Lesson 14: Acids and Bases

What Are Acids and Bases?
We divide people into two categories. Can you identify any of the people in this picture? While you probably do not know them personally, you can definitely separate the boys from the girls. We use male and female as one of the first identifying factors between human beings. Just like we acknowledge that there are two different groups of people, male and female, scientists acknowledge that there exist two types of compounds: acidsand bases. Acids and bases make up two very different groups of important compounds. What is an acid? What is a base? Acids and bases have been described differently by different scientists.
Arrhenius Svante Arrhenius was a Swedish chemist who won the Nobel Prize in Chemistry in 1903. Arrhenius described an acid as a compound that dissociates in water to produce hydrogen ions (H+). He defined a base as a compound that dissociates in water to produce hydroxide ions (OH-). These definitions can be expressed by the reactions below.   HCl (aq)     H+ (aq)   +   Cl- (aq)   In this equation, hydrogen chloride (HCl) is the acid. Remember that aq is the abbreviation for aqueous. This means the acid is in solution with water. The hydrogen chloride dissociates in the solution to produce a hydrogen ion (H+).   NaOH (aq)     OH- (aq)   +   Na+ (aq)   In this equation, sodium hydroxide (NaOH) acts as a base. The sodium hydroxide dissociates in solution to produce a hydroxide ion (OH-).
Bronsted-Lowry Johan Bronsted and Thomas Lowry are the scientists responsible for providing a more general description of acids and bases. A Bronsted-Lowry acid is any substance that donates a proton. A Bronsted-Lowry base is any substance that accepts a proton. A proton is a hydrogen ion. Remember that a hydrogen ion is a hydrogen atom that has lost an electron. A hydrogen atom only has one electron to lose. When it loses that electron, all that remains is a single proton, H+. Bronsted and Lowry went on to determine that acids and bases come together as conjugate acid-base pairs. These compounds differ from each other by a single proton (hydrogen ion). This means that on one side of the equation a substance acts as the acid, but on the other side it is the conjugate base. The conjugate base is the compound that forms when an acid donates a proton. For example:   HCl   +   H2O     H3O+   +   Cl-     acid       base       conjugate acid       conjugate base   This reaction involves hydrochloric acid. Hydrochloric acid (HCl) acts as the acid and water is the base. You also see a new compound called a hydronium ion (H3O+). A hydronium ion is formed when a proton is added to a water molecule. In this equation, the hydrochloric acid donates a proton to the water. The hydronium ion acts as the conjugate acid. A conjugate acid is the compound that forms when a base gains a proton. The chlorine ion acts as the conjugate base.
Lewis A more general definition of an acid came from a scientist by the name of Gilbert Lewis. Remember that Lewis is the scientist famous for his models of atoms called electron dot diagrams or Lewis structures. A Lewis acid is a substance that accepts an electron pair. A Lewis base is a substance that donates an electron pair.   +       H2O   acid       base           In this reaction, the base donates an electron pair to the acid. The hydroxide ion donates a pair of electrons to the hydrogen ion. Lewis's model of acids and bases is useful because it describes acids and bases that do not transfer hydrogen. Many more substances can be considered acids and bases because they do not have to occur in solution. Although the Lewis model is more general and includes all acids and bases, the most commonly used description of acids and bases is the Bronsted-Lowry definition. An acid is generally any substance that donates protons that are hydrogen ions. A base is generally any substance that accepts protons.
What Are the Properties of Acids and Bases?
Which solution is the acid? Which is the base? If someone gave you two glasses, one with milk and one with coffee, could you tell the difference between the two? Yes! Coffee is a dark brown liquid, and milk is a cloudy white liquid. This picture shows two beakers. Each beaker is filled with a clear solution. One beaker is filled with an acid, and one is filled with a base. If neither of the containers are labeled, how do you determine which solution is the acid and which solution is the base? Sometimes physical properties such as appearance are not useful when trying to distinguish between substances. You must then use their chemical properties.   Properties of Acids While many acids and bases look the same, there are many other properties that allow you to differentiate between the two. Acids are proton donors. They are the substances that give up a proton in reactions. Bases are proton acceptors and receive the proton from the acids in reactions. When dissolved in water, all acids dissociate to share certain chemical and physical properties. Acids contain hydrogen ions and can produce them in water. Hydrogen ions react with water to form hydronium ions. The general equation for a reaction with an acid in water is as follows:   HA (aq)   +   H2O(l)     H3O+ (aq)   +   A- (aq)     acid       base                   The reaction above can also be written as it is shown below. HA is the general formula for an acid.   HA (aq)     H+ (aq)   +   A- (aq)   The word acid comes from the Latin word acidus, which means sour. Acids taste sour. Foods such as lemons and pickles are sour because they contain acids. Acids are electrolytes, and in solution they conduct electricity. Some acids are corrosive, which means they are capable of damaging and destroying skin tissue and other materials on contact. They also react with certain metals to produce hydrogen gas and a metal compound. Acids react with compounds called indicators to yield specific color changes. An acid-base indicator is a substance that changes color when it comes in contact with an acid or base. Acids turn the indicator one color, while bases turn it a different color. One specific indicator is litmus. Litmus is often mixed with paper to form litmus paper. Acids turn litmus paper red.
Properties of Bases Another word for base is alkali. Bases are the opposite of acids. They are proton acceptors and produce hydroxide ions in water. The general equation for a base in a solution with water is shown below. In this example, sodium hydroxide is the base.   NaOH (aq)     Na+ (aq)   +   OH- (aq)   Bases taste bitter and feel slippery or soapy. They are caustic. Bases can burn body tissue and damage or destroy many materials on contact. They are electrolytes; in solution, they conduct electricity. Bases also cause indicators to change to specific colors. They turn litmus paper blue.
Common Acids and Bases Many compounds and substances you encounter on a daily basis contain acids and bases. Acids are found in vitamin C, soft drinks, coffee, and tea. They are also found in fruits such as lemons, grapefruits, apples, and strawberries. Acids are essential to your body. They play an important role in the digestion of foods. You would not be able to digest any food without a specific acid in your stomach called hydrochloric acid. Bases are found in many household cleaning products such as ammonia, bleach, and oven cleaner. They are also found in deodorant, soap, baking soda, and laundry detergent. Water is amphiprotic. This means it can act as both an acid and a base. Water can donate a proton and act like an acid. It can also accept a proton and act like a base.
What Is pH?
The power of the hydronium ion, pH for short, measures the concentration of hydronium ions in a solution. Values are expressed on the pH scale to indicate how acidic or basic a substance is. pH scale The pH scale is a number line that ranges in value from 0-14. Any pH below 7 is acidic, with 0 being the most acidic. A pH greater than 7 is basic, with 14 being the most basic. A pH of 7 is neutral. The pH of a substance can be determined in several different ways. It can be measured using a pH meter or pH electrode, which both use electrical currents to determine pH values. The pH can also be measured using indicators such as litmus paper. Litmus paper turns a specific color depending on how acidic or basic a substance is. The image below is a pH scale illustrating the pH values of some common substances.
pH values of Common Substances The most acidic substance on this pH scale is battery acid. The most basic substance shown on this pH scale is drain cleaner. Drain cleaner is a substance used to unclog pipes and has a pH close to 14.
Think About It In many TV ads such as shampoo commercials, companies claim that their product is pH balanced. What does this mean?
Strong or Weak?
Strong is an adjective that describes strength or power. Body builders and wrestlers are strong. Weak is opposite of strong. When something is weak, it has little power or lacks strength. On a team, the worst player is often called the weakest link. The terms strong and weak are also used to describe acids and bases. The strength of an acid or base depends on how completely the compound separates into ions when it is dissolved in water. This is not the same as concentration. Strong and weak describe how easy or difficult it is for an acid or base to form ions in solution.
Strong Acid versus Weak Acid A strong acid is an acid that completely dissociates when dissolved in water. One hundred percent of a strong acid's molecules separate into ions. Strong acids have very low pH values that are usually between zero and two. Weak acids are acids that only partially dissociate in water. Strong acids are better conductors than weak acids and carry more current. There are very few strong acids. Hydrochloric acid (HCl), nitric acid (HNO3), sulfuric acid (H2SO4), and hydrobromic acid (HBr) are the strong acids discussed in this lesson. Any acid that is not a strong acid is a weak acid. The majority of acids are weak acids. Two common weak acids are nitrous acid (HNO2) and acetic acid commonly known as vinegar (CH3COOH). Weak acids do not fully separate into ions. Reactions involving weak acids are written using double arrows such as in equilibrium reactions, because the reaction does not go to completion. Not all of the weak acid ionizes. Reactions with strong acids are written using normal chemical equations. The equations below portray two different acids dissociating in water. 1.   HNO3   +   H2O     H3O+   +   NO3-       strong acid                                                                                           2.   CH3COOH   +   H2O     H3O+   +   CH3COO-       weak acid                           The first equation shows nitric acid, a strong acid, completely dissociate in water. The second equation shows acetic acid, a weak acid, partially dissociate in water.
Strong Bases versus Weak Bases A strong base completely dissociates when dissolved in water. One hundred percent of the base molecules separate into ions. Strong bases have very high pH values, usually between 12 and 14. Weak bases are bases that only partially dissociate in water. Strong bases are better conductors than weak bases; they carry more current.   NaOH   +   H2O     Na+   +   OH-     strong base                           The equation above shows the dissociation of  a strong base, sodium hydroxide. The equation below shows the partial dissociation of a weak base, ammonia.   NH3   +   H2O     NH4+   +   OH-     weak base                           The double arrows indicate that ammonia only partially dissociates. Some common strong bases included sodium hydroxide, lithium hydroxide, and potassium hydroxide.
What Is a Neutralization Reaction?
Have you ever had an upset stomach and taken an antacid to relieve the symptoms? Hydrochloric acid (HCl) is a strong acid found in the body with a pH between one and two. It is produced and released into the stomach by special cells and functions to aid your body in digestion. An upset stomach is often a result of too much acid in the stomach. An antacid is a basic compound that helps reduce the acidity of the stomach. Antacids work by counteracting some of the acid. If you have ever taken an antacid, you experienced a type of chemical reaction called a neutralization. Neutralization is a specific type of double replacement reaction that occurs between an acid and a base. A neutralization reaction occurs when an acid and a base react to form a salt and water. The hydrogen atoms from the acid combine with the hydroxide atoms from the base to form water. The other ions involved in the reaction combine to form a salt.   acid   +   base     salt   +   water   The reaction between an acid and a base is called a neutralization reaction because the acid and base both lose their characteristic properties and balance each other out (neutralize) to form neutral compounds. After the reaction, the acid is no longer acidic and the base is no longer basic.
Salts Salts are ionic compounds usually composed of a metal and a non-metal. They are neutral compounds that form as a result of acid-base neutralization. Salts are electrolytes and are able to conduct electricity in solutions. The most common salt is sodium chloride, better known as table salt. Sodium chloride is a white crystal compound composed of sodium and chlorine.
How Does pH Affect Our Bodies?
Blood flows through your body carrying oxygen, absorbing and dissolving nutrients, and removing carbon dioxide. The pH of the blood must remain between 7.2 and 7.6 in order for it to carry out these many functions. Normal blood pH is 7.4. When pH is too high or too low, many of the important enzymes and organs within the body are unable to work properly. Serious health problems and even death can occur if the pH of the blood becomes too acidic or too basic. How can you eat foods with pH values as low as 2 or as high as 9, and not raise or lower the blood's pH outside of this range? Your blood contains compounds called buffers. Buffers allow small amounts of acids and bases to be absorbed without affecting the pH of your blood. Buffers are aqueous solutions containing both a weak acid and a weak base that are a conjugate acid-base pair. Buffer solutions have the ability to resist changes in pH. When small amounts of acid or base are added to a buffer solution, the buffer functions to keep the pH from changing.
The Buffer System at Work How does a buffer resist changes in pH when a strong acid such as hydrochloric acid is added to the solution? Acetic acid (CH3COOH) is a weak acid, and acetate ion (CH3COO-) is a weak base. Together, acetic acid and acetate ions form a conjugate acid-base pair. In solution, this conjugate acid-base pair act as a buffer solution. When small amounts of acid or base are added, buffer solutions are able to resist changes in pH. The conjugate acid-base pair work to protect the pH of the solution by balancing out the hydrogen ions supplied by the additional acid or balancing out the hydroxide ions supplied by the additional base. When an acid such as hydrochloric acid (HCl) is added to a buffer solution, only the hydrogen ions from the acid affect the solution. The remaining ions, in this case chlorine ions, are spectator ions; they do not participate in the reaction. The HCl reacts with the base component of the buffer and produces more of the buffer's acid component. Adding more acid decreases the pH of the solution, if the buffer solution does not remove these new hydrogen ions. The buffer removes the hydrogen ions, and they react with the hydroxide ions of the base. So the pH remains unchanged. When a base is added to a buffer, the opposite occurs. The base reacts with the acid component of the buffer. This removes the new hydroxide ions and prevents the base from raising the pH.
Making Connections
Gases released into the atmosphere from power plants mix with water in the clouds and precipitation to form acid rain. Rain is always a little acidic because it mixes with oxygen compounds in the air such as carbon dioxide. Non-polluted rain has a pH of about 5.6. Any rain that has a pH less than 5.6 is considered acid rain. Acid rain is the name given to rain or any other type of precipitation that is abnormally acidic as a result of air pollution. Acid rain has been measured at values as low as a pH of 2. It forms when gases such as sulfur trioxide and nitrogen dioxide are released into the atmosphere. These gases react with water in the air to form acid rain. Fossil fuels combust to emit one of these gases. Remember that combustion reactions occur when hydrocarbons react with oxygen. Fossil fueled power plants are our major source of electricity. Sulfur released into the atmosphere from fossil fuel power plants reacts with oxygen in the air to form sulfur trioxide. This compound then reacts with water in the air to form sulfuric acid. Which causes acid rain. Nitrogen dioxide is formed as a by-product of combustion reactions in automobile engines. Nitrogen dioxide reacts with water in the air to form nitric acid. Cars and other automobiles are major contributors to this type of acid rain. This statue has been damaged by acid rain. Acid rain is very harmful to the environment. It pollutes water and reduces the amount of freshwater available for consumption. It destroys the soil and kills trees that absorb the acidic water. Acid rain also flows into rivers and lakes, lowering the pH of  the water. When the pH of the water gets too low, it disturbs aquatic life. Many plants, fish, and other species that depend on water are no longer able to survive. Acid rain also damages metals, rocks, and stone. This includes metal buildings, machinery, stone buildings, statues, and automobiles. These materials react with the acids in the rain and begin to corrode. Acid rain causes buildings to decay at a faster rate and can also damage the paint on cars. In this lesson, you learned about acids and bases. You learned about pH as well as acid rain. You also learned about how a buffer works. This is the last of the chemistry lessons. With the next lesson you will begin the physics portion of this course.