Lesson 7:
Acids and Bases



Objective

In this lesson we will answer the following questions:

 

 

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:

H2O double-sided arrow H+ + OH-

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
vinegar and onion
Vinegar contains acetic acid, which makes it taste sour. 
Onions release a gas which turns into sulfuric acid when
it reaches your eyes, making them burn. 


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:

HCl yields H+ + Cl-

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. 

Diagrams of solutions with and without hydrochloric acid.

The diagrams above show what happens when hydrochloric acid is added to water.  The top diagram merely contains water.  Notice that most of the water is present in its un-ionized state, but that two water molecules have ionized into hydroxide and hydrogen ions.  Despite the presence of hydrogen ions, this solution is neutral (meaning that it is neither acidic nor basic) because the number of hydroxide ions equals the number of hydrogen ions. 

The bottom diagram shows what happens to the water after the addition of hydrochloric acid.  Notice that all of the hydrochloric acid has ionized, so now there are five hydrogen ions and only two hydroxide ions in the solution.  Since there are more hydrogen ions than hydroxide ions present, the solution has become acidic. 



Bases

Common 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:

NaOH yields Na+ + OH-

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

OH- + H+ yields H2O

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:

Reaction of sodium hydroxide with water.


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:

NH4OH
yields NH4+ + OH-

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. 

comparison of a strong and a weak acid

The top rectangle is an example of a solution containing sodium hydroxide. 
All of the molecules have ionized into their constituent ions.  The bottom rectangle is an example of a solution containing ammonia.  Most of the ammonia stayed in its current state, with only a small percentage breaking apart into ammonium and hydroxide ions. 

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:

H2O + NH3 yields NH4+ + OH-

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. 




Part 2: pH