Day 26

Context

We will now use some of the principles from our understanding of ionic compounds to understand how molecules are held together.

Explanation

Acids are interesting because they have some properties of ionic compounds, like dissolving in water, but they are not ionic, they form molecules.

After this class you should be able to:

• Explain the difference between an ionic and a covalent bond.
• Identify a property that acids have in common with ionic compounds.
• Describe the relationship between acid concentration and pH.
• Describe the pH scale in terms of acidic, neutral, and basic.
• Define what an acid-base indicator is.
• Describe what is produced in the reaction of an acid with a base.

Molecules and Covalent Bonds

A molecule is composed of two or more atoms that are hooked together, stay together, and act as one unit. They stay together because they are sharing electrons. For substances where the atoms share electrons the molecule is the smallest piece of the substance that retains the properties of that substance. When atoms stay together because they are sharing bonds it is called a covalent bond.

Our idea from last time that atoms will share electrons to have the same number of electrons as a noble gas still applies. When they share, each atom claims the electron so that they both have the number that is needed. A chlorine atom, for instance, has 17 protons and the neutral atom will have 17 electrons. Two chlorine atoms could combine by each atom contributing one electron. This electron will be shared with the other atom. Both atoms claim the electron shared by the other atom and so they "think" they have a total of 18 electrons each. All of the elements that form diatomic gases do something similar. Each bromine atom in diatomic bromine (Br2) contributes one electron to a shared pair of electrons and so each bromine effectively has 36 electrons (like the noble gas krypton).

Oxygen also forms diatomic molecules. In this case, however, each oxygen atom needs two more electrons. Two oxygen atoms could combine by each atom contributing two electrons. These electrons will be shared with the other atom so that both atoms will effectively have a total of 10 electrons, like the noble gas neon.

The sharing of electrons binds atoms together. When atoms share electrons they are hooked together, stay together, and act like one unit; they form molecules. Many substances stay together because they share electrons. Water (H₂O) is a common example that everyone is familiar with.

Acids

Last class we identified a basic solution as a solution that has extra hydroxide, OH^-, ions. A base is any substance that increases the hydroxide concentration. What identifies an acid? It is the hydrogen ion, H⁺. If a solution has extra H⁺ ions it is acidic. Any substance that increases the H⁺ concentration in an aqueous solution is an acid.

Even though acids are covalently bonded, the hydrogen ion separates from the rest of the molecule when it is added to water. Some examples are HCl, H₂SO₄, HNO₃. Acids always separate into an H⁺ ion and a negative ion when placed in water. This separation into ions is like an ionic compound, but remember that pure acids stay together because they share electrons and not because of opposite charges attracting. A famous acid is hydrochloric acid (HCl). When HCl is placed in water the molecule separates into H⁺ and Cl^- ions.

Common concentrations of H⁺ ions in solutions range from 1.0 x 10^-1 M (0.1 M) to 1.0 x 10^-14 M (0.00000000000001 M). The concentration is expressed in the common unit of molarity (M) which will not be discussed here. Remember that the negative in the exponent means "one over" or to take the reciprical. So, 10^-1 means 1/10 (0.1), 10^-2 means 1/10² or 1/100 (0.01), etc. For our purposes 10^-1 would be a high concentration of H⁺ and 10^-14 would be a very low concentration of H⁺. It turns out that when using this scale an equal number of H⁺ and OH^- occurs at a concentration of 10^-7.

A neutral solution would have an H⁺ concentration of 1.0 x 10^-7. If the concentration is greater than 1.0 x 10^-7, the solution is acidic. If the concentration is less than 1.0 x 10^-7, the solution is basic.

Sometimes I say that scientists are lazy and so they decided to simplify the notation and just use the numbers in the exponents, but it turns out that exponents can be obtained from a mathematical function called the logarithm. In the end the number in the exponent is given the name pH and is used to identify the concentration of H⁺ in a solution. A solution with a pH of 4 would have a concentration of 10^-4 and be acidic, pH 6 would have a concentration of 10^-6 and be acidic, etc. A solution with a concentration of 10^-8 would have a pH of 8 and be basic. Knowing the concentration we can determine the pH and whether the solution is acidic or basic. Knowing the pH we can determine the concentration and whether the solution is acidic or basic. The pH scale commonly in use ranges from 0 to 14 with pH = 7 being neutral, pH less than 7 being acidic and pH greater than 7 being basic.

Here are a few more examples. A concentration of 1.0 x 10^-7 would correspond to a pH of 7 (and be neutral). A concentration of 1.0 x 10^-3 would correspond to a pH of 3 (and be acidic), etc. A solution with a pH of 8 would correspond to a concentration of 1.0 x 10^-8 and would be a basic solution.

Acid-Base Indicators

Acid-base indicators are substances that have one color in an acidic solution and a different color in a basic solution. There are many of these indicators and the colors aren't always the same for the different substances. One that you can make yourself and that you should know about is red cabbage juice. Here is a link to an activity page using red cabbage indicator and one picture from that page is shown below.

This indicator is a red color when something acidic and a dark blue color when it is basic. As you can see they tested a variety of things from around the house. It looks like they also tested their solutions with litmus paper, which is another acid-base indicator that is also red for acid and blue for base.

To emphasize the point that not all indicators are the same colors in acids and bases, here is Bromothymol Blue, which is a yellowish color for acids and a light blue color for bases.

Acid-Base Reactions

Whenever an acid reacts with a base there is a chemical reaction that takes place that produces a salt and water. An example is hydrochloric acid, HCl, reacting with sodium hydroxide, NaOH.

HCl + NaOH → H₂O + NaCl

Homework

The homework associated with Day 26 is Class Review Homework Set #4.