Lecture 17

Acids & Bases

Acids and Bases in Earth Processes

Acid and base reactions are fundamental to many natural and human-driven processes. They regulate atmospheric CO2 levels, influence soil conditions for agriculture, and affect ocean chemistry.

• Acidic soils are often indicated by red coloration and are common in rainforests, as seen in landslides in El Yunque National Forest, Puerto Rico.

• Soil pH is a master variable for nutrient availability and agricultural productivity.

• Acid-base reactions help oceans act as carbon sinks, but also contribute to risks like ocean acidification.

Acid and Base Theories

Two main theories describe acids and bases: Arrhenius and Brønsted-Lowry.

• Arrhenius Theory:

  • Acid: A substance that ionizes in aqueous solution to produce hydrogen ions (H+).

  • Base: A substance that produces hydroxide ions (OH–) in aqueous solution.

• Brønsted-Lowry Theory:

  • Acid: Proton donor.

  • Base: Proton acceptor.

Common Acids and Bases

Acids and bases are classified as strong or weak depending on their degree of ionization in water.

Name	Formula	Acid Strength	Common Uses/Notes
Sulfuric acid	H2SO4	Strong	Battery acid; ore processing; fertilizer manufacturing; oil refining
Hydrochloric acid	HCl	Strong	Cleanin g metals and bricks; removing scale from boilers
Phosphoric acid	H3PO4	Moderate	Colas; rust removers
Lactic acid	CH3CHOHCOOH	Weak	Yogurt; food additive; lotion additive
Acetic acid	CH3COOH	Weak	Vinegar; food additive
Boric acid	H3BO3	Very weak	Antiseptic eyewash; roach poison
Hydrocyanic acid	HCN	Very weak	Plastics manufacture; extremely toxic

Name	Formula	Classification	Common Uses/Notes
Sodium hydroxide	NaOH	Strong	Acid neutralization; soap making
Potassium hydroxide	KOH	Strong	Liquid soap making; biodiesel fuels
Lithium hydroxide	LiOH	Strong	Alkaline storage batteries
Calcium hydroxide	Ca(OH)2	Strong	Plaster; cement; soil neutralizer
Magnesium hydroxide	Mg(OH)2	Strong	Antacid; laxative
Ammonia	NH3	Weak	Household cleaner; fertilizer

Acid-Base Reactions

Neutralization

When an acid reacts with a base, their properties are neutralized, producing water and a salt. This process is called neutralization.

• General equation: $\text{Acid} + \text{Base} \rightarrow \text{Water} + \text{Salt}$

• Example: $\text{NaOH} (aq) + \text{HCl} (aq) \rightarrow \text{NaCl} (aq) + \text{H}_2\text{O} (l)$

Neutralization reactions are used in titration to determine the concentration of acids or bases in solution.

Limitations of Arrhenius Theory

• H+ ions do not exist freely in water; they form hydronium ions (H3O+).

• Arrhenius theory does not explain the basicity of ammonia and similar compounds.

• It applies only to aqueous solutions.

Brønsted-Lowry Theory

The Brønsted-Lowry theory expands the definition of acids and bases:

• Acid: Proton donor

• Base: Proton acceptor

• Example: $\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$ (when H2O is the base it recieves another proton)

• Example: $\text{NH}_3 + \text{H}_2\text{O} \rightarrow \text{NH}_4^+ + \text{OH}^-$(when H2O is the acid, it forms OH)

Salts and Electrolytes

Formation and Properties of Salts

Salts are ionic compounds formed from the neutralization of acids and bases. They consist of cations other than hydrogen and anions other than hydroxide.

• Examples: Epsom salts (MgSO4), NaCl, KCl, ZnSO4

• Salts are concentrated in deserts by evaporation.

Electrolytes

Electrolytes are salts that conduct electricity when dissolved in water. Electrical conductivity measurements are used to determine salinity in water or soil.

• High sodium soils may have elevated salts but are not always saline.

• Salt marsh soils can have very high salinity.

Strong and Weak Acids and Bases

Ionization in Water

Acids and bases are classified as strong or weak based on their ionization in water.

• Strong acids: Ionize completely in water (e.g., HCl).

• Weak acids: Ionize only partially (e.g., HCN).

• Strong bases: Dissociate completely (e.g., NaOH).

• Weak bases: Ionize only partially (e.g., NH3).

The pH Scale

Definition and Calculation

The pH scale expresses the acidity or basicity of a solution. It is defined as:

• pH: $\text{pH} = -\log[\text{H}^+]$

• pH ranges from 0 (very acidic) to 14 (very basic).

Solution	pH
Hydrochloric acid (4%)	0
Gastric juice	1.6–1.8
Soft drink	2.0–4.0
Lemon juice	2.1
Vinegar (4%)	2.5
Urine	5.5–7.0
Rainwater	5.6
Saliva	6.2–7.4
Milk	6.3–6.6
Pure water	7.0
Blood	7.4
Fresh egg white	7.6–8.0
Bile	7.8–8.6
Milk of magnesia	10.5
Washing soda	12.0
Sodium hydroxide (4%)	13.0

Carbonic Acid and Global Processes

Carbonation Reactions

Carbon dioxide dissolves in water to form carbonic acid, which plays a key role in global carbon cycling and soil pH regulation.

• CO2 (g) → CO2 (aq)

• CO2 (aq) + H2O → H2CO3 (aq)

• Silicate rocks react with carbonic acid to form carbonate/bicarbonate and cations, contributing to global CO2 consumption.

• Soil pH is lower near the surface due to biological activity and CO2 respiration from roots and microbes.

Summary

• Acids and bases are defined by two main theories: Arrhenius and Brønsted-Lowry.

• Neutralization reactions produce water and salts.

• The pH scale quantifies acidity and basicity.

• Acid-base chemistry is central to environmental and biological processes.