P ROPERTIES OF ACIDS AND BASES

B RAINSTORM In partners answer the following questions: What are some common acids and bases that can be found in your home? What are some properties of acids and bases that you know?

A CIDS AND BASES Acids and bases have an important role in people’s everyday lives. Today, acids are used in the manufacturing of fertilizers, explosives, medicines, plastics, motor vehicles and computer circuit boards. One of Canada’s most important industries, the pulp and paper industry, uses huge quantities of a base called sodium hydroxide. Sodium hydroxide is also used to make soap, detergents, drain cleaner, dyes, medicines and many other products.

C OMMON ACIDS IN THE HOME ProductAcid(s) contained in the product Citrus fruitCitric acid and ascorbic acid Diary productsLactic acid VinegarAcetic acid PopCarbonic acid Underarm odour3-methyl-2-hexenoic acid

C OMMON BASES IN THE HOME ProductBase contained in the product Oven cleanerSodium hydroxide Baking sodaSodium hydrogen carbonate Washing sodaSodium carbonate Glass cleanerAmmonia

P ROPERTIES OF ACIDS Acids: Taste sour Are corrosive Turn blue litmus paper red Are molecular in structure Conduct electricity in aqueous solution React with bases to form salts and water React with certain metals (Zn, Fe, etc.) to produce hydrogen gas pH > 7

P ROPERTIES OF BASES Bases: Taste bitter Feel slippery (Strong bases convert oils in your fingers to soap, hence they feel slippery) React with oils and grease Conduct electricity in aqueous solution Turn red litmus paper blue React with acids to forms salts and water pH < 7

A CIDS AND B ASES Aqueous solutions of acids and bases conduct electricity. This is evidence that ions are present in acidic and basic solutions.

D EFINITIONS OF ACIDS AND BASES

T HE A RRHENIUS T HEORY OF A CIDS AND B ASES When hydrogen chloride dissolves in water it dissociates into hydrogen ions and chloride ions. HCl (aq) H + (aq) + Cl - (aq) When sodium hydroxide dissolves in water, it dissociates to form sodium ions and hydroxide ions. NaOH (aq) Na + (aq) + OH - (aq)

T HE A RRHENIUS T HEORY OF A CIDS AND B ASES The dissociation of other acids and bases in water reveal a pattern. This pattern was first noticed in the late nineteenth century by a Swedish chemist named Svanté Arrhenius. HBr (aq) H + (aq) + Br - (aq) H 2 SO 4(aq) H + (aq) + HSO 4 - (aq) HClO 4(aq) H + (aq) + ClO 4 - (aq) LiOH (aq) Li + (aq) + OH - (aq) KOH (aq) K + (aq) + OH - (aq) Ba(OH) 2(aq) Ba 2+ (aq) + 2OH - (aq)

T HE A RRHENIUS T HEORY OF A CIDS AND B ASES An acid is a substance that dissociates in water to produce one or more hydrogen ions, H + A base is a substance that dissociates in water to form one or more hydroxide ions, OH -

L IMITATIONS OF THE A RRHENIUS T HEORY The Arrhenius Theory is useful if you are interested in the ions that result when an acid or a base dissociates in water. It also helps explain what happens when an acid and a base undergo a neutralization reaction. In such a reaction, an acid combines with a base to form an ionic compound and water. HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O ( l ) Net ionic equation H + (aq) + OH - (aq) H 2 O ( l )

L IMITATIONS OF THE A RRHENIUS T HEORY One problem that arises with this theory involves the H + ion. HCl (aq) H + (aq) + Cl - (aq) This dissociation occurs in aqueous solution, but H 2 O is often left out of the equation. HCl (aq) + H 2 O ( l) H + (aq) + Cl - (aq) + H 2 O ( l) Notice that the water is unchanged when the reaction is represented this way. However, water is a polar molecule. The O atom has a partial negative charge and the H atoms have partial positive charges.

L IMITATIONS OF THE A RRHENIUS T HEORY Thus, H 2 O must interact in some way with the ions H + and Cl -. Chemists realized that protons (the hydrogen ion is simply a proton), do not exist in isolation in aqueous solutions. Protons are always hydrated; they are always attached to water molecules. A hydrated proton is called a hydronium ion, H 3 O +.

L IMITATIONS OF THE A RRHENIUS T HEORY There is another problem with the Arrhenius theory. NH 3(g) + H 2 O ( l ) NH 4 + (aq) + OH - (aq) Ammonia is one of several substances that produce basic solutions in water. The Arrhenius theory cannot explain the basic properties of ammonia.

T HE B RØNSTED -L OWRY T HEORY OF A CIDS AND B ASES In 1923, two chemists working independently of each other proposed a new theory of acids and bases. This theory overcame the limitations of the Arrhenius theory.

T HE B RØNSTED -L OWRY T HEORY OF A CIDS AND B ASES An acid is a substance from which a proton (H + ion) can be removed. A base is a substance that can remove a proton (H + ion) from an acid.

T HE B RØNSTED -L OWRY T HEORY OF A CIDS AND B ASES Like an Arrhenius acid, a Brønsted-Lowry acid must contain a H in its formula. This means that all Arrhenius acids are also Brønsted-Lowry acids. However, any negative ion (not just OH - ) can be a Brønsted-Lowry base. According to the Brønsted-Lowry theory, an acid- base reaction involves the transfer of a proton. (Note that the word proton refers to the nucleus of a hydrogen atom—an H + ion that has been removed from the acid molecule)

T HE B RØNSTED -L OWRY T HEORY OF A CIDS AND B ASES The idea of proton transfer has major implications for understanding the nature of acids and bases. According to the Brønsted-Lowry theory, any substance can behave as an acid, but only if another substance behaves as a base at the same time. Similarly, any substance can behave as a base, but only if another substance behaves as an acid at the same time.

C ONJUGATE A CID -B ASE P AIRS Two molecules or ions that are related by the transfer of a proton are called a conjugate acid- base pair. (Conjugate means “linked together”) The conjugate base of an acid is the particle that remains when a proton is removed from the acid. The conjugate acid of a base is the particle that results when the base receives the proton from the acid.

A MPHOTERIC S UBSTANCES Substances that can act as an acid in one reaction and as a base in a different reaction are said to be amphoteric. For example, the hydrogen carbonate ion is amphoteric. HCO 3 - (aq) + OH - (aq) CO 3 2- (aq) + H 2 O ( l ) HCO 3 - (aq) + H 3 O + (aq) H 2 CO 3(aq) + H 2 O ( l )

C OMPARING THE A RRHENIUS T HEORY AND THE B RØNSTED -L OWRY T HEORY OF A CIDS AND B ASES TheoryArrheniusBrønsted-Lowry Acid Base Example