Chapter 6 & 7: Chemical Reactions and Quantities

Equations for Chemical Reactions

• Definition of Chemical Change: A chemical change occurs when a substance is converted into one or more new substances that have different formulas and different properties.

• Learning Goal: To write a balanced chemical equation from the formulas of reactants and products and determine the number of atoms on both sides.

• Core Concepts:     * Chemical reactions involve chemical changes.     * Example: When iron ($Fe$) reacts with oxygen ($O_2$), the product is rust ($Fe_2O_3$).

• Observing Chemical Changes: Evidence of a reaction may include:     * Formation of bubbles (gas evolution).     * Change in color.     * Production of a solid (precipitate).     * Production or absorption of heat.

• Symbols in Chemical Equations:     * Arrows ($\rightarrow$) separate reactants (left side) from products (right side).     * Multiple reactants or products are separated by a plus sign ($+$).     * The delta sign ($\Delta$) indicates that heat is used to start the reaction.     * Physical States:         * Solid: $(s)$         * Liquid: $(l)$         * Gas: $(g)$         * Aqueous (dissolved in water): $(aq)$

Balancing Chemical Equations

• Principles of a Balanced Equation:     * No atoms are lost or gained (Law of Conservation of Mass).     * The number of atoms on the reactant side must equal the number of atoms on the product side for every element.

• Step-by-Step Guide to Balancing:     * Step 1: Write the equation using correct formulas for reactants and products.         * Example: $Al(s) + S(s) \rightarrow Al_2S_3(s)$     * Step 2: Count the atoms of each element in the reactants and products.     * Step 3: Use coefficients to balance each element. Start with the most complex formula. Never change subscripts in a formula.         * Example balancing: $2Al(s) + 3S(s) \rightarrow Al_2S_3(s)$     * Step 4: Check the final equation to confirm it is balanced. Ensure coefficients are in the lowest whole-number ratio.

• Balancing with Polyatomic Ions:     * When balancing reactions like $Na_3PO4(aq) + MgCl_2(aq) \rightarrow Mg_3(PO_4)_2(s) + NaCl(aq)$, balance the polyatomic ion (e.g., the phosphate ion, $PO_4^{3-}$) as a single unit if it appears on both sides.     * Balanced equation: $2Na_3PO_4(aq) + 3MgCl_2(aq) \rightarrow Mg_3(PO_4)_2(s) + 6NaCl(aq)$.

Types of Chemical Reactions

• Combination Reactions: Two or more elements or simple compounds combine to form one product.     * General form: $A + B \rightarrow AB$     * Example: $2Mg(s) + O_2(g) \rightarrow 2MgO(s)$     * Example: $SO_3(g) + H_2O(l) \rightarrow H_2SO_4(aq)$

• Decomposition Reactions: One substance splits into two or more simpler substances.     * General form: $AB \rightarrow A + B$     * Example: $2HgO(s) \rightarrow 2Hg(l) + O_2(g)$

• Single Replacement Reactions: One element takes the place of a different element in another reacting compound.     * General form: $A + BC \rightarrow AC + B$     * Example: $Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$

• Double Replacement Reactions: The positive ions in the reactant compounds switch places.     * General form: $AB + CD \rightarrow AD + CB$     * Example: $AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)$

• Combustion Reactions: A carbon-containing compound burns in oxygen ($O_2$) to form carbon dioxide ($CO_2$), water ($H_2O$), and energy (heat).     * Example: $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g) + \text{energy}$     * Example: $C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(g) + \text{energy}$

Oxidation-Reduction (Redox) Reactions

• Definition: A reaction where electrons are transferred from one substance to another.

• OIL RIG Mnemonic:     * Oxidation Is Loss of electrons ($e^-$).     * Reduction Is Gain of electrons ($e^-$).

• Redox in Practice:     * Rusting of iron: $4Fe(s) + 3O_2(g) \rightarrow 2Fe_2O_3(s)$     * Statue of Liberty Patina ($CuO$ formation):         * Oxidation: $2Cu(s) \rightarrow 2Cu^{2+}(s) + 4e^-$         * Reduction: $O_2(g) + 4e^- \rightarrow 2O^{2-}(s)$

• Characteristics and Biological Applications:     * Oxidation: Loss of electrons; addition of oxygen; OR loss of hydrogen.     * Reduction: Gain of electrons; loss of oxygen; OR gain of hydrogen.     * Methyl Alcohol Metabolism: The body metabolizes toxic methyl alcohol via successive oxidations:         1. $CH_3OH \rightarrow H_2CO + 2H$ (Methyl alcohol to Formaldehyde; loss of H).         2. $2H_2CO + O_2 \rightarrow 2H_2CO_2$ (Formaldehyde to Formic acid; addition of O).         3. $2H_2CO_2 + O_2 \rightarrow 2CO_2 + 2H_2O$ (Formic acid to Carbon dioxide; addition of O).     * Coenzyme FAD: Flavin adenine dinucleotide (FAD) is reduced to $FADH_2$ by transferring two hydrogen atoms ($2H^+$ and $2e^-$).

Toxicity of Carbon Monoxide (CO)

• Formation: Incomplete combustion occurs when oxygen supply is limited (burning gas, oil, or wood).     * Reaction: $2CH_4(g) + 3O_2(g) \rightarrow 2CO(g) + 4H_2O(g) + \text{heat}$

• Physical Properties: Colorless, odorless, poisonous gas.

• Biological Mechanism: CO attaches to hemoglobin molecules, preventing oxygen ($O_2$) from reaching cells.

• Health Symptoms based on COHb (Carboxyhemoglobin) Levels:     * 10%: Shortness of breath, mild headache, and drowsiness.     * 30%: Dizziness, mental confusion, severe headache, and nausea.     * 50%: Unconsciousness and death (requires immediate treatment with oxygen).

The Mole and Avogadro’s Number

• Definition: The mole is a counting unit for small particles (atoms, molecules, ions).

• Avogadro’s Number: $1\text{ mole} = 6.02 \times 10^{23}\text{ items}$.

• Conversion Factors:     * $\frac{6.02 \times 10^{23}\text{ particles}}{1\text{ mole}}$     * $\frac{1\text{ mole}}{6.02 \times 10^{23}\text{ particles}}$

• Relationships in Formulas: Subscripts in a chemical formula represent the number of moles of each element in 1 mole of the compound.     * Example: Aspirin ($C_9H_8O_4$) contains 9 moles of $C$, 8 moles of $H$, and 4 moles of $O$ per 1 mole of compound.

Molar Mass and Stoichiometry

• Molar Mass: The mass of 1 mole of an element or compound, expressed in grams ($g/mol$). It is numerically equal to the atomic mass.

• Calculating Molar Mass for Compounds (e.g., $Li_2CO_3$):     * $2 \times Li\text{ (6.941 g/mol)} = 13.88\,g$     * $1 \times C\text{ (12.01 g/mol)} = 12.01\,g$     * $3 \times O\text{ (16.00 g/mol)} = 48.00\,g$     * Total Molar Mass = $73.89\,g/mol$

• Law of Conservation of Mass: Matter cannot be created or destroyed. The total mass of reactants must equal the total mass of products.

• Mole-Mole Factors: Ratios derived from the coefficients of a balanced equation used to relate moles of any two substances.     * For $2Fe(s) + 3S(s) \rightarrow Fe_2S_3(s)$, factors include: $\frac{2\text{ moles Fe}}{3\text{ moles S}}$ or $\frac{1\text{ mole } Fe_2S_3}{2\text{ moles Fe}}$.

• Mass-to-Mass Calculation Process:     1. Convert mass of Substance A to moles using its molar mass.     2. Convert moles of Substance A to moles of Substance B using the mole-mole ratio.     3. Convert moles of Substance B to grams using its molar mass.

Limiting Reactants and Percent Yield

• Limiting Reactant: The reactant that is completely consumed first, limiting the amount of product that can form.

• Excess Reactant: The reactant that remains after the reaction stops.

• Theoretical Yield: The maximum amount of product calculated using the balanced equation and the limiting reactant.

• Actual Yield: The amount of product actually obtained from the reaction in a laboratory setting.

• Percent Yield Formula:     * $\text{Percent Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100\%$

• Sample Calculation:     * If $50.0\,g$ of $LiOH$ produces a theoretical yield of $142\,g$ of $LiHCO_3$, but only $72.8\,g$ is obtained, the percent yield is:     * $\frac{72.8\,g}{142\,g} \times 100\% = 51.3\%$

Questions & Discussion

• Study Check 7.1: Calculate atoms in $2C_2H_6(g) + 7O_2(g) \rightarrow 4CO_2(g) + 6H_2O(g)$.     * Response: Both sides contain 4 $C$ atoms, 12 $H$ atoms, and 14 $O$ atoms.

• Study Check 7.3: Balance $Pb(NO_3)_2(aq) + AlBr_3(aq) \rightarrow PbBr_2(s) + Al(NO_3)_3(aq)$.     * Answer: $3Pb(NO_3)_2(aq) + 2AlBr_3(aq) \rightarrow 3PbBr_2(s) + 2Al(NO_3)_3(aq)$.

• CO Toxicity Levels: What happens when hemoglobin bound to CO reaches 30%?     * Answer: The person may experience dizziness, mental confusion, severe headache, and nausea.

• Conversion Exercise: How many $\text{CO}_2$ molecules are in $0.500\text{ mole}$ of $\text{CO}_2$?     * Calculation: $0.500\text{ mole} \times (6.02 \times 10^{23}\text{ molecules} / 1\text{ mole}) = 3.01 \times 10^{23}\text{ molecules}$.