Double Displacement and Redox Reactions: Conductivity, Balancing, and Net Ionic Equations

Conductivity of Compounds in Solution

Introduction to Conductivity

Conductivity in aqueous solutions depends on the presence and mobility of ions. Compounds can be classified based on their ability to conduct electricity when dissolved in water.

• No Conductivity: Compounds that do not ionize in water (e.g., molecular compounds like alcohols).

• High Conductivity: Strong electrolytes that dissociate completely into ions (e.g., salts, strong acids).

• Low Conductivity: Weak electrolytes that partially ionize (e.g., weak acids, weak bases).

Example:

• HNO3: High conductivity (strong acid, fully ionizes)

• (CH3)2CHOH (isopropanol): No conductivity (molecular compound, does not ionize)

• KF: High conductivity (strong electrolyte, fully dissociates)

• HI: High conductivity (strong acid, fully ionizes)

Balancing Double Displacement Reactions

Overview of Double Displacement Reactions

Double displacement reactions involve the exchange of ions between two compounds, often resulting in the formation of a precipitate, gas, or water. These reactions are common in precipitation, acid-base, and some gas-forming reactions.

• General Form: $AB + CD ightarrow AD + CB$

• Each positive ion gets a new negative counterpart.

• Rebalance new ionic compounds to ensure neutrality.

• Identify strong vs. weak electrolytes.

• Note the physical phase (solid, liquid, gas, aqueous) for each compound/ion.

• Apply solubility rules to predict precipitate formation.

• Eliminate spectator ions to write the Net Ionic Equation.

Solubility Rules

Solubility rules help predict whether a compound will dissolve in water or form a precipitate.

• All nitrates, acetates, and most alkali metal salts are soluble.

• Most chlorides, bromides, and iodides are soluble except Ag+, Pb2+, and Hg22+.

• Sulfates are generally soluble except for Ba2+, Sr2+, Pb2+, and Ca2+.

• Carbonates, phosphates, and sulfides are generally insoluble except for alkali metals and NH4+.

Example: Double Displacement Reaction

Combine manganese(II) sulfate with sodium phosphate:

• Balanced Molecular Equation (BME): $3 ext{MnSO}_4(aq) + 2 ext{Na}_3 ext{PO}_4(aq) ightarrow ext{Mn}_3( ext{PO}_4)_2(s) + 3 ext{Na}_2 ext{SO}_4(aq)$

• Ionic Equation (IE): $3 ext{Mn}^{2+}(aq) + 6 ext{SO}_4^{2-}(aq) + 6 ext{Na}^+(aq) + 2 ext{PO}_4^{3-}(aq) ightarrow ext{Mn}_3( ext{PO}_4)_2(s) + 6 ext{Na}^+(aq) + 6 ext{SO}_4^{2-}(aq)$

• Net Ionic Equation (NIE): $3 ext{Mn}^{2+}(aq) + 2 ext{PO}_4^{3-}(aq) ightarrow ext{Mn}_3( ext{PO}_4)_2(s)$

Redox Reactions and Balancing

Introduction to Redox Reactions

Redox (reduction-oxidation) reactions involve the transfer of electrons between species, resulting in changes in oxidation numbers. These reactions are fundamental in chemistry and differ from double displacement reactions by involving electron exchange.

• Oxidation: Loss of electrons (increase in oxidation number).

• Reduction: Gain of electrons (decrease in oxidation number).

• Redox reactions can be identified by changes in oxidation numbers of elements from reactants to products.

Examples of Redox Reactions

• $ext{Zn}(s) + 2 ext{H}^+(aq) ightarrow ext{Zn}^{2+}(aq) + ext{H}_2(g)$

• $ext{Al}(s) + 3 ext{Ag}^+(aq) ightarrow ext{Al}^{3+}(aq) + 3 ext{Ag}(s)$

• $ext{Ba}(s) + ext{Cl}_2(g) ightarrow ext{BaCl}_2(aq)$

• $ext{SO}_4^{2-}(aq) + ext{MnO}_4^-(aq) ightarrow ext{SO}_2(aq) + ext{Mn}^{2+}(aq)$

Assigning Oxidation Numbers

Oxidation numbers are assigned based on a set of rules to track electron transfer in redox reactions.

• Elements in their standard state: 0

• Group 1 ions: +1; Group 2 ions: +2

• Fluorine: -1

• Hydrogen: +1 (except in hydrides: -1)

• Oxygen: -2 (except in peroxides: -1)

• Other elements: variable, determined after above rules are applied

Mnemonic for Redox

• OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons)

• LEO the Lion goes GER: Lose Electrons = Oxidation, Gain Electrons = Reduction

Example: Assigning Oxidation Numbers

• ClO4-: Cl = +7, O = -2

• Zn(ClO2)2: Zn = +2, Cl = +3, O = -2

• Na2S2O3: Na = +1, S = +2, O = -2

• Na2O2: Na = +1, O = -1 (peroxide)

• Sn(CO3)2: Sn = +4, C = +4, O = -2

• Cr2(Cr2O7)3: Cr = +6, O = -2

Summary Table: Solubility Rules

Compound Type	Solubility	Exceptions
Nitrates, Acetates	Soluble	None
Chlorides, Bromides, Iodides	Soluble	Ag+, Pb2+, Hg22+
Sulfates	Soluble	Ba2+, Sr2+, Pb2+, Ca2+
Carbonates, Phosphates, Sulfides	Insoluble	Alkali metals, NH4+

Additional info: The notes also include practical steps for writing net ionic equations and identifying spectator ions, which are essential for mastering double displacement and redox reactions in general chemistry.