Chapter 7 ACIDS BASES SALTS

Acid-Base Chemistry: Acid - Any substance that produces H + ions when dissolved in H 2 O. This was a definition discovered by a Swedish Chemist, Svante Arrhenius, in H + (called the hydrogen ion) is also considered identical with the hydronium ion, H 3 O + and can be written either way Acids: have a sour taste (think of orange juice). turn a dye called litmus from blue to red. (litmus is red in acid). dissolve active metals producing hydrogen gas react with bases to form water and ionic compounds called salts.

Base – Any substance that produces OH -1 ions when dissolved in H 2 O. This is also the Arrhenius definition. Bases: have a bitter taste Have a slippery feeling turn litmus from red to blue. (litmus is blue in base) react with acids to form water and salt.

When an acid and base react together, the H + and the OH -1 react to form H 2 O. The acid and base are being neutralized. Acidic properties are caused by H + while basic properties are caused by OH -1, thus when water is formed neither acid nor base properties are present (they have been neutralized). This type of reaction is called a Neutralization Reaction.

This brings up a better definition for an acid and a base. Acid - A substance that donates an H + ion (H + is actually just a proton, so the definition is usually given as a proton donor). This definition was proposed independently by 2 chemists, J.N. Bronsted and T.M. Lowry, and for that reason is called the Bronsted-Lowry theory. Base - A proton acceptor.

There always has to be a substance willing to accept the H + ion produced by an acid or to react with the OH -1 produced by a base. In other words acid and bases always occur together (You can’t have one without the other)

With these definitions some substances can behave as acids in one situation or as bases in other situations. The prime example is H 2 O, which we consider neither an acid nor a base, but in the strict definition it can chemically behave as both. HCl + H 2 O  H 3 O + + Cl -1 NH 3 + H 2 O  NH OH -1 The products of a neutralization reaction (not involving water as an acid or base) are always H 2 O plus a salt. There also might be other products, most often CO 2 Let’s identify the acid and base in each reaction, using both definitions.

Acids & Bases are classified as strong or weak. A strong acid or base completely decomposes into ions when dissolved in H 2 O. We say it is completely ionized. We say they are strong because they produce the maximum concentration of H + or OH -1 for that amount of acid or base added.

A weak acid or base only partially ionizes. Some whole molecules of the acid also remain in the solution. Thus less than the maximum amount of H + or OH -1 are produced in the solution. An example is acetic acid (active ingredient in vinegar): HC 2 H 3 O 2 + H 2 O  H 3 O + + C 2 H 3 O 2 -1 Only a small amount of HC 2 H 3 O 2 actually decomposes. All substances are still present in the solution, as opposed to a strong acid: HCl + H 2 O  H 3 O + + Cl -1 where no HCl is present in the solution.

Every water based solution, and water itself always contain both H + and OH -1. In fact always the same total amount. Thus if the concentration of H + is high the concentration of OH -1 will be low and vice versa. A neutral solution has exactly equal amounts of both. A solution with more H + than OH -1 will be acidic, while one with more OH -1 than H + will be basic.

A convenient unit to measure the concentration of H + and OH -1 is something called pH. Mathematically, pH = -log[H+], but we won’t worry about these calculations. Just understand that pH represents [H+] and if we know one, we can calculate the other. When the pH is less than 7 the solution is acidic and when it is greater than 7 it is basic. Exactly 7 is neutral

pOH does the same thing with the concentration of OH -1. pH + pOH always must = 14.0 Examples: If pH = 5.5, then pOH = 8.5 and the solution is acidic. If pH = 9.2, then pOH = 4.8 and the solution is basic.

Antacids are called that because they are bases (anti acid). Heartburn is caused by excess acid in the stomach or esophagus and antacids neutralize this excess acid. Frequently the antacid contains CO 3 -2 and during neutralization CO 2 is produced and causes the person to burp.