HS Chemistry - Reversible Reactions

Brønsted–Lowry Theory

Overview of The Page

This page will cover:

• What is the Brønsted–Lowry Theory?
• What are some characteristics of aicds and bases?
• What are titrations?

What are acids and bases? Different scientists have come up with different definitions over the years, but the one studied in this class is the Brønsted–Lowry definition:

• An acid is a proton $H^\+^$ donor.
• A base is a proton $H^\+^$ acceptor.

In a neutralization reaction, an acid neutralizes a base, creating a salt $an ionic compound$ and another substance. The acid donates, or loses, an H⁺ atom to the base, which accepts, or gains, an H⁺ atom. After losing the H⁺ atom, the acid turns into its conjugate base; after gaining the H⁺ ion, the base turns into its conjugate acid. As the conjugate base is negatively charged and the conjugate acid is positively charged, they react with each other, forming a salt $an ionic compound$.

For example, the conjugate base for the acid HCl is Cl^- $it lost an H^\+^ ion$, and the conjugate acid for the base NH₃ is NH~4⁺ $it accepted an H^\+^ ion$.

When an acid or base is dissolved in water, it ionizes into its conjugate acid or base. Different acids and bases do so to different extents, depending on how reactive they are. This can be measured by their pH.

pH is measured on a scale from 1 - 14, where 7 is neutral. A substance with a pH lower than 7 is acidic, and a substance with a pH higher than 7 is basic. The further the pH is from 7, the stronger the acid/base is, and the more it will ionize in solution. pH is a logarithmic scale - an acid with a pH of 1 is 10 times more reactive than an acid with a pH of 2, an acid with a pH of 2 is 10 times more reactive than an acid with a pH of 3, and so on.

Mathematically:

pH = −log⁡[H⁺]

Where [H⁺] is the concentration of H⁺ ions that dissociate, or ionize, from the substance.

Salts $ionic compounds$ also have a pH, as they also form a base or acid when dissolved. Their pH depends on the strength of the acid or base they form when dissolved.

• If the salt forms an acid when it dissolves in water, the salt is acidic $its pH is less than 7$.

• If the salt forms a base when it dissolves in water, the salt is basic $its pH is greater than 7$.

Additionally, there are some substances that can function as both acids and bases. These are called amphoteric substances.

There are two important properties of acids and bases that can be determined from the substance's pH:

• A substance with a pH that is far from 7 will ionize more in solution than one with a pH that is close to 7. Since there will be more free ions in solution, a substance with a pH farther from 7 will conduct electricity better.

• Stronger acids are also more reactive with reactive metals for the same reason.

The molar concentration of a substance is called its molarity. To calculate molarity:

M = $number of moles$ ÷ $Volume in Liters$

Where M is molarity. For example, if there is 1 mole of HCl gas in 10 Liters, the molarity would be 0.1. Thus, we would write that as 0.1M HCl $g$.

Titration

A titration experiment can be used to find out the pH of an unknown acid or base, provided we know how much of the unknown acid/base there is. Using a base/acid with a known pH, we can calculate how much of the base we need to add to completely neutralize the acid. That point is called the equivalence point. In a titration experiment, an indicator is used to determine when the equivalence point has been reached.

An indicator is a weak acid or base whose conjugate base has a different color in solution than the original acid, and which is weak enough to not affect the experiment. At the equivalence point, the conjugate base will be the only substance affecting the pH $as everything else will cancel each other out$, and so its color will become clearly visible. Thus, an indicator shows when the equivalence point has been reached in a titration experiment.

Once the equivalence point has been reached, the amount of base/acid we added $the one with a known pH$ can be used to determine the pH of the unknown acid/base.