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Acid–Base Reactions for AP Chemistry (page 2)

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By — McGraw-Hill Professional
Updated on Feb 9, 2011

Acid–Base Reactions

In general, acids react with bases to form a salt and, usually, water. The salt will depend upon which acid and base are used:

Reactions of this type are called neutralization reactions.

The first two neutralization equations are represented by the same net ionic equation:

    H+(aq) + OH(aq) → H2O (I)

In the third case, the net ionic equation is different:

    H+(aq) + NH3(aq) → NH4+ (aq)

As mentioned previously, certain salts have acid–base properties. In general, salts containing cations of strong bases and anions of strong acids are neither acidic nor basic. They are neutral, reacting with neither acids nor bases. An example would be potassium nitrate, KNO3. The potassium comes from the strong base KOH and the nitrate from the strong acid HNO3.

Salts containing cations not of strong bases but with anions of strong acids behave as acidic salts. An example would be ammonium chloride, NH4Cl.

Cations of strong bases and anions not of strong acids are basic salts. An example would be sodium carbonate, Na2CO3. It reacts with an acid to form carbonic acid, which would then decompose to carbon dioxide and water:

The same type of reaction would be true for acid carbonates, such as sodium bicarbonate, NaHCO3.

Another group of compounds that have acid–base properties are the hydrides of the alkali metals and of calcium, strontium, and barium. These hydrides will react with water to form the hydroxide ion and hydrogen gas:

Note that in this case, water is behaving as H+OH.

Acid–Base Titrations

A common laboratory application of acid–base reactions is a titration. A titration is a laboratory procedure in which a solution of known concentration is used to determine the concentration of an unknown solution. For strong acid/strong base titration systems, the net ionic equation is:

    H+(aq) + OH(aq) → H2O (I)

For example, suppose you wanted to determine the molarity of an HCl solution. You would pipet a known volume of the acid into a flask and add a couple drops of a suitable acid–base indicator. An indicator that is commonly used is phenolphthalein, which is colorless in an acidic solution and pink in a basic solution. You would then fill a buret with a strong base solution (NaOH is commonly used) of known concentration. The buret allows you to add small amounts of the base solution to the acid solution in the flask. The course of the titration can also be followed by the use of a pH meter. Initially the pH of the solution will be low, since it is an acid solution. As the base is added and neutralization of the acid takes place, the pH will slowly rise. Small amounts of the base are added until one reaches the equivalence point. The equivalence point is that point in the titration where the number of moles of H+ in the acid solution has been exactly neutralized with the same number of moles of OH:

    molesH+ = molesOH at the equivalence point

For the titration of a strong acid with a strong base, the pH rapidly rises in the vicinity of the equivalence point. Then, as the tiniest amount of base is added in excess, the indicator turns pink. This is called the endpoint of the titration. In an accurate titration the endpoint will be as close to the equivalence point as possible. For simple titrations that do not use a pH meter, it is assumed that the endpoint and the equivalence point are the same, so that:

    molesH+ = molesOH at the endpoint

After the equivalence point has been passed, the pH is greater than 7 (basic solution) and begins to level out somewhat. Figure 6.1 shows the shape of the curve for this titration.

Reaction stoichiometry can then be used to solve for the molarity of the acid solution. See the Stoichiometry chapter for a discussion of solution stoichiometry.

An unknown base can be titrated with an acid solution of known concentration. One major difference is that the pH will be greater than 7 initially and will decrease as the titration proceeds. The other major difference is that the indicator will start off pink, and the color will vanish at the endpoint.

Titration of a Strong Acid with a Strong Base

Practice problems for these concepts can be found at:

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