Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define and differentiate reaction types in aqueous solutions.
• Write balanced chemical equations with state symbols.
• Predict precipitate formation using solubility rules.
• Perform stoichiometric calculations (mole, mass, etc.).
• Determine the limiting reactant and theoretical yield.

Lesson summary

This week, we'll be diving into the exciting world of reactions happening in water (aqueous solutions) and how we can use stoichiometry to predict and understand the quantities of reactants and products involved. Understanding these concepts is crucial because many chemical reactions occur in aqueous solutions, from the reactions happening in our bodies to the processes used in water treatment plants. In South Africa, with our concerns about water quality and availability, understanding these reactions becomes even more important for informed decision-making and potentially contributing to solutions.

Lesson notes

2.1 Aqueous Solutions An aqueous solution is simply a solution where water is the solvent. This means water is the substance that dissolves another substance (the solute). Many ionic compounds and polar covalent compounds dissolve in water due to water's polar nature. Remember water is a polar molecule (has partial positive and negative charges). 2.2 Types of Reactions in Aqueous Solutions We will focus on three main types of reactions happening in aqueous solutions: Precipitation Reactions: These reactions occur when two aqueous solutions are mixed, and an insoluble solid (a precipitate) forms. The precipitate falls out of the solution. To predict whether a precipitate will form, we use solubility rules.

Solubility Rules (Simplified): These are general guidelines. There are exceptions, but they work well for our level. Most compounds containing alkali metals (Group 1: Li, Na, K, etc.) and ammonium (NH 4 + ) are soluble. Most nitrates (NO 3 - ), acetates (CH 3 COO - ), and perchlorates (ClO 4 - ) are soluble. Most chlorides (Cl - ), bromides (Br - ), and iodides (I - ) are soluble EXCEPT those of Ag + , Pb 2+ , and Hg 2 2+ . Most sulfates (SO 4 2- ) are soluble EXCEPT those of Ba 2+ , Sr 2+ , Pb 2+ , Hg 2 2+ , and Ca 2+ . Most hydroxides (OH - ) and sulfides (S 2- ) are insoluble EXCEPT those of Group 1 elements, Ca 2+ , Sr 2+ , and Ba 2+ (hydroxides) and Group 1 and Group 2 elements and ammonium sulfide (sulfides). Most carbonates (CO 3 2- ) and phosphates (PO 4 3- ) are insoluble EXCEPT those of Group 1 elements and ammonium.

Example: When aqueous solutions of lead(II) nitrate [Pb(NO 3 ) 2 (aq)] and potassium iodide [KI(aq)] are mixed, a yellow precipitate of lead(II) iodide [PbI 2 (s)] forms.

Equation: Pb(NO 3 ) 2 (aq) + 2KI(aq) → PbI 2 (s) + 2KNO 3 (aq)

Explanation: Lead(II) iodide is insoluble according to the solubility rules (iodides are generally soluble except for lead).

Acid-Base Neutralization Reactions: These reactions involve the reaction of an acid with a base. Acids donate protons (H + ions), and bases accept protons. When a strong acid reacts with a strong base, the products are a salt and water.

Example:* Hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH) to form sodium chloride (NaCl) and water (H 2 O).

Equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H 2 O(l)

Redox Reactions: These reactions involve the transfer of electrons. Oxidation is the loss of electrons, and reduction is the gain of electrons. One substance is oxidized (loses electrons), while another is reduced (gains electrons). Remember OIL RIG (Oxidation Is Loss, Reduction Is Gain). In aqueous solutions, redox reactions can involve the oxidation of metals by acids or the displacement of one metal by another.

Example: Zinc metal (Zn) reacts with hydrochloric acid (HCl) to produce zinc chloride (ZnCl 2 ) and hydrogen gas (H 2 ).

Equation: Zn(s) + 2HCl(aq) → ZnCl 2 (aq) + H 2 (g)

Explanation: Zinc is oxidized (loses electrons) to form Zn 2+ ions, and hydrogen ions (H + ) from the acid are reduced (gain electrons) to form hydrogen gas (H 2 ). 2.3 Stoichiometry in Aqueous Solutions Stoichiometry is the study of the quantitative relationships between reactants and products in a chemical reaction. We use balanced chemical equations to determine these relationships.

Key concepts: Mole Ratio: The ratio of the coefficients in a balanced chemical equation. For example, in the reaction 2H 2 (g) + O 2 (g) → 2H 2 O(g), the mole ratio of H 2 to O 2 is 2:1, and the mole ratio of H 2 to H 2 O is 2:2 (or 1:1).

Molar Mass (M): The mass of one mole of a substance, expressed in grams per mole (g/mol). You can find molar masses on the periodic table by adding up the atomic masses of each element in the compound.

Number of Moles (n): Related to mass (m) by the equation: n = m/M Concentration (c): The amount of solute dissolved in a given volume of solvent, typically expressed in moles per liter (mol/L or Molarity, M).

The relationship is: c = n/V, where V is the volume in liters.

Limiting Reactant: The reactant that is completely consumed in a reaction. It limits the amount of product that can be formed.

To determine the limiting reactant: Calculate the number of moles of each reactant. Divide the number of moles of each reactant by its stoichiometric coefficient in the balanced equation. The reactant with the smallest value is the limiting reactant.

Theoretical Yield: The maximum amount of product that can be formed based on the amount of the limiting reactant. 2.4 Worked Examples Example 1: What mass of silver chloride (AgCl) will precipitate when 100.0 mL of 0.200 M silver nitrate (AgNO 3 ) is mixed with 50.0 mL of 0.300 M sodium chloride (NaCl)?