Acids and Bases

Defining an Acid

There are a few different ways to define and recognize acids.

Arrhenius Acids start with Hydrogen as their cation and are the way we've been defining acids up to this point. These acids dissociate in water to make H+ ions while the bases they react with dissociate in water to make OH- ions. The H+ ions then combine with water to make hydronium ions (H3O+). An Arrhenius acid mixed with an Arrhenius base will always produce water and a salt when they react.

While Arrhenius acids are good to describe the general trend, there are some things that it doesn't consider bases even though they act similarly. A model to include these can be used through identification of Bronsted-Lowry acids which donate protons (H+) while their bases receive these protons (this means it includes water as an acid and base). It also includes materials in more phases than just the aqueous phase. When looking at the Bronsted-Lowry model we look at the products of acid-base reactions as conjugates. Conjugate acids are the ions or molecules formed when a base gains a hydrogen ion while conjugate bases are the ions or molecules that form when an acid donates a hydrogen. Together these form a conjugate acid-base pair.

Lewis acids are the broadest definition of acids, where Lewis acids accept pairs of electrons or release a proton that accepts a pair of electrons while Lewis bases donate pairs of electrons. Lewis acids and bases can form covalent bonds when combined (this means it includes unfilled octets as acids, such as things with Boron). With Lewis acids, drawing Lewis structures is the easiest way to tell if a reaction will happen.

The pH Scale

To measure acidity we use the pH scale, a logarithmic scale of the amount of hydronium (H3O+) ions found within a solution compared to the amount of hydroxide (OH-) ions. The scale is logarithmic, so each number is 1 order of magnitude higher/lower in concentration (a pH of 2 is 10 times more acidic than a pH of 3). The lower the number, the more acidic something is. The scale goes from 0-14 with 7 in the middle. 7 is the pH of pure water; it has an equal amount of hydronium and hydroxide within it.

Acids and bases have some distinct features. Both can be strong or weak and are related to one another. Strong acids have more cations floating around since they dissociate (detach) more from the anion portion of their compound. Both are also often used for cleaning products (since they can neutralize one another) and both can strongly react with other substances and dissolve them. 

• Acids generally taste sour and have low pH values (0-7)

• Bases are generally slippery or soapy and have high pH values (7-14)

Indicators

Titration is the process we use to neutralize an acid or base. To do this we first add an indicator to the solution we are neutralizing then slowly add an acid/base we know the concentration of until a color change indicates it’s the pH we want (usually 7).

Buffers

We can also make pH more constant by adding a buffer, a weak acid or base that makes a system more resistant to change since these weak compounds act like a "sponge" for the ions being added to the system. For example, one of the many buffers our body uses is carbonic acid in our blood, making sure our blood remains consistent with its pH so we don't undergo acidosis (too acidic blood) or alkalosis (too basic blood), both of which mess with our body. In this case carbon dioxide dissolving into the blood is the usual culprit since dissolved CO2 makes carbonic acid of its own. Buffers have a capacity (how much acid or base can be added to them without significant change happening) and range (the pH range where a buffer is effective).

Additional Resources

• Crash Course: Acids and Bases in Solutions

• Crash Course: pH and pOH

• Acid Types

• Sulfuric vs. Sulfurous Acids

Interesting Links

• Uses for Acids and Bases

• Making an Acid-Base Rocket

• Acid-Base Color Indicators