Acids and Bases Help (page 3)

Introduction to Acids and Bases

The most important thing to remember about acids is that they have very low pH values and burn skin, clothes, shoes, and almost everything they touch. They do not burn as a fire does with a flame, but by reacting strongly with the atoms of the substance and changing them permanently. Acids and bases cause chemical burns. Students are warned in their first laboratory safety class about strong acids and strong bases. Besides, (1) don’t eat or drink anything in the lab; (2) don’t run your fingers through the burner flame, and (3) don’t blow yourselves up; always remember to (4) handle strong acids and bases very carefully .

Hydrochloric acid, HCl, is one of the first acids a beginning chemistry student learns. This strong acid reacts with most metals and forms hydrogen ions. Many experiments use HCl as a reactant. It is easy to spot the sloppy chemistry student, just look at his or her clothes and shoes. If they have a lot of tiny holes in their clothes, chances are good they don’t have moths, but have splashed an acid or base on themselves while pouring. This is why chemists wear laboratory coats and eye goggles for protection. In this chapter, we will find out more about acids and bases and why they are so fierce.

Thousands of years ago, people learned that liquids were able to transform into different forms; grape juice fermented to wine and then if left too long to vinegar. In fact, the Greek word for vinegar is acetum , from which we get acetic acid, the main acid found in vinegar. Acidus is the Latin word for sour, a common characteristic of acids.

An acid is any solution that releases hydrogen (H) ions when added to water and has a pH of less than 7.0. A base is any solution that releases hydroxide (OH) ions in water and has a pH of greater than 7.0.

Ph

How do we figure out if a solution is acidic or basic? In 1909, a Danish chemist, P. L. Sorensen, calculated the negative logarithm of the hydrogen ion concentration. He wrote this as 1 × 10 ^{– n} with the pH equal to n . The name of the pH scale came from the French words for the “power of hydrogen” or “ pouvoir hydrogene .” The pH scale measures the amount of acidity in a solution.

pH scale measures the acidity of a liquid by measuring the concentration of hydrogen ions. Neutral pH is equal to 7.0 on the pH scale.

The most often used piece of equipment in any laboratory beside the beaker or Bunsen burner is the pH meter. The pH meter has a special sensor that is inserted into a liquid sample to be tested. By passing a current through the sample and measuring the resistance and change in current, the amount of hydrogen ions (positively charged ions) can be determined. Figure 14.1 shows a typical pH meter.

Fig. 14.1. A standard pH meter uses a probe to test the acidity of a sample.

Acids And Bases

Nearly 120 years ago, a Swedish graduate student in chemistry, Svante Arrhenius, came up with a few “rules of thumb” for deciding whether or not a solution is an acid:

1. In dilute solutions, acids taste sour. ( Never taste a lab sample!)

2. Litmus paper changes from blue to red.

3. Acids react with metals like iron, magnesium, and zinc and release hydrogen gas.

4. When combined with bases, the products are water and salt.

To find out if a solution is basic, Arrhenius wrote that:

1. In dilute solutions, bases taste bitter. (But you are never going to taste a lab sample, right?!)

2. Litmus paper changes from red to blue.

3. Bases react with acids to form water and salt.

4. Bases feel soapy or slippery between the fingers and on the skin.

Acids

When an acid completely ionizes in water and gives up all its protons to water to form a hydronium ion, H ₃ O ⁺ , it is considered a strong acid. Table 14.1 lists some common acids from strongest to weakest.

Table 14.1 Common acids from strongest to weakest should be handled with matching caution.

Since acids are formed from the ionization of samples and the release of H ⁺ ions, acids that contain carbon held together by covalent bonds are generally much weaker.

Sulfuric acid is the most widely produced acid in the United States. It is used to make fertilizers (70%) from ores containing phosphate rock. The remaining 30% is used in processing industrial metals, oil refining, car batteries, and in general cleaners.

Boric acid is a weak acid used in eye washes and to clean glass. Boron is also a natural insecticide, since many insects cannot metabolize the boron molecule. Boric acid when mixed with equal parts white flour and powdered sugar makes a tempting insect bait for cockroaches. It causes internal blockage and death, while being safe for animals and children.

Bases

Bases ionize almost completely in water. A base that does not ionize in water and is nearly insoluble in water may be thought of as a weak base, but it is really the amount of ionization that matters. Ca(OH) ₂ is not particularly soluble, but the little that does dissolve is completely ionized, so it is called a strong base.

Sodium hydroxide (NaOH) is the strongest base that most chemistry students will use in the laboratory. The metal sodium (Na ⁺ ) forms a tight bond with the (OH ^– ) ion. It is used as a strong cleaner, to stabilize soil in road construction, and in the making of soap.

Magnesium hydroxide Mg(OH) ₂ acts as a mild reactant by gently neutralizing stomach acids. When spicy or acidic foods, like tomatoes, cause problems, an antacid containing magnesium hydroxide neutralizes the acid and brings speedy relief. Table 14.2 lists a few common bases.

Table 14.2 It is just as important to become familiar with chemical bases as with acids.

Brönsted-lowry Acids And Bases

In 1923, two chemists, Johannes Brönsted and Thomas Lowry, described acids and bases in the scientific literature. They were studying how the transfer of hydrogen ions (protons) took place between reacting molecules. A Brönsted-Lowry acid donates a proton in a reaction, while a base is on the receiving end of the proton transfer. In the Brönsted-Lowry definition of acids and bases, ions as well as larger more complex molecules are included. The pairs of (H ₂ O, OH ⁻ ) and (NH ₃ , ) are called Brönsted–Lowry conjugate acid–base pairs .

Examples

A conjugate base of a sample or ion occurs when an H ⁺ ion is lost.

A conjugate base of a sample or ion occurs when an H ⁺ ion is added.

Some acids and bases are chameleons and can accept protons in one reaction and then turn around and donate them in a subsequent reaction. A lot of physiological reactions work like this with water or blood serving as the go-between solution. Ions or molecules, which can swing either way, depending on the environment and company present, are called amphiprotic. They can either lose or add a proton (H ⁺ ) in a reaction.

An ion or molecule is amphoteric when it can serve as either an acid or a base in a reaction, but has no protons (H ⁺ ).

To bring the Brönsted–Lowry acid and base idea all together, remember the following:

1. A base accepts protons.

2. An acid provides protons in a reaction.

3. Acidic and basic reactions don’t just occur with protons (H ⁺ ).

4. Ions as well as molecules can be acidic or basic.

5. Some reactants can swing either way, providing or accepting protons.