Chapter 7: Chemical and Physical Changes – Study Notes and Problem Guide

Chapter 7: Chemical and Physical Changes

Introduction to Chemical and Physical Changes

Chemical and physical changes are fundamental concepts in chemistry, distinguishing between processes that alter the identity of substances and those that do not. Understanding these changes is essential for interpreting chemical reactions, laboratory observations, and real-world phenomena.

Classification of Changes

• Physical Change: A change that does not alter the chemical composition of a substance. Examples include changes in state (melting, boiling), dissolving, and breaking.

• Chemical Change: A process in which one or more substances are converted into new substances with different properties. Indicators include color change, gas production, formation of a precipitate, and energy change.

Examples and Applications

• Burning natural gas in a stove: Chemical change (methane reacts with oxygen to form CO2 and H2O).

• Evaporation of liquid propane: Physical change (phase change, no new substance formed).

• Rusting of iron: Chemical change (iron reacts with oxygen and water to form iron oxide).

• Sugar dissolving in water: Physical change (no new substance, just dispersion of molecules).

• Surface tarnishing after exposure to air: Chemical change (formation of metal oxides or sulfides).

Properties: Physical vs. Chemical

• Physical Properties: Can be observed or measured without changing the substance’s identity (e.g., color, density, melting point, boiling point, solubility).

• Chemical Properties: Describe a substance’s ability to undergo a specific chemical change (e.g., flammability, reactivity with acids, tendency to rust).

Example Table: Properties of Isopropyl Alcohol

Property	Physical or Chemical?
Colorless	Physical
Flammable	Chemical
Liquid at room temperature	Physical
Density = 0.79 g/mL	Physical
Mixes with water	Physical

Writing and Balancing Chemical Equations

Chemical equations represent chemical reactions, showing the reactants and products, their physical states, and the stoichiometric relationships between them. Balancing equations ensures the law of conservation of mass is obeyed.

• Steps to Balance Equations:

  1. Write the correct formulas for all reactants and products.

  2. Count the number of atoms of each element on both sides.

  3. Add coefficients to balance the atoms (do not change subscripts).

  4. Check your work to ensure mass and charge are balanced.

Example Equations

• Combustion of hexane: $\mathrm{C_6H_{14}(g) + O_2(g) \rightarrow CO_2(g) + H_2O(g)}$

• Formation of ammonia: $\mathrm{N_2(g) + 3H_2(g) \rightarrow 2NH_3(g)}$

• Reaction of iron with hydrochloric acid: $\mathrm{Fe(s) + 2HCl(aq) \rightarrow FeCl_2(aq) + H_2(g)}$

Stoichiometry: Quantitative Relationships in Reactions

Stoichiometry involves the calculation of reactants and products in chemical reactions using balanced equations. It allows chemists to predict the amounts of substances consumed and produced.

• Mole Ratio: The ratio of coefficients in a balanced equation, used to convert between moles of reactants and products.

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

• Theoretical Yield: The maximum amount of product that can be formed from the given amounts of reactants.

• Percent Yield: The ratio of actual yield to theoretical yield, expressed as a percentage.

Example Table: Stoichiometry Calculation

Mol SiO2	Mol C	Mol SiC	Mol CO
3	6	3	6
2.8	--	--	10
1.55	--	--	--

Additional info: Missing values can be calculated using the balanced equation: $\mathrm{SiO_2(s) + 3C(s) \rightarrow SiC(s) + 2CO(g)}$

Limiting Reactant, Theoretical Yield, and Percent Yield

• Finding the Limiting Reactant: Compare the mole ratio of reactants used to the balanced equation. The reactant that produces the least amount of product is limiting.

• Calculating Theoretical Yield: Use the limiting reactant to determine the maximum amount of product possible.

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

Combustion, Alkali Metal, and Halogen Reactions

• Combustion Reaction: A substance reacts with oxygen, releasing energy, and forming oxides (e.g., CO2, H2O).

• Alkali Metal Reaction: Alkali metals react vigorously with water and halogens, forming hydroxides and halides.

• Halogen Reaction: Halogens react with metals to form ionic halides.

Example Equations

• $\mathrm{2Na(s) + Br_2(g) \rightarrow 2NaBr(s)}$

• $\mathrm{4Al(s) + 3O_2(g) \rightarrow 2Al_2O_3(s)}$

• $\mathrm{S(s) + O_2(g) \rightarrow SO_2(g)}$

Conceptual and Challenge Problems

• Classify changes as physical or chemical based on molecular diagrams and descriptions.

• Apply stoichiometry to solve for limiting reactants, theoretical yields, and percent yields in complex reactions.

• Interpret molecular diagrams to determine the outcome of reactions and the limiting reactant.

Summary Table: Key Concepts

Concept	Definition	Example
Physical Change	No new substance formed	Melting ice
Chemical Change	New substance formed	Burning wood
Physical Property	Observed without changing identity	Density
Chemical Property	Describes reactivity	Flammability
Limiting Reactant	Reactant used up first	O2 in combustion
Theoretical Yield	Maximum possible product	Calculated from stoichiometry

Key Equations

• General Stoichiometry: $\mathrm{\text{mol A} \rightarrow \text{mol B} \rightarrow \text{mass B}}$

• Percent Yield: $\mathrm{Percent\ Yield = \frac{Actual\ Yield}{Theoretical\ Yield} \times 100\%}$

Additional info: These notes are based on a comprehensive set of practice and conceptual problems, including molecular diagrams, property classification, and quantitative reaction calculations, as typically found in a General Chemistry college course.