Neutralization Process and Differences Between Acids and Bases
Water solutions of any acid taste sour and react with certain metals to liberate hydrogen gas. Bases in water have a bitter taste and feel slippery.
In this project, you will examine the differences between acids and bases and the neutralizing effect of adding the two chemicals. The titration process will be used to determine the amount of acetylsalicylic acid in various brands of aspirin tablets as well as the strength of different antacid tablets. You will also look at the pH-pOH scale and explore methods of determining the pH and pOH.
Purpose: To determine how an acid is neutralized.
- 3 baby-food jars
- sheet of white paper
- 3 eyedroppers
- white vinegar
- phenolphthalein (see Appendix 5)
- household ammonia
CAUTION: Handle the ammonia with care and work in a well-ventilated area. Ammonia is poisonous, and its fumes can damage skin and the mucous membranes of the nose, mouth, and eyes.
- To prepare for the experiment, set the baby-food jars on a sheet of white paper so that you can detect color changes easily. Set aside a separate eyedropper for the vinegar, phenolphthalein, and ammonia.
- Add 25 drops of vinegar (an acid) to one jar.
- Add three or four drops of phenolphthalein and stir to test for the effect of this liquid on an acid.
- Record any color changes.
- Add 25 drops of ammonia (a base) to the second jar.
- Add three or four drops of phenolphthalein and stir to test for the effect of this liquid on a base.
- Record any color changes.
- Add 25 drops of vinegar to the third jar.
- Add three or four drops of phenolphthalein and then neutralize this acid with a base by adding a few drops of ammonia.
- Continue to add drops of ammonia until the entire solution retains a pale pink color after stirring (see Figure 12.1).
There is no change in color when phenolphthalein is added to the vinegar, but the solution turns pink when phenolphthalein is added to the ammonia. At first, adding ammonia causes no color change in the vinegar. Eventually, the area around the ammonia drop turns pink, but the color disappears when the solution is mixed. A quantity of ammonia is finally added that results in a pale pink color that does not disappear.
Vinegar is an acid, and like all acids, it contains hydrogen ions (H+). Household ammonia is a basic solution and is identified as a base by the presence of hydroxide ions (OH-). Indicators are chemicals that change colors in the presence of hydrogen and/or hydroxide ions. They change because the chemical structure of a material determines how it absorbs and reflects light waves, and therefore its color. Phenolphthalein is an indicator that specifically determines the presence of a base because when combined with hydroxide ions, its chemical structure changes and reflects pink light, giving a pink color.
In a neutralization reaction, an acid and a base react to form a salt and water. In this experiment, vinegar and household ammonia combine to form the salt ammonium acetate, and the H+ and OH- ions combine to form water (HOH) molecules. That is,
When all of the hydrogen ions from the acid combine with all of the hydroxide ions from the base, the solution is considered neutral. The solution is neither acidic nor basic and thus does not affect the indicator. In this experiment, the hydroxide ions in each drop of ammonia combine with the hydrogen ions in the vinegar until all of the hydrogen ions are neutralized. The resulting pink color of the solution indicates that all of the acid is neutralized and that a few extra hydroxide ions are present to react with the phenolphthalein indicator.