Unit 7: Acid-Base and Oxidation-Reduction Reactions
In this unit, we study two important types of chemical reactions: acid-base and oxidation-reduction. We will discuss how these types of reactions occur in all aspects of science and in everyday life. We will also review the properties of acids and bases and introduce two acid-base definitions: Arrhenius and Brønsted-Lowry.
We will perform pH calculations and learn how to use the pH scale to identify acidic and alkaline solutions. Then, we will discuss oxidation and reduction, also known as electron transfer reactions. We will also learn how to write and balance equations for oxidation-reduction reactions and introduce some common oxidizing and reducing agents.
Completing this unit should take you approximately 4 hours.
Upon successful completion of this unit, you will be able to:
- use the Arrhenius and Brønsted-Lowry definitions to identify acids and bases;
- write and balance equations for neutralization reactions;
- conduct pH calculations and use pH scale to classify solutions as acidic, basic, or neutral;
- contrast strong acids and bases with weak acids and bases;
- perform calculations for a titration of a strong acid with a strong base;
- compare and contrast processes of oxidation and reduction;
- calculate oxidation numbers for each element in a given compound; and
- write and balance equations for oxidation-reduction reactions.
7.1: Acids and Bases
Before we begin examining acid-base chemistry, we need to define acids and bases. As stated above, there are multiple definitions of acids and bases. First, let's take a look at the Arrhenius definition. The Arrhenius definition states that an acid is a substance that produces hydrogen ions (H+ ions) in an aqueous (water) solution. A base is a substance that produces hydroxide ions (OH- ions) in an aqueous solution. The reaction where an acid loses a hydrogen to produce H+ (or a base loses an OH to produce OH-) is called a dissociation reaction.
Read this text. Pay attention to the dissociation reactions for acids and bases when dissolved in water. Section 4 introduces neutralization reactions, where an acid and a base react together to form a salt (ionic compound) and water. Pay attention to the form of the neutralization reactions, and make sure you can identify the acid, base, and salt in each reaction.
Watch this video for more discussion of the Arrhenius definition. Notice that Arrhenius acids increase the concentration of hydrogen ions in aqueous solutions, while Arrhenius bases increase the concentration of hydroxide ions.
Read this text, which introduces the pH scale that scientists use to determine whether a substance is acidic, basic, or neutral. The pH scale ranges from 0 to 14. A solution with a pH of 7 is neutral. If the pH is less than 7, the solution is acidic. If the pH is greater than 7, the solution is basic. Since the pH scale is logarithmic, each whole pH value below 7 is ten times more acidic than the next higher value. We determine pH based on the concentration of H+ ions in the solution.
In Part 1, pay close attention to the ion product of water (Kw) – this is an important value used in chemistry. In Part 2, you should know the equations that define pH. Try Problem Example 2.
This text also introduces titrations, an important analytical technique chemists use to determine the concentration of an unknown substance, called the analyte. When we do a titration, we slowly add a substance of known concentration, called the titrant, to the analyte and allow them to react until the reaction is complete. The point when the reaction is complete is called the equivalence point. Color-changing indicators are often used to visually show the equivalence point. If we know the amount of titrant used, we can use stoichiometry to determine the amount of analyte.
There are many types of titrations, but the most well-known are acid-base titrations, in which a neutralization reaction takes place during the titration. In Part 3, pay close attention to Problem Example 3, since this is a typical titration problem. You should also note the shape of a titration curve, and how to determine the equivalence point from a titration curve.
Watch this video to learn how acids and bases neutralize each other to form a salt and water.
Watch these next four videos, which provide more detailed information and examples of pH and titration calculations. Our first video defines pH and the pH scale and shows examples of how to conduct pH calculations.
Our second video shows how to do pH calculations for solutions of strong acids or bases.
Our third video shows how to write an equilibrium expression for an acid-base reaction and how to evaluate the strength of an acid using Ka.
Finally, our fourth video demonstrates an example of a strong acid-strong base titration. It covers indicators, endpoint, equivalence point, and calculating the unknown analyte's concentration.
Now, we are ready to learn about a second definition of acids and bases, the Brønsted-Lowry definition. According to the Brønsted-Lowry definition, an acid is a substance that can donate a proton (H+ ion) to another molecule. A base is a substance that can accept that donated H+.
After the Brønsted-Lowry acid donates its proton, it becomes the conjugate base of the acid. After the Brønsted-Lowry base accepts a proton, it becomes the conjugate acid of the base. We think of Brønsted-Lowry acids and bases in terms of their reactions with other molecules rather than just their structure. This is a more general definition and broadens the compounds that can be considered acids or bases.
Watch this video, which shows examples of how we can interpret a chemical reaction to determine, which substance is the acid, base, conjugate acid, and conjugate base. Water acts as a base because it is reacting with a strong acid. Water is an amphiprotic compound, which means it can act as an acid or a base. Hydrogen ions do not exist in water on their own, but immediately get grabbed by water molecules to form hydronium ions. A conjugate pair is always one acid and one base.
Read this text about acid-base reactions which discusses the information we studied in the previous video. Make sure you are able to define conjugate acid and bases and identify the conjugate acid-base pairs for a given reaction.
Water acts as a base when reacted with a strong acid. This seems odd, since we know water is neutral. Water is an amphiprotic compound, meaning it can act as either an acid or a base. When an acid reacts with water, the water will act as a base. When a base reacts with water, the water will act as an acid. Water also autoionizes, meaning it can form H+ and OH-. This video shows how two water molecules react to form hydronium ion (H3O+, an acid), and OH-, but is still overall pH-neutral.
Watch this video, which reviews how to determine the conjugate acid-base pairs in an acid-base neutralization reaction. You should be able to recognize conjugate acid-base pairs for a given reaction.
7.2: Oxidation-Reduction Reactions
Now let's explore oxidation-reduction reactions. These reactions are tremendously important in our every-day lives. For example, they are the basis of batteries, explain why rusting occurs, and are common reactions in our human metabolism. Oxidation-reduction reactions involve the transfer of electron(s) from one reactant to the other.
Oxidation-reduction reactions are commonly called redox reactions. When a redox reaction occurs, two half-reactions occur simultaneously. One of the reactants undergoes oxidation, meaning that it loses electron(s). The other reactant undergoes reduction, meaning it gains electron(s). Oxidation and reduction must occur together – one half-reaction cannot happen alone. The reactant that undergoes oxidation is called the reducing agent, while the reactant that undergoes reduction is called the oxidizing agent.
Read this text. The author breaks the redox reaction up into the two separate half-reactions. Each half-reaction is either oxidation or reduction. In the oxidation half-reaction, electrons are lost. In the reduction half-reaction, electrons are being gained.
In the example in the previous text, we could tell, which species were gaining and losing electrons because the species were single elements and elemental ions. However, most redox chemistry is much more complex and cannot be determined by quickly looking at the reaction.
To determine if a substance is being oxidized or reduced in a reaction, we use oxidation numbers. Oxidation numbers are simply a record-keeping tool. We can calculate the oxidation numbers for all elements in a reaction. If an element's oxidation number increases from reactant to product, it is being oxidized. If an element's oxidation number decreases from reactant to product, it is being reduced.
Read this text, which lists the rules we use to determine oxidation numbers. You need to know these rules and be able to apply them to determine the oxidation numbers of elements in compounds. After you study the rules, try the example problems.
Balancing redox reactions is more difficult than regular reaction balancing because we need to account for the electron transfer. We also need to balance the reaction differently depending on whether the solution is acidic or basic. This text reviews the steps used to balance redox reactions. You should know these steps for balancing a redox reaction and be able to balance a redox reaction using these steps.
Watch these videos, which provide worked examples of how to balance a redox reaction in an acidic and in a basic solution. Be sure you are able to follow all of the steps in this balancing process.
Read these sections, which look at some common oxidizing and reducing agents, and substances that can act as an oxidizing agent or reducing agent depending on what they react with. Water is one of the compounds that can act as an oxidizing or reducing agent.