Acids and Bases: An Introduction

Acids are found in food, your stomach, and the environment. Bases are found in soap, household cleaners, and antacid tablets. Acids often taste sour, are corrosive, and will turn blue litmus paper (an indicator) red. Bases often taste bitter, feel slippery, are corrosive, and will turn red litmus paper blue.

Common acids: Citrus fruits have citric acid. Yogurt has lactic acid. Vinegar is acetic acid. Your stomach uses hydrochloric acid. Car batteries use sulfuric acid. Some soft drinks use phosphoric acid. Nitric acid is used as a fertilizer.

Common bases: Glass cleaner may contain ammonia. Draino uses sodium hydroxide. Tums uses calcium hydroxide. Milk of magnesia uses magnesium hydroxide. Baking soda is sodium bicarbonate.

What makes an acid an acid or a base a base can vary depending on definition being used. The first definition was created by Svante Arrhenius in Arrhenius said compounds that will ionize to give off H + ions are acids and compounds that will ionize to give off OH - ions are bases. (Of course to ionize they must be dissolved in water - aquated!)

HCl → H + + Cl - NaOH → Na + + OH -

HCl → H + + Cl - NaOH → Na + + OH - ACID BASE

In the first reaction water acts as a base to accept the H + from HCl, and water can also act as an acid to give H + to another compound. Substances that can act as both an acid and base are called amphoteric.

Strengths of Acid and Bases

The strength of an acid or base has nothing to do with the concentration. A strong acid is a compound where 100% of all the little molecules will ionize. A weak acid is a compound where much less than 100% of all the little molecules will ionize. Thus a strong acid falls apart completely and a weak acid still has some of the molecules stuck together.

A concentrated acid has a lot of acid dissolved in water, and a dilute acid has only a little. The concentration is usually measured in molarity (moles divided by liters). It is possible to have a concentrated strong acid (12M HCl) and a concentrated weak acid (12M HCN). It is also possible to have a dilute strong acid (0.1M HCl) and a dilute weak acid (0.1M HCN).

There is NO connection between concentrated/dilute and strong/weak!

19.3 What is pH?

Because you cannot use the concentration to know the strength of the acid, and because you cannot use the strength to know the concentration of the acid, chemists have determined a way to get one all- inclusive number to use. This number is called pH. (The “p” means a logarithm is used and the “H” refers to the H + that acids give off.)

The pH scale goes from 0 to is considered neutral. Anything from 0-7 on the pH scale is an acid and anything from 7-14 is a base. In addition, anything from 0-4 is a strong acid and anything from 4-7 is a weak acid. Anything from is a weak base and anything from is a strong base.

To determine the pH of a substance requires an indicator. Indicators are substances that turn colors at certain pH’s. Most indicators can only turn one color. For example, phenolphthalein is colorless in acids and pink-purple with bases. In order to quickly determine a pH, pH paper has many different indicators soaked into it. This allows pH paper to turn 5 or more colors so 5 or more pHs can be determined.

The best way to determine the pH is with a pH meter or calculation. pH = -log [H + ], which means the negative logarithm of the concentration of the H + ions. If the [H + ] is known, then a calculator can provide the pH number, but if the [H + ] is not known, determining it without the use of technology is a long a grueling process.

pH meters have been created to do these calculations quickly and show the exact pH results almost immediately. Extreme care must be taken with pH meters as they are fragile and will not give correct results if not properly cared for.

19.4 Neutralization

When an acid and base are added together in appropriate amounts, the resulting pH is near 7 - neutral. This is why acids and bases are said to “neutralize” each other. In a neutralization reaction the acid and base (according to the Arrhenius definition) react to form water and a salt.

HCl + NaOH → HOH + NaCl WaterSalt

The salt does not have to be table salt (NaCl). A salt is almost any metal with an anion. What’s the salt in the below reaction? H 2 SO 4 + Ba(OH) 2 → 2HOH + BaSO 4

Neutralization has many purposes. If a lake becomes too acidic because of acid rain, then a base can be added to return the pH to the level fish need to live. If a base spills in lab an acid can be added to make the spill less dangerous to clean up.

Certain compounds in your blood neutralize the carbonic acid in your body to maintain homeostasis. Most importantly for chemists, if an unknown base is neutralized with a known acid, the amount of OH - in the base can be calculated. Likewise if an unknown acid is neutralized with a known base, the amount of H + in the acid can be calculated.

Neutralizing an acid and base to determine an unknown amount of H + or OH - is called titration. In a titration a small amount of indicator is added that will change color when the neutralization is complete.