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.

Chapter 7

ACIDS

BASES

SALTS

Acid-Base Chemistry:

• Acid - Any substance that produces H+ ions when dissolved in H2O. This was a definition discovered by a Swedish Chemist, Svante Arrhenius, in 1887.

• H+ (called the hydrogen ion) is also considered identical with the hydronium ion, H3O+ and can be written either wayAcids:

• 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 H2O. 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 H2O. 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 H2O, which we consider neither an acid nor a base, but in the strict definition it can chemically behave as both.

HCl + H2O H3O+ + Cl-1

NH3 + H2O NH4+ + OH-1

• The products of a neutralization reaction (not involving water as an acid or base) are always H2O plus a salt. There also might be other products, most often CO2

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 H2O. 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):

HC2H3O2 + H2O H3O+ + C2H3O2-1

Only a small amount of HC2H3O2 actually decomposes. All substances are still present in the solution, as opposed to a strong acid:

HCl + H2O H3O+ + 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 CO3-2 and during

neutralization CO2 is produced and causes the person to

burp.