3: Aqueous Solutions & Reactions

Lecture Announcements

• Lecture 3 covered general properties of aqueous solutions, precipitation reactions, acids, bases, neutralization reactions, and oxidation-reduction reactions, as well as concentrations.
• Problem Set 2 was due before Exercise #3 and uploaded to Moodle.
• Problem Set 3 was posted today on Moodle and is due before Exercise 4, next week.
• The study center started yesterday, Wednesdays 6:00 PM to 8:00 PM in ETA F5.
• Office hours are Thursdays 5:00 PM to 6:00 PM in LEE P 210.
• The lecture introduced several concepts that were covered in depth later. It also contained information on calcium deposits ("Kalk") and their removal.

Lecture 4

• The next week's lecture will cover the nature of chemical energy, the first law of thermodynamics, enthalpy, enthalpies of reaction, calorimetry, Hess's Law, and enthalpies of formation.
• The equation ΔE = q + w was presented.

Review

• Lecture 2 discussed gases, variables needed to describe gases (P, V, T, n), pressure, barometers, manometers, gas laws (Boyle's, Charles's, and Avogadro's laws), ideal gas law (PV = nRT), standard temperature and pressure (STP), partial pressures, mole fraction, kinetic-molecular theory of gases, velocities and velocity distributions, molecular diffusion, collisions, and mean free path.

Nonidealities in Gases

• The lecture discussed non-ideal gas behavior, demonstrating how real gases deviate from ideal gas behavior as pressure increases.
• The Van der Waals equation (P + a(n²/V²))(V - nb) = nRT was introduced, with constants (a and b) for different gas molecules provided in a table.

Reactions in Aqueous Solutions

• Solutions are homogeneous mixtures of two or more substances.
• A solution consists of a solvent (the substance present in the greatest amount) and solutes (substances dissolved in the solvent).
• Aqueous solutions have water (H₂O) as the solvent.

Electrolytes

• Electrolytes are substances that dissolve in water (H₂O) to form ions.
• An example is solid NaCl, which dissociates into individual cations (Na⁺) and anions (Cl⁻).
• Aqueous solutions of electrolytes conduct electricity.

Why is H₂O a Good Solvent for Ions?

• Water's polarity plays a role in dissolving ionic compounds.
• Oxygen in water has a partial negative charge, while hydrogen has a partial positive charge.
• Water molecules orient themselves around ions to stabilize their charges.

Nonelectrolytes

• Nonelectrolytes dissolve in water (H₂O) without forming ions.
• An example is sucrose (table sugar).
• Sucrose molecules dissolve individually in water.

Why Do Substances Dissolve in H₂O?

• Dissolution involves both energetic and entropic factors.
• Thermodynamics is crucial in determining which substances dissolve.

Precipitation Reactions

• Precipitation reactions form an insoluble solid (precipitate) when two solutions are mixed.
• Examples include reactions of silver nitrate (AgNO₃) and potassium chloride (KCl) to form silver chloride (AgCl).
• Other reactions involve the creation of soap scum.

Complete Ionic vs. Net Ionic Equations

• Complete ionic equations show all ions present in a reaction.
• Net ionic equations only show ions that participate in the reaction.

Solubility of Common Ionic Compounds in H₂O

• A table shows soluble and insoluble ionic compounds in water, with important exceptions.

Acids

• Acids are substances that ionize in water to produce hydrogen cations (H⁺).
• Examples include hydrochloric acid (HCl), nitric acid (HNO₃), and sulfuric acid (H₂SO₄), categorized as monoprotic or diprotic.

Bases

• Bases accept hydrogen cations (H⁺) and produce hydroxide ions (OH⁻) when in water.
• Examples include sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)₂), and ammonia (NH₃).

Strength of Acids and Bases

• The strength of acids and bases depends on the quantity of H⁺ and OH⁻ ions.
• pH quantifies acidity and basicity. Acid reactivity depends not just on the H⁺ amount but on the anion as well.

Neutralization Reactions

• Neutralization reactions occur when an acid and a base react, combining H⁺ and OH⁻ to form water.
• The product is typically water, along with a salt.

Cleaning Limescale

• Limescale (CaCO₃) is typically removed through reactions with acids such as acetic acid.
• Precipitation and neutralization reactions are involved in removing limescale.

Oxidation-Reduction Reactions (Redox Reactions)

• In redox reactions, one substance loses electrons (oxidation) while another gains electrons (reduction). A key example is corrosion.
• Rust, tarnishing are common examples.
• Redox reactions are critical in batteries, corrosion, fuel cells, and electroplating.

Oxidation and Reduction

• Oxidation happens when an atom, ion, or molecule loses electrons, becoming more positively charged.
• Reduction happens when an atom, ion, or molecule gains electrons.

Oxidation Numbers

• Oxidation numbers (or oxidation states) are used to track electrons in redox reactions.
• Rules for assigning oxidation numbers are presented.

Activity Series

• Activity series lists metals in order of their ease of oxidation.
• Metals higher in the series are more easily oxidized than those lower in the series.

Using Activity Series

• The activity series aids in predicting if a reaction will occur between metals.

Concentration

• Molarity (M) is a common way to express the concentration of a solute in a solution.

What We Learned Summary

• Key concepts learned in the lecture are summarized in a list format.