From this lesson, you will understand the neutralization process between acids and bases. Learn how a hydroxide ion from a base reacts with a hydronium ion from an acid to neutralize each other and form water. Discover what conjugate acids and bases are and what the definition of amphoteric is.
You have learned that an acid is a compound that produces hydronium ions, H3O+, and that a base is a compound that produces hydroxide ions, OH-. Strong acids and bases are those compounds that ionize nearly 100% in solution, meaning they release 100% of their hydronium and hydroxide ions into the solution.
pH is a measure of how acidic or basic something is. The pH scale runs from 1 to 14, with the value 7 representing neutral. The lower the pH, the more acidic something is; the higher the pH, the more basic it is. If something is said to be neutral, then it contains equal concentrations of hydronium and hydroxide ions. A neutralization reaction is when a hydronium ion from an acid reacts with a hydroxide ion from a base to make water and a salt.
The pH scale measures the acidity of a solution.
When a strong acid solution filled with a bunch of H3O+ ions combines with a strong base filled with OH- ions, almost all of the ions combine to make water. The reaction looks like this:
OH- + H3O+ --> H2O
This very same reaction happens no matter what the acid or base is. For instance:
HCl + NaOH --> NaCl + H2O
A fun experiment to do at home is to have a glass full of limewater (Ca(OH)2 - calcium hydroxide), which is a base, and add a little phenolphthalein, a pH indicator, to it. Amazon sells both of these items. The solution will be pink because the phenolphthalein is a pH indicator that turns pink when it is added to a basic solution. Take a straw, put it in the glass and blow into it. The carbon dioxide in your breath will act as an acid and neutralize the limewater base, turning the solution clear because all the acid and base have turned to water.
Another home experiment you can do is combining baking soda and lemon juice. The citric acid of the lemon juice is neutralized by the sodium bicarbonate of the baking soda in a bubbling reaction that creates salty water.
Another home example of an acid-base reaction is baking powder. Baking powder contains both sodium bicarbonate and tartaric acid. When the two components combine in baked goods, such as biscuits or cookies (mmm...cookies), they neutralize and produce carbon dioxide along with water. The carbon dioxide, which is a gas, causes the baked goods to rise.
Bread rises from an acid-base reaction that produces carbon dioxide.
Another example is TUMS for the tummy-ache. The antacid in the TUMS neutralizes the extra acid in the stomach to make you feel better.
Conjugate Acid and Conjugate Base
According to Bronsted and Lowry, who you have learned about before, an acid-base reaction is simply when one molecule gives another molecule a proton. When ammonia and water react, water gives a proton to the ammonia. The ammonia becomes an ammonium ion and the water becomes a hydroxide ion.
Bronsted and Lowry define a conjugate acid as the acid that forms when a base gains a proton. A conjugate base is a base that forms when an acid loses a proton. In the reaction below, the ammonium ion is called the conjugate acid and the hydroxide ion is the conjugate base.
Ammonium ion (conjugate acid) and hydroxide ion (conjugate base).
Another example is when hydrofluoric acid reacts with water.
HF + H2O --> F- + H3O+
HF is the acid. H2O is the base. F- is the conjugate base because it formed when the hydrofluoric acid lost a proton. H3O+ is a conjugate acid because it formed when the base, water, gained a proton.
An example of the production of a conjugate acid and base.
Every Bronsted-Lowry acid has a conjugate base, and every Bronsted-Lowry base has a conjugate acid. In general, the stronger the acid is, the weaker the conjugate base it produces will be. This is because a strong acid gives up its protons easily, so the base in the reaction will have no need to strongly attract those protons. I mean, why work hard for something when someone is willing to just give it to you? The same is true for bases - the stronger the base, the weaker the conjugate acid it produces will be.
Sometimes, there are substances that can act as both acid and base by being both a proton acceptor and a proton donor. This is called amphoteric. Water is an example. It can donate a proton to become OH- or accept a proton to become H3O+.
If something is said to be neutral, then it contains equal concentrations of hydronium and hydroxide ions. A neutralization reaction is when a hydronium ion from an acid reacts with a hydroxide ion from a base to make water and a salt.
According to Bronsted and Lowry, an acid-base reaction is simply when one molecule gives another molecule a proton. They define a conjugate acid as the acid that forms when a base gains a proton. A conjugate base is a base that forms when an acid loses a proton. An amphoteric substance is something that can act as both acid and base by being both a proton acceptor and a proton donor.
From this lesson, you should learn to:
- Define amphoteric and define neutral in terms of pH
- Explain the Bronsted-Lowry definition of conjugate acid and conjugate base
- Explain what happens during a neutralization reaction