion.  However, some acids and bases work slightly differently.  Technically, an acid is any substance which donates a hydrogen ion to a solution and a base is any substance which accepts a hydrogen ion. 

The table below lists some weak acids and bases:

Weak Acids	Weak Bases
Name Formula Acetic acid HC2H3O2 Phosphoric acid H3PO4 Carbonic acid H2CO3 Hydrofluoric acid HF Fluosilicic acid H2SiF6	Name Formula Ammonia NH4OH Magnesium hydroxide Mg(OH)2 Aluminum hydroxide Al(OH)3 Lime CaO Sodium silicate Na2SiO2 HTH NaOCl and Ca(OCl)2 Soda ash Na2CO3

Some of these weak acids and bases ionize just like their strong counterparts, donating hydrogen or hydroxide ions to the solution.  For example, ammonia will ionize as follows:

The difference between strong and weak bases (and between strong and weak acids) is that weak bases do not ionize completely when placed in solution.  So, while every molecule of sodium hydroxide will break down into a sodium ion and a hydroxide ion when placed in solution, only some of the ammonia molecules will ionize under the same conditions.   The illustration below  gives a schematic representation of what happens under these two circumstances. 


Some weak bases not only do not ionize fully, they also do not release a hydroxide ion.  For example, ammonia in its gaseous form reacts as follows:


Notice that the ammonia accepted a hydrogen ion from the water, decreasing the acidity of the solution.  So it acted as a base even though it didn't produce hydroxide by ionization.