The Arrhenius Concept

Despite being a very old and well-studied branch of chemistry, there is still a good deal of disagreement among chemists as to the best way to understand and represent certain aspects of acid-base chemistry. Different observations are best explained using different conceptual frameworks, with three major frameworks having emerged over time. These are the Arrhenius, Brønsted-Lowry, and Lewis concepts of acid and base behavior. Each has its own advantages and disadvantages. When thinking about acids and bases in this class, considering what framework is best to use will be important. Each one has a somewhat different explanation of:

• how we categorize things as acids or bases

• the processes acids and bases undergo when they react

• the structures formed by the action of acids and bases

Dissociation

The Arrhenius definition of acids and bases centers on two ions: the hydrogen ion (H⁺) and the hydroxide ion (OH^-). We will soon learn that the first of these ions (H⁺) is far more complicated than it may look at first. However, this framing will suffice for now.

Under the Arrhenius concept, acids and bases are electrolytes, which we have discussed in previous lessons. As a refresher, an electrolyte is a substance that dissociates into ions when dissolved in water. In the case of acids and bases, the ions in question are H⁺ and OH^-. An acid is a substance which releases H⁺ ions in water, and a base is a substance which releases OH⁺ ions in water.

Acid: HCl (aq) → H⁺ (aq) + Cl^- (aq)

Base: NaOH (aq) → Na⁺ (aq) + OH^- (aq)

As we know, when ionic compounds are dissolved in water they separate into their component ions. Indeed, NaOH is an ionic compound, and above we see it separating into sodium and hydroxide ions. Thus, we understand that "NaOH (aq)" should always be interpreted to mean "separated sodium and hydroxide ions."

It's important to note that acids like HCl are not ionic compounds! They are molecular substances, which only dissociate into ions once dissolved in water. This is an important but subtle distinction: in a pure sample of NaOH, the Na⁺ and OH^- ions already exist, and dissolution simply separates them; in a pure sample of HCl, there are no H⁺ and Cl^- ions, and those only form through a reaction that happens in water.

Below are a couple more examples of how acids and bases dissociate under the Arrhenius concept. You will be expected to be able to write equations like these, given the formula of an acid.

H₂S (aq) → H⁺ (aq) + HS^- (aq)

KOH (aq) → K⁺ (aq) + OH^- (aq)

HF (aq) → H⁺ (aq) + F^- (aq)

NH₄OH (aq) → NH₄⁺ (aq) + OH^- (aq)

Names and Formulas

In the next two lessons, we will go into depth on the naming of acids and bases. For now, we will give a cursory introduction so that we have some foundation for talking about these compounds.

Acids

Since acids are substances that dissociate to give H⁺ in solution, their formulas always include at least one hydrogen, sometimes more. Usually, the hydrogen atom(s) can be found at the beginning of the formula, as shown in these examples.

HF HCl HBr HI H₂S

It's important to know that not all substances containing hydrogen are acids. For example, methane, ammonia, and other compounds shown below are not acids, and for this reason their hydrogen atoms are not written first in their formulas. These are not formulas you need to know, just illustrations that "hydrogen-containing" does not necessarily mean "acid."

CH₄ NH₃ B₂H₆ CsH CaH₂

All the acids above (HF, HCl, and so on) are what we would call binary acids. This means they contain hydrogen and just one other non-metal element (usually a halogen). When naming acids like this, you take the name of the element, change its ending to "-ic," and add "hydro-" at the beginning. So the names of those five acids are as follows.

HF: hydrofluoric acid
HCl: hydrochloric acid
HBr: hydrobromic acid
HI: hydroiodic acid
H₂S: hydrosulfuric acid

The number of hydrogen atoms in the formula comes from the valence of the elements. The halogens all have 7 valence electrons and need one bond to get a full octet (yes, you still have to remember the octet rule from CH 104!), so they bond to one hydrogen atoms. Sulfur has 6 valence electrons, so it needs two bonds to hydrogen.

In addition to binary acids, there are ternary acids, also known as oxyacids. As "oxyacid" implies, these are acids containing hydrogen, oxygen, and some third element, which is almost always between H and O in the formula. ("Ternary" means "three" - a reference to the three elements present in these acids). Some examples are shown below.

H₂SO₄ HNO₃ H₃PO₄ H₂CO₃ HClO₃

The naming of ternary acids is a more complicated subject, which we will cover in the next lesson. To give you a preview, these acids are named sulfuric, nitric, phosphoric, carbonic, and chloric acid, respectively.

As a final note, you should be aware that sometimes you will see acids written with hydrogen at the end of the compound. This is most common in the case of organic acids (acids that include carbon atoms), some examples of which are shown below. You won't need to write formulas like this, but you should be aware of them. In each case below, it is the hydrogen in blue which is lost by the molecule in the process of dissociation.

CH₃CO₂H HCO₂H C₃H₇CO₂H

Bases

Names and formulas of bases should be a bit simpler for you, because you have already learned in CH 104 how to name bases and write their formulas. Under the Arrhenius definition, almost all the bases we will encounter are metal hydroxides, meaning they are combinations of some metal ion (like Na⁺, Ba²⁺, or Fe³⁺) with the hydroxide ion (OH^-), giving formulas like NaOH, Ba(OH)₂, and Fe(OH)₃. As you should remember (yes, once again, I am making you remember things from CH 104!), the names and formulas of these compounds can be predicted using the periodic table.

When it comes to formulas, it's important to remember the rules for charge. The reason sodium is Na⁺ in the example above is that it is in Group IA of the periodic table. Other Group IA metals, like K, Li, etc. would also have a +1 charge, meaning the formula will have a 1:1 ratio of metal ion to hydroxide. Being in Group IIA, barium is Ba²⁺, and its formula needs two hydroxides for every one barium ion to balance the charges. For the unpredictable metals, like iron, you will need some external indication of the charge, such as the compound name, to know that Fe³⁺ is the ion you are dealing with, and that a 3:1 ratio of ions is needed.

To name a hydroxide base, things are pretty simple: the name will always be "_______ hydroxide," so NaOH is "sodium hydroxide" and Ba(OH)₂ is "barium hydroxide." Remember not to use prefixes like "mono" or "di" since those are only for molecular compounds. Lastly, remember that unpredictable metals need a roman numeral to indicate the charge. So Fe(OH)₃ would be "iron (III) hydroxide."

Polyprotic Acids and Bases

To start this section, a note on terminology. Hydrogen has atomic number 1, meaning the nucleus of every hydrogen atom contains exactly one proton. In a neutral hydrogen atom, there is also an electron. In the H⁺, that electron has been lost, so that proton is really the only part of the atom left. For this reason, it is common to refer to H⁺ ions as "protons" in the context of acids.

As some of the formulas above show, acids can have different numbers of hydrogens (or "protons") that can be lost in dissociation. It us usually possible to determine how many protons can dissociate from the formula. For example, HNO₃ can lose one proton, H₂SO₄ can lose two, and H₃PO₄ can lose three.

H₂SO₄ (aq) → 2 H⁺ (aq) + SO₄^2- (aq)

H₃PO₄ (aq) → 3 H⁺ (aq) + PO₄^3- (aq)

Acids that can lose only one proton are called monoprotic. Ones that can lose two are diprotic and ones that can lose three are triprotic. Numbers above three do occur, but are uncommon. Anything above one can be called polyprotic.

Likewise, bases can be classified by the number of hydroxides in their formulas. A base like NaOH is monobasic (sometimes "monoprotic" is used), while Ba(OH)₂ would be dibasic (or diprotic) and Al(OH)₃ would be tribasic (triprotic).

Your lab workbook contains example problems you can use to practice writing dissociation equations using the Arrhenius concept, including for polyprotic acids and bases. That's a great place to look for some practice.