Acids and Bases

In this chapter, you will learn about the properties of acids and bases. You will learn how these properties change when acids and bases react together. You will also have a chance to estimate and measure the acidity of aqueous solutions.

Acids and bases are common products in the home. Sometimes it is easy to identify some products as acids. Often the word “acid” appears in the list of ingredients. Identifying bases is a bit more difficult. Acids and bases have different properties that enable you to distinguish between them.

Acids Product Acids in the Product Citrus fruits (lemons, limes, oranges, tomatoes) Citric acid, ascorbic acid Dairy products (cheese, milk, yogurt) Lactic acid Vinegar Acetic acid Soft drinks Carbonic acid, and sometimes phosphoric and citric acids

Bases Product Bases in the Product Oven cleaner Sodium hydroxide Baking soda Sodium hydrogen carbonate (sodium bicarbonate) Washing soda Sodium carbonate Glass cleaner ammonia

Properties of Acids Acids Taste sour No characteristic feel Conducts electricity Reacts with metals to produce hydrogen gas Reacts with carbonated compounds to produce carbon dioxide gas Change color when mixed with colored dyes called indicators (turns blue litmus paper red)

Properties of Bases Bases Taste bitter Feel slippery Conducts electricity Does not react with metals Does not react with carbonated compounds Also change color when mixed with indicators (turns red litmus paper blue)

You should never taste or touch acids, bases or any other chemical. Early chemists used their senses of taste and touch to observe the properties of many chemicals. This dangerous practice often lead to serious injury, and sometimes death.

The Arrhenius Theory In 1887, Arrhenius published a theory to explain the nature of acids and bases. In the theory of acids and bases, he states that An acid is a substance that dissociates in water to produce one or more hydrogen ions (H+) HCl (aq)  H+(aq) + Cl-(aq) HClO4 (aq)  H+(aq) + ClO4-(aq) A base is a substance that dissociates in water to form one or more hydroxide ions (OH-) LiOH (aq)  Li+(aq) + OH-(aq) Ba(OH)2 (aq)  Ba2+(aq) + 2 OH-(aq)

According to the Arrhenius theory, acids increase the concentration of H+ ions in aqueous solutions. An Arrhenius acid must contain hydrogen. Bases, on the other hand, increase the concentration of OH- in aqueous solutions. An Arrhenius base must contain the hydroxide ion.

The Arrhenius theory is useful if you are interested in the ions that result when and acid or a base dissociate in water. It also helps explain what happens when an acid and a base undergo a neutralization reaction. HCl(aq) + NaOH_(aq)  NaCl(aq) + H2O(l)

The net ionic reaction is In a neutralization reaction, an acid combines with a base to form an ionic compound and water. The net ionic reaction is H+(aq) + OH-(aq)  H2O(l)

Limitations to the Arrhenius Theory There are problems with the theory. One problem involves the ion that is responsible for acidity (H+). Look at the dissociation of HCl. HCl(aq)  H+(aq) + Cl-(aq) The dissociation happens in an aqueous solution, but chemists often leave out the H2O component of the reaction. They simply assume it is there. But what is you put it in? HCl(aq) + H2O(l)  H+(aq) + Cl-(aq) + H2O(l)

The water is unchanged when the reaction is represented this way, but you know that water is a polar molecule. The O atom has a partial negative charge and the H atoms have a partial positive charge. The H2O interacts some way with the ion H+ and Cl- ions.

Chemists realized that protons do not exist in isolation in aqueous solution (the hydrogen ion is simply a proton, a positively charged nuclear particle). Instead protons are always hydrates (attached to water) and the result is a hydronium ion, H3O(aq).

Another Problem with Arrhenius’ Theory Consider the reaction of ammonia with water. NH3 (g) + H2O(l)  NH4+(aq) + OH-(aq) Ammonia is one of several substances that produce basic solutions in water.

Ammonia does not contain the hydroxide ion, however it does produce these ions when it reacts with water. Arrhenius’ theory does not explain the basic properties of ammonia.

Arrhenius’ Theory only explains acid-base reactions in water Arrhenius’ Theory only explains acid-base reactions in water. Many acid-base reactions take place in other solvent.

Brønsted-Lowry Theory In 1923, two chemists proposed a new theory of acids and bases. This theory overcame the problems related to the Arrhenius theory. A B-L acid is a substance from which a protons (H+) can be removed. A B-L base is a substance that can remove a proton from an acid.

Like the Arrhenius theory, a Brønsted-Lowry acid must contain H in its formula. So basically all Arrhenius acids are B-L acids. However, any negative ion (not just OH-) can be a B-L base. Also note that water is not the only solvent that can be used. According to B-L, there is only one requirement for an acid-base reaction. One substance must provide a proton and the other substance must receive the same proton. Basically, it is a transfer of a proton.

Look at the following reaction: proton transfer According to B-L, any substance can behave as an acid, but only if another substance behaves as a base at the same time. The same is true of the reverse, any substance can behave as a base, but only if another substance behaves as an acid at the same time. Look at the following reaction: proton transfer HCl(aq) + H2O(l)  H3O(aq) + Cl-(aq) acid base HCl is an acid because it provides a proton. The water is the base because it receives the proton.

Two molecules or ions that are related by the transfer of a proton are called a conjugate acid-base pair. Conjugate means that they are linked together. The conjugate base of an acid is the particle that remains when a protons is removed from the acid. The conjugate acid of a base is the particle that results when the base receives the proton.

HCl(aq) + H2O(l)  H3O(aq) + Cl-(aq) conjugate acid conjugate base Every acid has a conjugate base and every base has a conjugate acid. The conjugate base of an acid-base pair has one less hydrogen than the acid The conjugate acid of an acid-base pair has one more hydrogen than the base.

Conjugate Acid-Base Pair Example Hydrogen Bromide is a gas at room temperature. It is soluble in water, forming hydrobromic acid (HBr). Write the equation, determine the products, and then identify the acid, base, as well as the conjugate acid and conjugate base. HBr(g) + H2O(l)  H3O(aq) + Br-(aq) Acid Base Conjugate Conjugate acid base

Another Example Ammonia is a pungent gas at room temperature. Its main use is in the production of fertilizers and explosives. It is very soluble is water. It forms a basic solution that is used in common products, such as glass cleaners. Identify the acid, the base, the conjugate acid, the conjugate base, and the conjugate acid base pairs in the following reaction. NH3 (g) + H2O(l)  NH4+(aq) + OH-(aq)

NH3 (g) + H2O(l)  NH4+(aq) + OH-(aq) Base Acid Conjugate Conjugate acid base

Practice The following are considered bases. Write the formula’s for their conjugate acids a) F_ b) NH3 c) HSO4- d) CrO42- Answers: a) HF b) NH4+ c) H2SO4 d) HCrO4-

Practice The following are considered acids. Write the formula’s for their conjugate bases a) HClO3 b) HSO3 c) H2O d) HCO32- Answers: a) ClO3- b) SO3- c) OH- d) CO33-

Practice Identify the conjugate acid-base pairs in the following reactions: a) NH4+ (aq) + CO32- (aq)  NH3 (aq) + HCO3- (aq) b) H2O(l) + HS- (aq)  H3O+ (aq) + S2- (aq) C. acid C. base Acid Base C. base Base Acid C. acid

Lewis Theory Lewis Acid Lewis Base A substance that is capable of accepting electrons and are considered to be electron deficient ie. Positive ions, substances with less than a full octet, those with expandable valence shells A substance that is capable of donating electrons and are considered electron rich ie. Negative ions, substances with unshared electron pairs, substances with double bonds.

Lewis theory is ‘all-embracing’, so the term Lewis acid usually is reserved for substances that are not also Bronsted-Lowry acids. Many of these do not even contain hydrogen. BF3 + F− → BF4− Al2Cl6 + 2Cl− → 2AlCl4− AlF3 + 3F− → AlF63−

The lewis theory also covers substances like transition metals which do not contain protons, yet are capable of accepting electron pairs from ligands (neutral molecules with non-bonding pairs of electons ‘electron rich’)