Chemical Reactions and Aqueous Solutions: Compounds in Aqueous Solution Notes

Describe and write chemical equations representing the dissociation of ionic compounds upon dissolution in water.
Identify compounds as being strong electrolytes, weak electrolytes, or nonelectrolytes based on their chemical formulas.

Solubility Definitions:
- Soluble: A solid compound that readily dissolves when mixed with water is described as being soluble.
- Insoluble: A compound that remains in the solid state when mixed with water is said to be insoluble.
Chemical Notation:
- The notation $NaCl(aq)$ indicates that sodium chloride has been mixed with water and has successfully dissolved.
- In the context of a reaction involving aqueous substances, a notation such as $AgCl(s)$ indicates that silver chloride has not dissolved, even though it is mixed with water.

Reactive Dynamics: Many chemical reactions occur in aqueous solutions that would not occur if the same reactants were simply mixed together as solids.
Reasons for Increased Reactivity in Solution:
1. Mobility: Dissolved compounds possess significantly more mobility than they do in a solid state. This increased movement makes it much more likely for reactant particles to collide or "bump into each other."
2. Dissociation Behavior: This relates specifically to how ionic compounds behave at the molecular level when they interact with water.
Process of Dissociation:
- Ionic compounds dissociate into individual ions and disperse among the water molecules.
- Example: Sodium Chloride ( $NaCl$ ): When $NaCl$ dissolves, it forms separate $Na^{+}$ and $Cl^{-}$ ions. These ions are surrounded by water molecules and are specifically referred to as hydrated ions.
- Dissociation Equation: $NaCl(s) → NaCl(aq) → Na^{+}(aq) + Cl^{-}(aq)$
- The use of the notation $(aq)$ after the formula of an ionic compound explicitly indicates that the compound is present in the form of hydrated ions.
Structural Changes:
- In its solid state, an ionic compound exists as an ionic lattice ( $± ± ±$ structure).
- Upon dissolution, this lattice breaks down into individual hydrated ions dispersed in the solvent.

Definition of Electrolytes: Substances that conduct electricity when dissolved in water are known as electrolytes.
Conductivity Mechanism: Aqueous solutions of ionic compounds conduct electricity because of the presence of mobile hydrated ions.
Strong Electrolytes: Ionic compounds are classified as strong electrolytes because they undergo $100\%$ dissociation, producing solutions that conduct electricity readily.
The Mobility of Ions (Figure 4.15):
- For a substance to be written in the form of separate ions, it must be both soluble and ionic.
- In a solution of ions, the positive and negative particles are dispersed.
- When charged electrodes are introduced (positive and negative electrodes), the ions move toward the electrode of the opposite charge.
- In an ionic solid, ions are fixed in place and cannot carry a current effectively.

Nonelectrolytes: Most molecular compounds that dissolve in water form solutions that do not conduct electricity. These are termed nonelectrolytes because they dissolve as whole molecules rather than breaking into ions.
Acids as Molecular Exceptions: Acids are molecular compounds that undergo ionization when dissolved in water to produce ions, which allows the solution to conduct electricity.
- Ionization Example (Hydrochloric Acid): $HCl(aq) → H^{+}(aq) + Cl^{-}(aq)$
Strong Acids: These compounds ionize $100\%$ in water and are classified as strong electrolytes.

There are seven common strong acids that ionize completely in aqueous solution:

Weak Acids:
- Definition: Any acid that is not one of the seven strong acids listed above is considered a weak acid.
- Behavior: Weak acids ionize only partially in water.
- Composition in Solution: They dissolve mainly as whole molecules with only a very small percentage of ions present.
- Electrolytic Property: They form solutions that conduct electricity only slightly and are classified as weak electrolytes.
Bases:
- Strong Bases: These are ionic compounds containing hydroxide ions ( $OH^{-}$ ). They dissociate $100\%$ in water and are strong electrolytes.
- Weak Bases: These are molecular compounds (such as ammonia). They react with water to a small extent to produce hydroxide ions. They are classified as weak electrolytes.

Task: Classify the following water-soluble substances as strong electrolytes, weak electrolytes, or nonelectrolytes in aqueous solution.

$CaCl_2$ : Strong electrolyte (It is an ionic salt).
$C_6H_12O_6$ : Nonelectrolyte (It is a molecular compound/sugar).
$HNO_2$ : Weak electrolyte (It is a weak acid; it is not on the list of 7 strong acids).
$NH_3$ (weak base): Weak electrolyte (Molecular compound that reacts slightly with water).
$KOH$ : Strong electrolyte (It is a strong base/ionic compound containing hydroxide).
$HBr$ : Strong electrolyte (It is one of the 7 strong acids).

Ionic Compounds: Dissociate $100\%$ in water to produce hydrated ions.
Strong Bases: Ionic compounds containing the hydroxide ion that dissociate $100\%$ .
Molecular Compounds:
- Soluble molecular compounds may or may not ionize.
- Strong acids ionize $100\%$ .
- Weak acids and weak bases dissolve mainly as molecules but ionize to a very small extent.
- Other molecular compounds (like sugars) do not ionize or dissociate at all.
Electrolyte Solutions: Solutions containing dissociated ions that can conduct electricity.
Categorization Recap:
- Strong Electrolytes: Ionic compounds and strong acids.
- Weak Electrolytes: Weak acids and weak bases.
- Nonelectrolytes: Molecular compounds that are neither acids nor bases.