Chapter 7: Chemical Reactions and Chemical Quantities – Structured Study Notes

Chapter 7: Chemical Reactions and Chemical Quantities

Climate Change and Combustion of Fossil Fuels: The Greenhouse Effect

The greenhouse effect is a critical environmental phenomenon influenced by chemical reactions, especially combustion. Understanding the chemistry behind greenhouse gases helps explain global warming.

• Greenhouse gases in the atmosphere:

  • Allow sunlight to enter the atmosphere.

  • Warm Earth's surface.

  • Prevent some of the heat generated by sunlight from escaping.

• The balance between incoming and outgoing energy from the sun determines Earth's average temperature.

Global Warming

Global warming is linked to chemical changes in atmospheric composition, particularly the increase in carbon dioxide due to combustion reactions.

• Average atmospheric temperature has risen by 0.8 °C (1.4 °F) since 1860.

• Atmospheric CO2 levels have risen 37% in the same period.

• Scientists investigate whether these trends are causally related.

Physical Changes in Matter

Physical changes affect only the state or appearance of matter, not its composition.

• Physical change: A change in state or appearance without altering chemical composition.

• The identity of atoms or molecules remains unchanged.

• Example: Boiling water changes H2O from liquid to gas, but the molecules remain H2O.

Chemical Changes in Matter

Chemical changes alter the composition of matter, resulting in new substances.

• Chemical change: Atoms rearrange, transforming original substances into different substances.

• Example: Rusting nail – iron atoms exchange electrons with oxygen, forming iron(III) oxide (Fe2O3).

Practice Problems on Physical and Chemical Changes

• Identify whether a process is a physical or chemical change based on changes in composition and appearance.

• Interpret molecular diagrams to distinguish between physical and chemical changes.

Writing and Balancing Chemical Reactions

Chemical reactions involve the rearrangement and exchange of atoms to produce new molecules. Writing and balancing equations is essential for describing these changes.

• Reactants → Products

• A chemical equation states the quantities, chemical identities, and physical states of substances.

• Example: $\mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)}$

Chemical Equations: Shorthand for Describing a Chemical Reaction

Chemical equations provide concise information about reactions.

• Molecular or ionic formulas of reactants and products.

• States: gas (g), liquid (l), solid (s), aqueous (aq).

• Relative numbers of molecules (stoichiometry).

• Used to determine masses of reactants and products.

The Quantities in Chemical Reactions

The amount of each substance in a reaction is related to all others by the law of conservation of mass. Balancing equations ensures this law is obeyed.

• Law of conservation of mass: Mass is conserved in chemical reactions.

• Balancing equations: Ensures equal numbers of each atom on both sides.

• Stoichiometry: The study of numerical relationships between chemical quantities.

Reactants	Products
1 C atom in CH4	1 C atom in CO2
4 H atoms in CH4	4 H atoms in 2 H2O
4 O atoms in 2 O2	2 O atoms in CO2 + 2 in 2 H2O

Practice Problem: Balancing and Writing Chemical Equations

Balancing equations involves adjusting coefficients to ensure equal numbers of each atom on both sides.

• Write unbalanced (skeletal) equation.

• Balance atoms in complex substances first, then free elements.

• Adjust coefficients to clear fractions if necessary.

• Check by counting atoms on both sides.

Left	Right
4 Co atoms	4 Co atoms
6 O atoms	6 O atoms
3 C atoms	3 C atoms

Balancing Equations with Polyatomic Ions

When polyatomic ions appear unchanged on both sides, balance them as units.

• Balance metal ions first, then polyatomic ions, then remaining ions.

• Check by counting each type of ion.

Left	Right
3 Sr2+ ions	3 Sr2+ ions
6 Li+ ions	6 Li+ ions
2 PO43- ions	2 PO43- ions
6 Cl- ions	6 Cl- ions

Reaction Stoichiometry

Stoichiometry uses balanced equations to relate amounts of reactants and products.

• Chemical coefficients specify relative amounts in moles.

• Example: $2\,\mathrm{C_8H_{18}(l)} + 25\,\mathrm{O_2(g)} \rightarrow 16\,\mathrm{CO_2(g)} + 18\,\mathrm{H_2O(g)}$

• 2 mol C8H18 : 25 mol O2 : 16 mol CO2 : 18 mol H2O

Cooking Stoichiometry: Making Pizza (Analogy)

Stoichiometry is analogous to recipes: the amount of product depends on the limiting ingredient.

• 1 crust + 5 oz. tomato sauce + 2 cups cheese → 1 pizza

• Mathematical relationship: $\frac{6\,\text{cups cheese}}{2\,\text{cups cheese}} \times 1\,\text{pizza} = 3\,\text{pizzas}$

Mole-to-Mole Conversions

Stoichiometric ratios convert between moles of reactants and products.

• Use coefficients from balanced equations as conversion factors.

• Example: $\frac{22.0\,\text{mol}\,\mathrm{C_8H_{18}}}{2\,\text{mol}\,\mathrm{C_8H_{18}}} \times 16\,\text{mol}\,\mathrm{CO_2} = 176\,\text{mol}\,\mathrm{CO_2}$

Mole-to-Mass and Mass-to-Mass Conversions

Stoichiometric ratios can be used to convert between mass and moles of reactants and products.

• General strategy:

  • Mass of A → Moles of A → Stoichiometric ratio → Moles of B → Mass of B

• Formula:

$\text{mass A} \times \left( \frac{1\,\text{mol A}}{\text{molar mass A}} \right) \times \left( \frac{\text{mol B}}{\text{mol A}} \right) \times \left( \frac{\text{molar mass B}}{1\,\text{mol B}} \right)$

• Example: $3.6 \times 10^{15}\,\text{g}\,\mathrm{C_8H_{18}} \times \left( \frac{1\,\text{mol}\,\mathrm{C_8H_{18}}}{114.22\,\text{g}\,\mathrm{C_8H_{18}}} \right) \times \left( \frac{16\,\text{mol}\,\mathrm{CO_2}}{2\,\text{mol}\,\mathrm{C_8H_{18}}} \right) \times \left( \frac{44.01\,\text{g}\,\mathrm{CO_2}}{1\,\text{mol}\,\mathrm{CO_2}} \right) = 1.1 \times 10^{16}\,\text{g}\,\mathrm{CO_2}$

Limiting Reactant and Theoretical Yield

The limiting reactant is the reactant that is completely consumed first, determining the maximum amount of product (theoretical yield).

• Limiting reactant: Reactant that makes the least amount of product.

• Theoretical yield: Maximum product based on limiting reactant.

• Actual yield: Amount of product actually produced.

• Percent yield: $\frac{\text{actual yield}}{\text{theoretical yield}} \times 100$

Practice Problems: Limiting Reactant and Theoretical Yield

• Identify the limiting reactant by calculating the amount of product each reactant can produce.

• The reactant that produces the smallest amount of product is the limiting reactant.

• Use the limiting reactant to calculate theoretical yield.

Practice Problems: Stoichiometry and Excess Reactant

• Excess reactant: Any reactant present in a quantity greater than required to react with the limiting reactant.

• Calculate the amount of excess reactant remaining after the reaction.

Combustion Reactions

Combustion reactions involve a substance reacting with oxygen to form oxygen-containing compounds, typically releasing energy.

• Products often include water (H2O) and heat.

• Example: $\mathrm{CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g) + \text{heat}}$

Alkali Metal and Halogen Reactions

Alkali metals (Group 1) and halogens (Group 17) undergo vigorous reactions to form ionic compounds.

• Alkali metals form 1+ cations; halogens form 1- anions.

• Alkali metals react with halogens to form salts (e.g., NaCl).

• Alkali metals react with water to form hydroxide ions and hydrogen gas.

• Halogens react with metals to form metal halides and with hydrogen to form hydrogen halides.

Summary Table: Key Terms and Definitions

Term	Definition
Physical Change	Change in state or appearance without altering composition
Chemical Change	Change that alters the composition of matter
Stoichiometry	Numerical relationship between chemical quantities in a reaction
Limiting Reactant	Reactant that is completely consumed first
Theoretical Yield	Maximum amount of product possible
Actual Yield	Amount of product actually obtained
Percent Yield	Ratio of actual yield to theoretical yield, expressed as a percentage

Key Equations

• General Stoichiometry:

• $\text{mass A} \times \left( \frac{1\,\text{mol A}}{\text{molar mass A}} \right) \times \left( \frac{\text{mol B}}{\text{mol A}} \right) \times \left( \frac{\text{molar mass B}}{1\,\text{mol B}} \right)$

• Percent Yield:

• $\frac{\text{actual yield}}{\text{theoretical yield}} \times 100$

Additional info: These notes are based on lecture slides and textbook images for Chapter 7, covering chemical reactions, stoichiometry, limiting reactants, and related quantitative concepts in General Chemistry.