Lesson 5: Arrhenius Concept

Conceptual Definitions of Acids and Bases

There are three questions about acids that we might ask:

• What is it about all the different acids that makes them have so many properties in common?
• What is the underlying nature of acids?
• What is it that makes them do what they do?

We can ask the same questions about all the different bases, also.

To consider these things we have to use a conceptual approach and come up with some conceptual definitions.

We will deal with three conceptual definitions of acids and bases. They are the Arrhenius, Brønsted-Lowry, and Lewis concepts. Each one has its own merits and disadvantages. Each one provides a perspective on the nature of acids and bases that the others do not. Certain properties of acids and bases are best explained using different concepts. Because of this, those properties will be discussed along with the appropriate concept.

The Arrhenius Concept | Nomenclature | Multiprotism | Neutralization

 

The Arrhenius Concept

One of the properties that acids and bases have in common is that they are electrolytes--they form ions when they dissolve in water. Svante Arrhenius, a Swedish chemist who received a Nobel prize in 1903 for his work on electrolytes, focused on what ions were formed when acids and bases dissolved in water. He came up with the concept or idea that acids dissociated in water to give hydrogen ions and that bases dissociated in water to give hydroxide ions. This definition is very useful when we talk about acids and bases as being electrolytes in solution.

These equations (also shown in example 5 in your workbook) serve as examples.

An acid, like HCl, is something that dissociates in water to give hydrogen ion.	Acid: HCl H+ + Cl-
A base, like NaOH, is something that dissociates in water to give hydroxide ion.	Base: NaOH Na+ + OH-
Acids and bases neutralize one another because the hydrogen ion and the hydroxide ion combine with one another to form water. The top equation shown here is the net-ionic version of acid-base neutralization. The complete-formula equation for this reaction (shown below) will depend on which acid and which base are reacting. In general, however, it will show that an acid and a base neutralize one another to form water and a salt. If the salt is soluble, it won't show up in the net ionic equation, but the ions are there in solution as spectator ions.	Neutralization: H+ + OH- H2O HCl + NaOH H2O + NaCl

Arrhenius focused on the idea that acids and bases split into ions when they dissolved in water. In a sense, the Arrhenius concept focuses on what the chemical contains or what is there in solution.

The names of acids and bases correlates closely with their formulas and we will look at that in the section on nomenclature.

Two important features of acids and bases are readily explained using Arrhenius' approach. They are multiprotism and neutralization.

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Nomenclature

For the purposes of naming acids and bases we will put these chemicals into three categories: binary acids, ternary acids (sometimes called oxy-acids), and bases.

Binary Acids

Binary acids contain hydrogen and one other nonmetallic element. Their names follow the pattern of "hydroelementic acid" where element is replaced by the root of the name of the element. These acids contain no oxygen. Here are some examples.

Note that the names of binary acids do not indicate how many hydrogen atoms are in the formula. That is determined by the valence of the other element.

Ternary Acids or Oxy-acids

Ternary acids contain hydrogen and oxygen and one other (usually) nonmetallic element. The names of the most common ones follow the pattern of "elementic acid" where element is replaced by the root of the name of the other element. Note that these acids contain both hydrogen and oxygen but their names make no reference to either hydrogen or oxygen. Here are some examples.

H2SO4	sulfuric acid
HClO3	chloric acid
H2CO3	carbonic acid
H2CrO4	chromic acid

Note that the names of ternary acids do not indicate how many hydrogen atoms are in the formula, nor do they indicate how many oxygen atoms are in the formula. That needs to be learned by memorization or repetition. However, if you remember the formulas of the common polyatomic ions, these acids contain the same number of oxygen atoms as those, and the number of hydrogen atoms matches the negative charge on the ions. We will look at that relationship more fully later in this lesson and also in a later lesson.

There are also ternary acids for which the names follow the pattern "elementous acid." The formulas of these acids contain one less oxygen than the "elementic acids." Here are two examples.

H2SO3	sulfurous acid
HClO2	chlorous acid

Bases

Bases are simply named as ionic compounds containing the hydroxide ion. Here are some examples.

NaOH	sodium hydroxide
Ca(OH)2	calcium hydroxide
Fe(OH)3	iron(III) hydroxide ferric hydroxide
NH4OH	ammonium hydroxide

Practice

We will deal more with nomenclature of acids, bases, salts and polyatomic ions in a later lesson. But for now, for practice, try your hand at naming the acids and bases listed below (and shown in exercise 6 in your workbook). To keep it simple, none of the acids has an -ous ending in its name. Hint: for the acids, first identify it as binary or ternary, then use the appropriate pattern for naming.

HCl	HNO3	H2SO4
H3PO4	NaOH	KOH
Mg(OH)2	Al(OH)3	NH4OH

 

Answers

HCl	hydrochloric acid
HNO3	nitric acid
H2SO4	sulfuric acid
H3PO4	phosphoric acid
NaOH	sodium hydroxide
KOH	potassium hydroxide
Mg(OH)2	magnesium hydroxide
Al(OH)3	aluminum hydroxide
NH4OH	ammonium hydroxide

 

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Multiprotism

Hydrogen ions are quite often referred to as protons, because that is all that is left of a hydrogen atom when its electron has been removed. Some acids have the capability of providing more than one hydrogen ion from each molecule. Such acids are called multiprotic.

If an acid has more than one proton, it is called multiprotic. Sulfuric acid, which has the formula H₂SO₄, has two hydrogen ions which can dissociate from the molecule. It is called diprotic because it has two protons which can dissociate. There are a number of other acids that can dissociate to give two or even three protons. Those that give three protons are called triprotic.

Bases that contain more than one hydroxide ion in their formula are called multibasic. Magnesium hydroxide is called dibasic because it has two hydroxide ions that can dissociate from each magnesium. There are several other dibasic bases. There are even a few tribasic bases that dissociate to give three hydroxide ions.

Practice (Ex. 8)

For practice, use the Arrhenius concept to write equations showing the dissociation of these acids and bases (which are also listed in exercise 8 in your workbook.) Check your answers below, then continue.

	KOH
	HBr
	H3PO4
	Al(OH)3
	Ca(OH)2
	H2CO3

 

Answers (Ex. 8)

Here are the answers to exercise 8.

	KOH K+ +OH-
	HBr H+ + Br-
	H3PO4 3 H+ + PO43-	(H3PO4 is triprotic)
	Al(OH)3 Al3+ + 3 OH-	(Al(OH)3 is tribasic)
	Ca(OH)2 Ca2+ + 2 OH-	(Ca(OH)2 is dibasic)
	H2CO3 2 H+ + CO32-	(H2CO3 is diprotic)

 

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Neutralization

It is possible to predict the products of acid-base neutralization reactions in the same way that you were able to predict the products of double-displacement precipitation reactions. Let's use the neutralization of stomach acid (HCl) by milk of magnesia (Mg(OH)₂) as an example.

Mentally (or on paper) divide the acid into H+ ions and negative ions. Similarly, divide the base into positive ions and OH- ions. For products, the H+ and OH- ions are going to combine to make H2O.  At first, it may look like you have an excess of H+ or OH- ions - you need to change the coefficients in front of the acid and the base to make an equal number of each ion so that you end up with a whole number of H2O molecules. The next step is to combine the positive and negative ions to make an ionic compound (a salt) taking into account the charges on each ion. In this case, Mg2+ and Cl- make MgCl2. If you correctly adjusted the coefficients in front of the acid and the base to make water, you should have the correct number of each ion to make the salt. You should confirm that the formula of the salt is correct, and that you have correctly balanced the equation.

Reactants Products HCl + Mg(OH)2   H+  Cl-  Mg2+   OH-   2 HCl + Mg(OH)2     + 2 H2O   MgCl2 + 2 H2O 2 HCl + Mg(OH)2 MgCl2 + 2 H2O

 

Practice (Ex. 9)

Now try your hand at this by completing and balancing equations for these reactions (which are also shown in exercise 9 in your workbook). Check your answers below, then continue.

	KOH + H2CO3
	HBr + Al(OH)3
	H3PO4 + Ca(OH)2

 

Answers (Ex. 9)

Here are the answers (to exercise 9). If you have trouble with any of these you should stop now and get help from your instructor.

	2 KOH + H2CO3 2 H2O + K2CO3
	3 HBr + Al(OH)3 3 H2O + AlBr3
	2 H3PO4 + 3 Ca(OH)2 6 H2O +  Ca3(PO4)2

 

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