Chaoter 5 Stugy Guide

Stoichiometry and Solution Concentration

Balancing Chemical Equations

Balancing chemical equations ensures the law of conservation of mass is obeyed. Each side of the equation must have the same number of atoms for each element.

• Example: The balanced equation for the reaction between lead(II) nitrate and hydrochloric acid is:

$\mathrm{Pb(NO_3)_2 (aq) + 2HCl (aq) \rightarrow PbCl_2 (s) + 2HNO_3 (aq)}$

Calculating Molarity

Molarity (M) is defined as the number of moles of solute per liter of solution.

• Formula: $M = \dfrac{\text{moles of solute}}{\text{liters of solution}}$

• Example Calculation: If 7.55 g of HCl is dissolved in 350.0 mL of solution:

  • Convert grams to moles: $\dfrac{7.55\ \text{g}}{36.46\ \text{g/mol}} = 0.207\ \text{mol}$

  • Convert mL to L: $350.0\ \text{mL} = 0.3500\ \text{L}$

  • Calculate molarity: $M = \dfrac{0.207\ \text{mol}}{0.3500\ \text{L}} = 0.592\ \text{M}$

Stoichiometry in Solution Reactions

Stoichiometry allows calculation of the required volume or mass of reactants for a complete reaction.

• Example: To react 0.155 L of 0.592 M HCl with 0.525 M Pb(NO3)2 solution:

  • Calculate moles HCl: $0.155\ \text{L} \times 0.592\ \text{mol/L} = 0.0916\ \text{mol}$

  • From the balanced equation, 2 mol HCl reacts with 1 mol Pb(NO3)2:

  • Moles Pb(NO3)2 needed: $0.0916\ \text{mol HCl} \times \dfrac{1\ \text{mol Pb(NO}_3)_2}{2\ \text{mol HCl}} = 0.0458\ \text{mol}$

  • Volume of Pb(NO3)2 solution: $\dfrac{0.0458\ \text{mol}}{0.525\ \text{mol/L}} = 0.0873\ \text{L} = 87.3\ \text{mL}$

Solubility Rules

Predicting Solubility of Ionic Compounds

Solubility rules help determine whether an ionic compound will dissolve in water.

• Soluble: Compounds that dissolve in water to form ions.

• Insoluble: Compounds that do not dissolve appreciably in water.

• Examples:

  • K2S: Soluble (all potassium salts are soluble)

  • BaSO4: Insoluble (most sulfates are soluble, but BaSO4 is an exception)

Writing and Balancing Chemical Equations

Double Displacement (Metathesis) Reactions

In double displacement reactions, the cations and anions of two compounds exchange partners.

• General form: $AB + CD \rightarrow AD + CB$

• Example: $2\mathrm{LiOH}_{(aq)} + \mathrm{CuCl}_2_{(aq)} \rightarrow 2\mathrm{LiCl}_{(aq)} + \mathrm{Cu(OH)}_2_{(s)}$

• Example: $\mathrm{Ca(NO_3)_2}_{(aq)} + \mathrm{BaSO_4}_{(s)} \rightarrow \mathrm{CaSO_4}_{(s)} + \mathrm{Ba(NO_3)_2}_{(aq)}$

Oxidation States and Redox Reactions

Assigning Oxidation States

Oxidation states (numbers) indicate the degree of oxidation of an atom in a compound.

• Rules:

  • Elemental form: 0

  • Monatomic ion: charge of the ion

  • Oxygen: usually -2

  • Hydrogen: +1 (with nonmetals), -1 (with metals)

  • Sum of oxidation states equals the charge of the molecule or ion

• Examples:

  • In CaCr2O7: Ca = +2, O = -2, Cr = +6

  • In PO43-: P = +5, O = -2

Identifying Redox Processes

Redox (reduction-oxidation) reactions involve the transfer of electrons between species.

• Oxidation: Loss of electrons (increase in oxidation state)

• Reduction: Gain of electrons (decrease in oxidation state)

• Oxidizing agent: Causes oxidation, is reduced

• Reducing agent: Causes reduction, is oxidized

• Example: $SiCl_4 + 2Mg \rightarrow 2MgCl_2 + Si$

  • Mg is oxidized (0 to +2), Si is reduced (+4 to 0)

  • Oxidizing agent: SiCl4, Reducing agent: Mg

Net Ionic Equations

Writing Net Ionic Equations

Net ionic equations show only the species that actually change during the reaction.

• Steps:

  1. Write the balanced molecular equation.

  2. Write the complete ionic equation (all strong electrolytes as ions).

  3. Cancel spectator ions to get the net ionic equation.

• Example: Reaction of HCl with Ca(OH)2:

  • Molecular: $2\mathrm{HCl}_{(aq)} + \mathrm{Ca(OH)}_2_{(aq)} \rightarrow 2\mathrm{H}_2O_{(l)} + \mathrm{CaCl}_2_{(aq)}$

  • Net ionic: $2\mathrm{H}^+_{(aq)} + 2\mathrm{OH}^-_{(aq)} \rightarrow 2\mathrm{H}_2O_{(l)}$

Summary Table: Solubility of Selected Compounds

Compound	Solubility in Water	Notes
K2S	Soluble	All potassium salts are soluble
BaSO4	Insoluble	Exception to sulfate solubility