Definitions and Properties of Acids and Bases
Acids and Bases. Reference: Text sections 8.1, 8.2, 8.3, 8.5
Essential Points: Chapters 8 and 9 deal with acids and bases, which are substances with certain properties such that one or both of them are involved in many chemical reactions.
1. There are a couple of common ways that hydrogen ions in water are depicted. One of these is simply H+(aq). Another way is to represent it is as what is called ahydronium ion, H3O+(aq). These can be considered as two different names for the same thing. In an earlier lesson dealing with water, we saw that ions in water are surrounded by water molecules due to attraction of the polar water molecules to the charges of the ions. An H+ion, which is a proton, is a very concentrated positive charge and thus in water is surrounded by water molecules. The hydronium ion representation , which is a H+ion associated with an H2O molecule, emphasizes that the ion has water molecules attracted to it. It is not the actual situation as current information indicates that a H+ion in water may have 4 water molecules surrounding it so that a representation of this would be H9O4+(aq). This representation, although most likely, is not used commonly used. Generally , as indicated above, H+(aq) or H3O+(aq) are used to represent whatever the nature of protons in water is.
2. There is more than one definition of the termacid.One, that is restricted to solutions with water solvent, is that anacidis a substance that increases the hydronium ion (or hydrogen ion) concentration in water. Another definition that is not restricted to water solutions is that an acid is a proton donor or an equivalent definition that an acid is a hydrogen ion donor. These definitions apply to any solvent and even the gas phase. Since we will deal with solutions in water in these chapters, the different definitions are equivalent. Just because a substance contains hydrogen in its formula does not necessarily mean that it is an acid. To be an acid a substance must be able to donate some or all of its H+ions. CH4and NH3both contain hydrogen, but in water they do not produce H+or H3O+ions. They are not acids!
3. Some acids arestrong acidswhich means that in water they ionize 100% to hydronium (or hydrogen ) ions and the anion of the acid. There are two ways of representing this andthey mean the same thing. One indicates the acid species as hydronium ion, and the other indicates it a hydrogen ion For example, considering the strong acid, hydrochloric acid, HCl:
One way: HCl(g) + H2O(l)H3O+(aq) + Cl-(aq)
The other way: HCl(g)H+(aq) + Cl-(aq)
The ionization reactions of strong acids are written with a single arrow from the reactants to the products.
There are only a few common strong acids. They are:
HClO4 / perchloric acid
H2SO4 / sulfuric acid
HNO3 / nitric acid
HCl / hydrochloric acid
HBr / hydrobromic acid
HI / hydroiodic acid
4. Since we have defined strong acids, you can correctly conclude that there are weak acids. Essentiallyweak acidsare those that ionize less than 100% in water. Most of them ionize far less than 100%. When these acids ionize, they have areversible reactionwhich means that the reaction of ionization occurs not only in the forward direction, but also in the reverse direction with the ions combining to form the acid. Rapidly, the system reaches a point ofequilibriumwhere the rates of the forward and reverse reactions are exactly the same. This means that there is no observable change in the amounts of unionized acid and its ions. Reversible reactions are written with arrows in both directions between the reactants and products. Considering acetic acid, which is often written as CH3COOH to indicate that only one of the 4 hydrogen atoms , the end one, can be lost. It is a weak acid such that, depending on its concentration in water, only about 0.5% to 1.0% of it ionizes. Two ways of representing its ionization are:
CH3COOH(l) + H2O(l)H3O+(aq) + CH3COO-(aq)
or
CH3COOH(l)H+(aq) + CH3COO-(aq)
5. Polyprotic acids are those that can donate more than one H+ion per unit. This can occur in steps of losing one proton at a time. The text has a reaction on page 276 for the ionization of sulfuric acid, H2SO4written as:
H2SO4(l) + 2H2O(l)2H3O+(aq) + SO42-(aq)
Let’s look at this in a little more detail. As mentioned above, such reactions can be considered as occurring in steps. Here, the first step is:
H2SO4(l) + H2O(l)H3O+(aq) + HSO4-(aq)
This occurs 100%. Note that when one H+is removed from the acid, the anion formed, HSO4-has a negative one charge.
The second step is:
HSO4-(aq) + H2O(l)H3O+(aq) + SO42-(aq)
Note the double arrow here. That is because for the loss of its second H+, the HSO4-is not a strong acid, it ionizes less than 100%.
The textbook also has an overall reaction for the loss of three H+ions from phosphoric acid, H3PO4.
H3PO4(l) + 3H2O(l)3H3O+(aq) + PO43-(aq)
As was the case for H2SO4,this is an overall reaction. H3PO4ionizes in 3 steps, each involving a weak acid. Can you write the three steps?
6. Bases are species for which, like acids, there is more than one definition. One states thata baseis a substance that increases the concentration of hydroxide ion, OH-, in water.
For example: NaOH(s)Na+(aq) + OH-(aq).
Note that this definition is limited to water as a solvent, A second definition, the one we will use, is thata baseis a proton acceptor or a H+ion acceptor. Using the definition of an acid as proton donor and a base as a proton acceptor, we can consider reactions of acids and bases in water as involvingconjugate acid-base pairs. What this means is that when an acid reacts with water, the water acts as a base in the forward direction, and the product formed, hydronium ion, is an acid and the anion from the acid is a base for the reverse reaction. Considering a general acid, represented as HA, we can write:
HA(aq) + H2O(l)H3O+(aq) + A-(aq)
In the forward reaction the HA is a proton donor (an acid) and the H2O is a proton acceptor(a base). In the reverse reaction the H3O+is a proton donor (an acid) and the A-ion is a base (proton acceptor). The HA and A-are called a conjugate acid-base pair, and the H3O+and the H2O are a conjugate acid-base pair. In other words, a conjugate base is the species left after an acid loses a proton and a conjugate acid is the species produced when a base accepts a proton. In the case of a strong acid such as HCl, its conjugate base, Cl-, is an extremely weak base; so weak that we can consider the reaction
HCl(aq) + H2O(l)H3O+(aq) + Cl-(aq)
occurs only in the forward direction and not in the reverse direction. On the other hand, the anion formed when a weak acid loses a proton has a significant tendency to react in the reverse direction; thus, the weak acid is not completely ionized. The weaker an acid, the stronger the tendency for its anion (its conjugate base) to undergo the reverse reaction , or said differently, the stronger the conjugate base of the acid. Recalling the case of acetic acid, a weak acid:
CH3COOH(l) + H2O(l)H3O++ CH3COO-
The CH3COO- ion is a strong base so that only 0.5-1% of the reaction above lies to the right. 99-99.5% of the system is in the form of the reactants on the left.
7. Just as there are strong and weak acids, there are strong and weak bases. The strong bases are soluble hydroxides of metals, which are essentially Group IA metal hydroxides and to a limited degree Group 2A metal hydroxides. The reason why we say to a limited degree for the Group 2A metal hydroxides is that some of them have only limited solubilities in water. For the strong bases the ionization reactions are written with a unidirectional arrow to indicate essentially 100% reaction. For example:
KOH(s)K+(aq) + OH-(aq)
Weak bases often do not have any OH in their formulas, yet they produce OH-ion to some extent by reacting with water. Such as:
NH3(aq) + H2O(l)NH4+(aq) + OH-(aq)
Since this reaction does not occur 100% to the right (it only is 0.5-1% to the right), a double arrow is used. Since the NH3is a base in the forward direction, the NH4+ion is its conjugate acid and the OH-ion is the conjugate base of the H2O which is the acid in the forward direction.
8. Water has a dual nature in that it can act as an acid or a base depending upon what is reacting with it. When an acid reacts with water, the water accepts a proton to form the hydronium ion. The water is thus acting as a base. On the other hand looking at the reaction of NH3with water above the water donates a proton to form NH4+ion. The water is thus acting as an acid here. Water and other substances that can react as an acid or a base are said to beamphiprotic. We will consider this nature of water in more detail a bit later.
9. A particular reaction of acids is one that you can see at home as described on page 278 of the text. Placing an egg in vinegar, which is essentially an aqueous solution of acetic acid, results in the decomposition of the shell of the egg which is essentially calcium carbonate according to:
CaCO3(s) + 2CH3COOH(aq)Ca2+(aq) + CO2(g) + H2O(l) + 2CH3COO-(aq)
If you have an egg, vinegar, and the time, you might try this yourself. This is the type of reaction that occurs when you use vinegar to remove the deposits in a coffee maker or teapot. When water is heated in these devices, some solids such as CaCO3deposit, and in time can become quite substantial, but vinegar to the rescue can remove the deposits Strong acids could do the same thing, but are more dangerous and expensive than vinegar to use.
10. In the previous chapter we used a unit of concentration of solutions called molality which is the moles of a solute per kilogram of a solvent. There are other ways of expressing the concentrations of solutions, one of which ismolarity, which is the moles of solute per liter of solution. It is represented by a capital letter M. For example a 1.50M solution contains 1.5 moles of the solute per liter of solution. Like molality, molarity has situations where it is the practical concentration unit to use, one area being solutions of acids and bases. Since acid and base solutions often have molarities that involve concentrations that are expressed as a number mulplied by 10 to a negative power, such as 1.0 x 10-3M, a special unit related to molarity has been developed that expresses the concentration in what is considered to be a more convenient form. This is the pH of a solution defined as pH = -log(H+molarity). To simplify things, molarities of ions are commonly expressed in the form [H+] and thus, pH = -log[H+].When the [H+] is 1.0 times 10 to a power, the pH is simply the power of ten with its sign reversed. Some examples:
[H+] / pH
1.0 x 10-3 / 3.00
1.0 x 10-9 / 9.00
1.0 x 10-12 / 12.00
When the pH is a number other than one multiplied by 10 to a power, your electronic calculator is the best way to determine the pH. To do so: 1. key in the H+molarity, 2. punch the “log” button and 3. punch the button that changes the sign. Depending on the make of your calculator, steps 1 and 2 may have to be reversed. The answer has as many digits after the decimal point as there are significant figures in the number before the power of 10.
Example: What is the pH of a solution with [H+] = 3.75 x 10-4
1. Key in 3.75 x 10-4
2. Hit the “log” key---this should give you –3.425968732 (if your calculator carries10 digits)
3. Hit the +/- button and round off to the correct significant figures which gives 3.426in this case.
It is also possible to determine the molarity of H+ion in solution from the pH using your calculator. The pH is actually 10 to the –pH power i.e. 10-pH. The procedure to determine the [H+] is to 1. key in the pH and change its sign to -, then 2. hit the button or buttons necessary to give the value of 10x. Again, depending on your make of calculator you may need to reverse these steps. The answer should have a many significant figures as there are digits past the decimal point in the pH.
Example: What is [H+] when pH = 5.17?
[H+] = 10-5.17= 6.8 x 10-6M
One last thing to note about pH is that as the [H+] becomes higher, the pH value becomes less. As a simple example, a solution with [H+] of 1.0 x 10-3with a pH of 3.00 has a higher [H+] than a solution with [H+] of 1.0 x 10-6with a pH of 6.00.
Assignment 9.1
ASSIGNMENT:
1. Write equations for the ionization in water of the strong acids, HNO3and HClO4when
A. H+ions are formed
B. H3O+ions are formed.
2. Write an equation showing the reaction of the weak acid, HCN, with water.