Chapter 2: Acids, Bases, and Salts
Introduction to Acids, Bases, and Salts
Acids
Acids are chemical substances that exhibit distinct properties in aqueous solutions. They have a sour taste and can turn blue litmus paper red. One of the fundamental properties of acids is their ability to neutralize bases, forming water and salts as a result. This neutralization reaction is crucial in various chemical processes. Acids are often identified by their ability to release hydrogen ions (H⁺) in solution, which is a key characteristic of their chemical behavior.
Examples of common acids include:
- Hydrochloric Acid (HCl): Found in gastric juice and used in various industrial processes.
- Sulphuric Acid (H₂SO₄): Widely used in the manufacture of fertilizers, batteries, and cleaning agents.
- Nitric Acid (HNO₃): Used in the production of explosives, fertilizers, and in metal processing.
Bases
Bases are substances that, when dissolved in water, yield solutions that taste bitter and turn red litmus paper blue. Bases can neutralize acids, resulting in the formation of water and salts. Aqueous solutions of bases are often slippery to the touch due to their saponification of oils on the skin. Bases are characterized by the presence of hydroxide ions (OH⁻) in solution.
Common examples of bases include:
- Sodium Hydroxide (NaOH): Known as caustic soda, used in soap making, paper production, and as a drain cleaner.
- Potassium Hydroxide (KOH): Used in the manufacture of soaps, batteries, and as an electrolyte.
- Calcium Hydroxide (Ca(OH)₂): Also called slaked lime, used in the construction industry, water treatment, and in the preparation of ammonia.
Salts
Salts are neutral substances formed from the reaction of acids and bases. In an aqueous solution, salts typically have no effect on litmus paper, indicating their neutral pH. Salts are composed of the cations from the base and the anions from the acid, resulting in a wide variety of compounds with diverse properties and uses.
Classification of Matter
Composition
- Elements: Pure substances consisting of only one type of atom. Examples include hydrogen, oxygen, and gold.
- Compounds: Substances composed of two or more different types of atoms chemically bonded together. Examples include water (H₂O) and carbon dioxide (CO₂).
- Mixtures: Physical combinations of two or more substances that retain their individual properties. Examples include air, saltwater, and soil.
State
- Solids: Matter with a definite shape and volume. Examples include ice, iron, and diamond.
- Liquids: Matter with a definite volume but no definite shape, taking the shape of its container. Examples include water, oil, and alcohol.
- Gases: Matter without a definite shape or volume, filling the entire space available. Examples include oxygen, nitrogen, and carbon dioxide.
Solubility
- Suspensions: Mixtures in which particles are dispersed throughout but will settle over time. Examples include muddy water and flour in water.
- Colloids: Mixtures with particles that are larger than those in solutions but do not settle out. Examples include milk, mayonnaise, and gelatin.
- Solutions: Homogeneous mixtures with particles at the molecular level, remaining evenly distributed. Examples include saltwater, sugar in water, and vinegar.
Types of Mixtures
- Homogeneous Mixtures: Mixtures that are uniform throughout, with no visible separation of components. Examples include air and saline solution.
- Heterogeneous Mixtures: Mixtures with visibly different components and non-uniform composition. Examples include sand and water, and salad.
Types of Compounds
- Covalent Compounds: Compounds formed by the sharing of electrons between atoms. Examples include methane (CH₄) and carbon dioxide (CO₂).
- Ionic Compounds: Compounds formed by the transfer of electrons from one atom to another, resulting in positive and negative ions. Examples include sodium chloride (NaCl) and magnesium oxide (MgO).
What Is an Acid and a Base?
Compounds can be ionisable or non-ionisable based on their ability to dissociate into ions.
- Ionisable Compounds: These dissociate into ions almost entirely in water or molten state. Examples include sodium chloride (NaCl), hydrochloric acid (HCl), and potassium hydroxide (KOH).
- Non-Ionisable Compounds: These do not dissociate into ions in water or molten state. Examples include glucose and acetone.
Arrhenius’ Theory of Acids and Bases
- Arrhenius Acid: A substance that dissociates in water to give hydrogen ions (H⁺) or hydronium ions (H₃O⁺).
- Arrhenius Base: A substance that dissociates in water to give hydroxide ions (OH⁻).
Bronsted-Lowry Theory
- Bronsted Acid: A substance that donates a proton (H⁺).
- Bronsted Base: A substance that accepts a proton (H⁺).
For example, in the reaction: HCl (aq) + NH₃ (aq) → NH₄⁺ (aq) + Cl⁻ (aq)
- HCl acts as a Bronsted acid (H⁺ donor).
- Cl⁻ is the conjugate base of HCl.
- NH₃ acts as a Bronsted base (H⁺ acceptor).
- NH₄⁺ is the conjugate acid of NH₃.
Physical Tests for Acids and Bases
- Taste
- Acids: Sour (Note: Tasting chemicals is not advisable due to potential toxicity and corrosiveness).
- Bases: Bitter (Again, tasting is not recommended for safety reasons).
- Effect on Indicators
- Litmus Paper:
- Acids turn blue litmus paper red.
- Bases turn red litmus paper blue.
- Phenolphthalein:
- Colorless in acidic solutions.
- Pink in basic solutions.
- Methyl Orange:
- Red in acidic solutions.
- Yellow in basic solutions.
Chemical Properties of Acids
Neutralization Reaction
A neutralization reaction occurs when an acid reacts with a base to produce salt and water. This reaction is fundamental in chemistry and is used in various applications, including titrations to determine concentrations of solutions and in everyday products like antacids.
Example: HCl + NaOH → NaCl + H2O
In this reaction, hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH) to produce sodium chloride (NaCl) and water (H2O).
Reaction with Metals
Acids react with certain metals to produce hydrogen gas and a corresponding salt.
Example: The reaction between hydrochloric acid and zinc:
Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂ (g)
Reaction with Metal Carbonates
Acids react with metal carbonates to produce a salt, carbon dioxide, and water.
Example: The reaction between calcium carbonate and hydrochloric acid:
CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + CO₂ (g) + H₂O (l)
Reaction with Metal Oxides
Acids react with metal oxides to produce a salt and water.
Example: The reaction between hydrochloric acid and calcium oxide:
CaO (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l)
Chemical Properties of Bases
Reaction with Acids
Bases react with acids to produce a salt and water. This reaction is called neutralization.
Example: The reaction between sodium hydroxide and hydrochloric acid:
NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O (l)
Reaction with Non-Metal Oxides
Bases react with non-metal oxides to produce a salt and water.
Example: The reaction between calcium hydroxide and carbon dioxide:
Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O (l)
Importance of pH in Everyday Life
The pH scale is a measure of the acidity or basicity of a solution, ranging from 0 to 14. A pH of 7 is considered neutral, while a pH less than 7 indicates acidity and a pH greater than 7 indicates basicity.
- pH Sensitivity of Plants and Animals: Living organisms require specific pH levels for optimal functioning. Enzyme activities, metabolic processes, and overall health are pH-dependent.
- pH of Soil: Soil pH affects nutrient availability and plant growth. Most plants thrive in soil with a pH between 6.5 and 7.0.
- Digestive System: The stomach has a highly acidic environment with a pH between 1.5 and 4, essential for digesting food and killing harmful bacteria.
- Tooth Decay: Tooth enamel begins to demineralize at pH levels below 5.5, leading to tooth decay. Consuming acidic foods and beverages can lower the pH in the mouth, promoting decay.
- Self-Defense Mechanisms: Some animals and plants secrete acidic or basic substances as a defense mechanism against predators. For example, ants release formic acid, and some plants secrete toxic alkaloids.
Manufacture of Acids and Bases
Acids
Acids can be synthesized by reacting non-metal oxides with water.
Example: Sulfur trioxide reacts with water to produce sulfuric acid:
SO₃ + H₂O → H₂SO₄
Bases
Bases can be produced by reacting metal oxides with water or by reacting ammonia with water.
Example: Sodium oxide reacts with water to produce sodium hydroxide:
Na₂O + H₂O → 2NaOH
Example: Ammonia reacts with water to produce ammonium hydroxide:
NH₃ + H₂O → NH₄OH
Salts
Formation and Examples
Salts are formed by the combination of the anion of an acid and the cation of a base. Common examples include potassium chloride (KCl), sodium nitrate (NaNO₃), and calcium sulfate (CaSO₄).
Common Salt
Sodium chloride (NaCl), or common salt, is widely used in cooking and food preservation. It is also a raw material for producing various chemicals.
Family of Salts
Salts with the same cation or anion are considered to belong to the same family. For example, sodium chloride (NaCl), potassium chloride (KCl), and lithium chloride (LiCl) all belong to the chloride family.
pH of Salts
- Strong acid + strong base: Neutral salt (pH ≈ 7)
- Weak acid + strong base: Basic salt (pH > 7)
- Strong acid + weak base: Acidic salt (pH < 7)
Chemicals from Common Salt
Sodium Hydroxide (NaOH)
Sodium hydroxide, commonly known as caustic soda or lye, is a versatile chemical compound with significant industrial applications. It is produced through the Chlor-alkali process, a fundamental industrial method for generating sodium hydroxide, chlorine gas, and hydrogen gas simultaneously from saltwater (brine).
Production Process (Chlor-alkali Process):
- Electrolysis of Brine: Saltwater solution (brine), primarily composed of sodium chloride (NaCl), undergoes electrolysis.
- Electrochemical Reactions:
- Anode (Positive Electrode): Chlorine gas (Cl₂) is produced.
- Cathode (Negative Electrode): Hydrogen gas (H₂) is released.
- Solution: Sodium hydroxide (NaOH) is left in the electrolyte solution.
- Chemical Equation: 2NaCl + 2H₂O → Cl₂ + H₂ + 2NaOH
Uses:
- Chemical Manufacturing: Essential in the production of various chemicals, including detergents, soaps, and pharmaceuticals.
- Pulp and Paper Industry: Used in paper manufacturing processes, particularly in pulping and bleaching.
- Textile Industry: Employed in textile processing, such as mercerization and dyeing.
- Aluminum Production: Used in alumina production and as a pH regulator in electrolytic baths.
- Cleaning Agents: Found in household and industrial cleaning products due to its strong alkaline nature.
- Water Treatment: Used in water treatment facilities for pH adjustment and chemical neutralization.
Sodium hydroxide’s diverse applications underscore its importance across multiple industries, making it a cornerstone of chemical manufacturing and industrial processes worldwide.
Bleaching Powder (Ca(OCl)Cl)
Bleaching Powder, chemically known as calcium oxychloride (Ca(OCl)Cl), is produced by reacting calcium hydroxide (Ca(OH)₂) with chlorine gas (Cl₂). It serves multiple essential roles in various industries and household applications due to its powerful oxidizing and disinfectant properties.
Preparation:
- Chemical Reaction: Ca(OH)₂ + Cl₂ → Ca(OCl)Cl + H₂O
Uses:
- Bleaching Agent: Used extensively in the textile and paper industries to bleach and whiten materials.
- Disinfectant: Effective in sterilizing drinking water and surfaces due to its ability to release chlorine.
- Oxidizing Agent: Utilized in chemical processes where oxidation reactions are required.
- Household Cleaner: Used for household cleaning tasks where disinfection and whitening are necessary.
Bleaching powder’s versatility and effectiveness make it a crucial component in various industrial processes and essential for maintaining hygiene and cleanliness in everyday settings.
Baking Soda (NaHCO₃)
Produced through the Solvay process, where sodium chloride reacts with ammonia and carbon dioxide in water.
Preparation (Solvay Process):
- Chemical Equation: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl
Uses:
- Reduces stomach acidity, antacid, water softener.
Washing Soda (Na₂CO₃·10H₂O)
Washing soda, chemically represented as sodium carbonate decahydrate (Na₂CO₃·10H₂O), is produced through the Solvay process and finds versatile applications across several industries and domestic settings.
Preparation (Solvay Process):
- Raw Materials: Sodium chloride (NaCl, common salt), ammonia (NH₃), carbon dioxide (CO₂), and water (H₂O) are used.
- Chemical Reaction: These ingredients react in a series of steps to produce sodium carbonate (Na₂CO₃) and ammonium chloride (NH₄Cl).
- NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl
- NaHCO₃ + NaOH → Na₂CO₃ + H₂O
Uses:
- Glass Industry: Used as a flux in glass production to lower the melting point of silica.
- Soap Industry: Essential in soap manufacturing processes, aiding in cleaning and emulsifying properties.
- Paper Industry: Employed in paper production to improve the efficiency of pulp bleaching.
- Water Softening: Effective in removing hardness ions (calcium and magnesium) from water, preventing scale formation.
- Domestic Cleaning: Acts as a cleaner and deodorizer in various household cleaning applications.
Washing soda’s broad utility underscores its significance in industrial processes and everyday cleaning tasks, making it a versatile chemical compound with numerous practical applications.
Crystals of Salts
Water of crystallization refers to the water molecules that are integral to the crystal structure of certain salts, defining their crystalline form. Examples of substances that exhibit water of crystallization include table salt (NaCl), sugar, and snowflakes.
- Table Salt (NaCl): Commonly known as sodium chloride, table salt exists in cubic crystals. Water of crystallization is not present in NaCl crystals, as it does not incorporate water molecules into its crystal lattice.
- Sugar: Various types of sugar, such as sucrose (table sugar), form crystals that include water of crystallization. The water molecules are incorporated within the crystal structure, contributing to the overall stability and appearance of sugar crystals.
- Snowflakes: Snowflakes are intricate ice crystals that form when water vapor condenses and freezes in the atmosphere. The unique and delicate structure of each snowflake is influenced by temperature and humidity conditions during formation.
Understanding water of crystallization enhances our appreciation of the diverse forms and properties of crystals in nature and industry.
Plaster of Paris (CaSO₄·½H₂O)
It is produced by heating gypsum (CaSO₄·2H₂O) at 100°C to remove water molecules. It is used in casts for healing fractures, sculpting, and as a gauze bandage material.
Plaster of Paris, chemically represented as CaSO₄·½H₂O, is derived from gypsum (CaSO₄·2H₂O) through a controlled heating process. Here’s an elaboration on its preparation and uses:
- Chemical Formula: CaSO₄·½H₂O
- Preparation:
- Gypsum (CaSO₄·2H₂O) heated at 100°C → CaSO₄·½H₂O
- Heating Gypsum: Gypsum (CaSO₄·2H₂O) is heated to approximately 100°C.
- Water Removal: During heating, gypsum loses water molecules, transforming into Plaster of Paris (CaSO₄·½H₂O).
Uses:
- Medical Applications: Used in casts for healing fractures due to its ability to set into a hard, stable form that supports injured limbs.
- Artistic and Craft Uses: Widely utilized in sculpting and artistic endeavors due to its moldability and ability to capture fine details.
- Industrial and Household Uses: Commonly employed in the production of molds, as a component in pottery and ceramics, and in the creation of decorative elements.
Plaster of Paris plays a crucial role in various fields, offering versatility in both medical and creative applications.
Chapter 1: Chemical Reactions and Equations Notes
