Unit 7: Acid-Base and Oxidation-Reduction Reactions
Now let's look at two types of chemical reactions: acid-base and oxidation-reduction. We will review the properties of acids and bases and introduce two acid-base definitions: Arrhenius and Brønsted-Lowry. We will also examine pH calculations, and learn how to use the pH scale to identify acidic and alkaline solutions. Next, we discuss oxidation and reduction, 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 12 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;
- explain the process of self-ionization of water molecules;
- conduct pH calculations and use pH scale to classify solutions as acidic, basic, or neutral;
- explain the process of titration;
- explain relationship between conjugated acids and bases;
- compare and contrast processes of oxidation and reduction;
- write and balance equations for oxidation-reduction reactions; and
- identify common oxidizing agents, common reducing agents, and substances that can act both as oxidizing and reducing agents.
7.1: Acids and Bases
Read this page, which introduces the concepts of acids, bases, salts, and neutralization reactions. The Arrhenius theory views acids and bases as substances that produce hydrogen ions or hydroxide ions on dissociation.
Watch this video, which explains Arrhenius' definition of acids and bases. Arrhenius acids increase the concentration of hydrogen ions in aqueous solutions, while Arrhenius bases increase the concentration of hydroxide ions.
In this virtual lab, you will solve problems related to strong acids and bases. To see the instructions, click the name of the problem ("Strong Acid Textbook Problems") on the virtual lab screen.
In this virtual lab, you will solve problems related to weak acids and bases. To see the instructions, click the name of the problem ("Weak Acid Textbook Problems") on the virtual lab screen.
Read this page, which introduces the pH scale and explains how titration is done. We can titrate an acid with a base or a base with an acid. The substance whose concentration we are determining is called the analyte. The substance we are adding in measured amounts is called the titrant.
Watch these videos. The first explains pH and the pH scale and explains how to conduct pH calculations. 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.
The second covers common strong acids and strong bases and presents examples of pH calculations. Do not forget that strong acids and strong bases nearly completely dissociate in ions in aqueous solutions.
The third explains how titration is conducted and presents an example of titrating strong acid with a strong base. It covers indicators, endpoint, equivalence point, and calculating the unknown concentration. Note that the endpoint of titration is defined as the moment when enough titrant has been added to the titrand to facilitate a color change. The equivalence point is defined as the moment when the amount of titrant added is enough to completely neutralize the analyte solution.
In this virtual lab, you will standardize an unknown NaOH solution four significant figures via titration with 25.00 mL of a standard solution of KHP. To see the instructions, click the name of the problem ("Standardization of NaOH") on the virtual lab screen.
Read this page, which presents auto-ionization of water molecules, explains Brønsted-Lowry definition of acids and bases, and introduces the formation of conjugate acid-base pairs. Note that Brønsted-Lowry theory expands categories by defining acids are proton donors and bases are proton acceptors.
Watch these videos. The first explains the Brønsted-Lowry definition of acids and bases. The second explains how auto-ionization of water molecules happens. Water is an amphiprotic compound: it can act as either 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. The third explains the relationship between conjugate acids and bases. A conjugate pair is always one acid and one base.
Read this page, which covers different kinds of neutralization reactions. Make sure you can sketch proton-energy diagrams for a strong acid, a weak acid or base, and a strong base.
Watch this video, which covers different kinds of neutralization reactions. We use the term "neutralization" to define these reactions because the acid and base properties of hydrogen and hydroxide ions are destroyed or neutralized. Hydrogen and hydroxide ions combine to form water molecules.
7.2: Oxidation-Reduction Reactions
Read this page, which presents oxidation-reduction reactions and explains how oxidation-reduction reaction can be written in the form of half-reactions. Note that oxidation is the loss of electrons, while reduction is the gain of electrons. A species that donates electrons in a redox reaction is called a reducing agent, or reductant. A species that accepts electrons in a redox reaction is called an oxidizing agent, or oxidant.
Read this page, which explains how oxidation states are determined and how they change due to electron transfer in the oxidation-reduction reactions. Make sure you understand the rules. You might also want to memorize the list of elements that almost always have the same oxidation states in their compounds, as well as the exceptions to those rules.
Read this page, which explains how to balance equations of oxidation-reduction reactions. Different steps should be used depending on whether the solution is acidic or basic.
Watch these videos. The first introduces oxidation states, oxidation, and reduction. It also provides some tips for remembering oxidation and reduction. The second explains how to write the half-reactions in a combustion reaction. Combustion is a reaction between hydrocarbon and oxygen that produces carbon dioxide and water. Focus on what is being oxidized and what is being reduced during combustion.
Read this page, which introduces common oxidizing agents. All of them share one property: the ability to accept electrons readily.
Read this page, which introduces common reducing agents. All of them share one property: the ability to donate electrons readily.
Read this page, which introduces substances that can act both as oxidizing and reducing agents. One of these substances (water) is essential for your life. Another substance (hydrogen peroxide) is also commonly found in household products.
Watch this video, which explains how to identify the oxidizing and reducing agents in an oxidation-reduction reaction. The substance being oxidized is a reducing agent; the substance being reduced is an oxidizing agent.
In this virtual lab, you will design an experiment to determine which metals are the strongest and weakest reducing agents. To see the instructions, click the name of the problem ("Redox") on the virtual lab screen.