Jumat, 09 Oktober 2009

A Review of Acid-Base Chemistry

One of the first reactions students learn about in an introductory chemistry course is the acid-base reaction between hydrochloric acid and sodium hydroxide, Equation 1.

This seemingly simple reaction is, in fact, much more complex than Equation 1 suggests . We're going to take a closer look at this familiar reaction, but before we do, let's review a couple of basic concepts about chemical bonds and molecular properties.

Ionic Molecules

Ionic and covalent bonds constitute the ends of a spectrum of bonding possibilities. Ionic molecules are formed by the transfer of one or more electrons from one atom to another. This transfer results in the formation of cations and anions. These oppositely charged particles are then held tightly together by very strong Coulombic attractions. Equation 2 illustrates the formation of sodium chloride from the reaction of a sodium atom with a chlorine atom.

There are two properties that are characteristic of ionic molecules: 1. They are generally high melting solids 2. When dissolved in water, they produce solutions which conduct an electrical current.

Covalent Molecules

As we saw in our discussion of valence bond theory, covalent bonds involve the sharing of electrons between atoms rather than the transfer of electrons from one atom to another. Equation 3 depicts the formation of a covalent bond between a hydrogen atom and a chlorine atom.

In contrast to ionic compounds, covalent molecules may be gases, liquids, or solids. Those that are solids generally have melting points below 300oC. Furthermore, aqueous solutions of most covalent molecules do not conduct a current. There are some notable exceptions however. HCl is one.

HCl A Chemical Jekyl and Hyde

Hydrogen chloride is a gas at room temperature. Its boiling point is -85oC. This certainly indicates that HCl is a covalent molecule. But when HCl is bubbled into water, the resulting solution conducts an electrical current as though it were an ionic compound. In order to understand these seemingly incompatible behaviors, you have to remember that HCl is a polar molecule. In aqueous solution there are strong dipole-dipole interactions between HCl and water molecules. Figure 1 animates a process in which one such interaction becomes strong enough to lead to a transfer of a hydrogen atom from an HCl molecule to a water molecule, producing a hydronium ion, H3O+ and a chloride ion, Cl-.

Figure 1

Dipole-Dipole Interactions

Scheme 1 offers a static-image description of the process shown in Figure 1. The dipole-dipole interaction between a water molecule and a molecule of HCl is depicted by the arrow labeled 1. Arrow 2 implies that the bond between the hydrogen and the chlorine begins to break as the bond between the oxygen and the hydrogen begins to form. The structure shown in brackets describes a point along the reaction pathway where the O-H bond is partially formed and the H-Cl bond is partially broken. Bond making and bond breaking continue from this point until the final products are formed. Chemists call this type of scheme a reaction mechanism.

Scheme 1

A Simple Reaction Mechanism

The fact that an aqueous solution of HCl is highly conductive suggests that a lot of hydronium ions and chloride ions are formed. In fact, conductivity measurements indicate that essentially all of the covalent HCl molecules that dissolve also dissociate into ions. Compounds that dissolve in water to produce high concentrations of hydronium ions are called strong acids.

NaOH A Straight Shooter

Sodium hydroxide is a white solid. It melts at 318oC and boils at 1390 oC. It dissolves in water to produce a solution that conducts a current. In other words, NaOH is an ionic molecule. It dissolves in water because of the charge-dipole interactions between the ions and the water. Figure 2 shows a sodium cation and a hydroxide anion each surrounded by a cluster of water molecules. This blanket of water molecules acts as insulation, reducing the Coulombic attractions between the oppositely charged ions, allowing them to seperate from each other. Ions that are surrounded by solvent molecules in this way are said to be solvated. The blanket of water molecules surrounding each ion is called a solvent shell.

Figure 2

Charge-Dipole Interactions

Equation 1 implies that the reaction between HCl and NaOH occurs in aqueous solution. In light of the ideas presented in Figures 1 and 2, it is reasonable to rewrite Equation 1 to show the species that really participate in the acid-base reaction. This is done in Equation 4.

A mechanistic description of reaction 4 is shown in Scheme 2.

Scheme 2

Here We Go Again

According to Scheme 2, when a solution of sodium hydroxide is mixed with a solution of hydrochloric acid, Coulombic forces draw the oppositely charged ions towards each other. If a hydroxide ion collides with one of the hydrogen atoms of the hydronium ion (arrow 1), a bond is formed between that hydrogen atom and the oxygen atom of the hydroxide ion. Note that the hydroxide ion is contributing both of the electrons that are used to form the new bond. As the new O-H bond begins to form, the H-O bond in the hydronium ion begins to break (arrow 2). If this bond didn't break, the hydrogen would have 4 electrons in its valence shell, violating the filled shell rule.The electron pair from the bond that breaks becomes a non-bonding pair on the oxygen atom.

The OH Group- Acidic, Basic, or Neutral?

Many compounds contain an OH group. In some of them the OH group behaves as an acid, in others it acts as a base, while in yet others it is neither an acid or a base. To make matters worse, the OH group in a given molecule can act as an acid in some situations and as a base in others. Click here to go to a set of guidelines for deciding whether a compound that contains an OH group will be an acid, a base, or neutral.

One difference between Equations 1 and 5 involves the bonds that are made and broken. In reaction 1 an H-Cl bond is broken and an H-O bond is made. In reaction 5 a C-Cl bond is broken and a C-O bond is formed. These seeming differences are not differences in the eyes of an organic chemist: In both reactions, the bond that is broken is the bond between the chlorine atom and its bonding partner, H or C. The bond that's made is the bond between the oxygen atom and its new bonding partner, H or C. In other words, an organic chemist sees both Equations 1 and 5 as the same thing, namely Equation 6.

This viewpoint is legitimate because both reactions are a result of the charge-dipole interactions that occur between HO- and RCl. See Figure 3.

Figure 3

It's All the Same to Me

When looking at a reaction like this, it is good practice to express the equation in words. A statement of Equation 5 might say "Chloromethane reacts with sodium hydroxide to produce methanol and sodium chloride. In the process, a C-Cl bond is broken and a C-O bond is formed."

Since this is a course in organic chemistry, we can't end this introductory discussion of chemical reactions without talking about a reaction of an organic molecule. The reaction shown in Equation 5 has many similarities to the process described in Equation 1. There are some notable differences, too.

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