chem exam 4 (ch 14)-Acids, Bases, and Their Reactions: Naming, Properties, and the pH Scale

Acids and Bases: Definitions and Properties

Arrhenius Acids and Bases

The Arrhenius definition classifies acids and bases based on their behavior in water. An Arrhenius acid is a substance that produces hydrogen ions (H+) when dissolved in water, while an Arrhenius base produces hydroxide ions (OH−).

• Acids taste sour, can corrode metals, and neutralize bases.

• Bases taste bitter or chalky, feel slippery, and neutralize acids.

Example: Sulfuric acid (H2SO4) dissolves in water to produce H+ ions and sulfate ions (SO42−).

Example: Sodium hydroxide (NaOH) dissolves in water to produce Na+ and OH− ions.

Naming Acids and Bases

Naming Binary Acids

Binary acids consist of hydrogen and a nonmetal. They are named with the prefix hydro- and the suffix -ic acid.

• Formula: H + nonmetal

• Naming: hydro + [root of nonmetal] + ic acid

• Example: HCl is hydrochloric acid; HBr is hydrobromic acid.

Naming Oxyacids

Oxyacids contain hydrogen, oxygen, and another element (usually a nonmetal). Their names are based on the polyatomic ion present:

• If the ion ends in -ate, the acid name ends in -ic acid (e.g., HNO3: nitric acid).

• If the ion ends in -ite, the acid name ends in -ous acid (e.g., HNO2: nitrous acid).

• Prefixes per- and hypo- are used for acids with more or fewer oxygen atoms, respectively (e.g., HClO4: perchloric acid; HClO: hypochlorous acid).

Naming Bases

Most bases are named as hydroxides, consisting of a metal ion and the hydroxide ion (OH−).

• Example: NaOH is sodium hydroxide; Mg(OH)2 is magnesium hydroxide.

Brønsted–Lowry Acids and Bases

Brønsted–Lowry Theory

The Brønsted–Lowry definition expands on Arrhenius by focusing on proton transfer:

• Acid: Proton (H+) donor

• Base: Proton (H+) acceptor

In water, free H+ ions do not exist; instead, they bond to water molecules to form the hydronium ion (H3O+).

Conjugate Acid–Base Pairs

When an acid donates a proton, it forms its conjugate base. When a base accepts a proton, it forms its conjugate acid.

• Example: HF (acid) + H2O (base) ⇌ F− (conjugate base) + H3O+ (conjugate acid)

The pH Scale

Definition and Calculation

The pH scale quantifies the acidity or basicity of a solution. It is defined as the negative logarithm of the hydronium ion concentration:

• $pH = -\log[H_3O^+]$

• pH < 7: acidic; pH = 7: neutral; pH > 7: basic (at 25°C)

Measuring pH

pH can be measured using a pH meter, pH paper, or indicators that change color at specific pH values.

Significant Figures in pH

The number of decimal places in a pH value equals the number of significant figures in the coefficient of [H3O+].

pOH and the Relationship to pH

The pOH scale is related to the hydroxide ion concentration:

• $pOH = -\log[OH^-]$

• At 25°C, $pH + pOH = 14.00$

Reactions of Acids and Bases

Acids with Metals

Acids react with active metals (e.g., K, Na, Ca, Mg, Al, Zn, Fe, Sn) to produce hydrogen gas and a salt. This is a single replacement reaction.

• General equation: $\text{Acid} + \text{Metal} \rightarrow \text{Hydrogen gas} + \text{Salt}$

Acids with Carbonates and Bicarbonates

Acids react with carbonates (CO32−) and bicarbonates (HCO3−) to produce carbon dioxide gas, water, and a salt.

• General equation: $\text{Acid} + \text{Carbonate/Bicarbonate} \rightarrow CO_2(g) + H_2O(l) + \text{Salt}$

Neutralization Reactions

Neutralization is the reaction between an acid and a base to produce water and a salt. One H+ from the acid reacts with one OH− from the base.

• General equation: $\text{Acid} + \text{Base} \rightarrow H_2O(l) + \text{Salt}$

Summary Table: Common Acids, Bases, and Their Properties

Substance	Type	Name	Formula	Properties
HCl	Acid	Hydrochloric acid	HCl	Sour, reacts with metals
H2SO4	Acid	Sulfuric acid	H2SO4	Strong acid, used in industry
NaOH	Base	Sodium hydroxide	NaOH	Slippery, caustic
NH3	Base	Ammonia	NH3	Pungent, cleaning agent

Key Equations

• $pH = -\log[H_3O^+]$

• $pOH = -\log[OH^-]$

• $pH + pOH = 14.00$ (at 25°C)

• $[H_3O^+] = 10^{-pH}$

• $[OH^-] = 10^{-pOH}$

Review and Applications

• Acids and bases are essential in biological, environmental, and industrial processes.

• Understanding naming conventions, reactions, and the pH scale is fundamental for predicting chemical behavior and for laboratory work.