Chapter 7: Chemical Reactions and Chemical Quantities – Study Notes

Chapter 7: Chemical Reactions and Chemical Quantities

Physical Changes in Matter

Physical changes are transformations that affect only the state or appearance of a substance, without altering its chemical composition.

• Definition: A physical change is a change in which the atoms or molecules that compose a substance do not change their identity.

• Examples:

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

  • Melting ice: Solid H2O becomes liquid H2O.

Chemical Changes in Matter

Chemical changes result in the formation of new substances with different compositions and properties.

• Definition: A chemical change alters the composition of matter; atoms rearrange to form new substances.

• Example: Rusting of iron: Iron atoms react with oxygen to form iron(III) oxide (Fe2O3).

Writing and Balancing Chemical Reactions

Chemical reactions are represented by chemical equations, which show the reactants and products, their quantities, and physical states.

• Reactants are substances consumed; products are substances formed.

• Example equation: $\mathrm{CH}_4(g) + 2\mathrm{O}_2(g) \rightarrow \mathrm{CO}_2(g) + 2\mathrm{H}_2O(l)$

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

Chemical Equations: Shorthand for Describing a Chemical Reaction

Chemical equations provide essential information about a reaction.

• Molecular or ionic formulas of reactants and products

• Physical states of each substance

• Relative numbers of molecules or moles involved

• Used to calculate masses of reactants and products

The Quantities in Chemical Reactions

The amount of each substance in a chemical reaction is related to the amounts of all other substances, according to the law of conservation of mass.

• Balancing equations: Ensures the same number of each atom on both sides.

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

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	4 O atoms in CO2 + 2 H2O

Practice: Balancing Chemical Equations

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

• Example: Iron(III) oxide and carbon: $\mathrm{Fe}_2\mathrm{O}_3(s) + 3\mathrm{C}(s) \rightarrow 2\mathrm{Fe}(s) + 3\mathrm{CO}_2(g)$

• Example: Combustion of butane: $2\mathrm{C}_4\mathrm{H}_{10}(g) + 13\mathrm{O}_2(g) \rightarrow 8\mathrm{CO}_2(g) + 10\mathrm{H}_2O(g)$

Left	Right
8 C atoms	8 C atoms
20 H atoms	20 H atoms
26 O atoms	26 O atoms

Balancing Equations with Polyatomic Ions

When balancing equations containing polyatomic ions, treat the ion as a unit if it appears unchanged on both sides.

• Example: $3\mathrm{SrCl}_2(aq) + 2\mathrm{Li}_3\mathrm{PO}_4(aq) \rightarrow \mathrm{Sr}_3(\mathrm{PO}_4)_2(s) + 6\mathrm{LiCl}(aq)$

• Balance cations and anions separately, then check total numbers.

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 is the calculation of reactants and products in chemical reactions using balanced equations.

• Definition: Stoichiometry is the numerical relationship between chemical amounts in a reaction.

• Coefficients in a balanced equation indicate the relative amounts in moles.

• Example: $2\mathrm{C}_8\mathrm{H}_{18}(l) + 25\mathrm{O}_2(g) \rightarrow 16\mathrm{CO}_2(g) + 18\mathrm{H}_2O(g)$ 2 mol C8H18 : 25 mol O2 : 16 mol CO2 : 18 mol H2O

Predicting Amounts from Stoichiometry

Amounts of any substance in a reaction can be calculated from the amount of one substance using mole ratios.

• Example: How much CO2 from 22.0 mol C8H18? $22.0\ \text{mol}\ \mathrm{C}_8\mathrm{H}_{18} \times \frac{16\ \text{mol}\ \mathrm{CO}_2}{2\ \text{mol}\ \mathrm{C}_8\mathrm{H}_{18}} = 176\ \text{mol}\ \mathrm{CO}_2$

Mass Calculations in Stoichiometry

To convert between mass and moles, use molar mass. Concept plans help organize multi-step calculations.

• Example: Mass of CO2 from 3.4 × 1015 g C8H18: Concept Plan: g C8H18 → mol C8H18 → mol CO2 → g CO2 $3.4 \times 10^{15}\ \text{g}\ \mathrm{C}_8\mathrm{H}_{18} \times \frac{1\ \text{mol}\ \mathrm{C}_8\mathrm{H}_{18}}{114.22\ \text{g}} \times \frac{16\ \text{mol}\ \mathrm{CO}_2}{2\ \text{mol}\ \mathrm{C}_8\mathrm{H}_{18}} \times \frac{44.01\ \text{g}\ \mathrm{CO}_2}{1\ \text{mol}\ \mathrm{CO}_2}$

Volume Calculations Using Density

To convert mass to volume, use the density of the substance.

• Example: Water produced from glucose combustion: $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6(s) + 6\mathrm{O}_2(g) \rightarrow 6\mathrm{CO}_2(g) + 6\mathrm{H}_2O(l)$ Density of water = 1.00 g/mL Calculation: $9.0\ \text{g}\ \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 \times \frac{1\ \text{mol}\ \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6}{180.0\ \text{g}} \times \frac{6\ \text{mol}\ \mathrm{H}_2O}{1\ \text{mol}\ \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6} \times \frac{18.0\ \text{g}\ \mathrm{H}_2O}{1\ \text{mol}\ \mathrm{H}_2O} \times \frac{1\ \text{mL}}{1.00\ \text{g}} = 5.4\ \text{mL}\ \mathrm{H}_2O$

Limiting Reactant and Excess Reactant

In reactions with multiple reactants, the limiting reactant is the one that is completely consumed first, thus limiting the amount of product formed.

• Limiting reactant: The reactant that determines the maximum amount of product.

• Excess reactant: Reactants not completely consumed.

• Theoretical yield: Amount of product formed from the limiting reactant.

Actual Yield and Percent Yield

Actual yield is the measured amount of product obtained from a reaction, while percent yield expresses the efficiency of the reaction.

• Actual yield: Amount of product actually obtained.

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

Limiting and Excess Reactants in the Combustion of Methane

Example: CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)

• 1 molecule CH4 reacts with 2 molecules O2

• If 5 molecules CH4 and 8 molecules O2 are present, O2 is the limiting reactant (less CO2 can be made from O2 than CH4).

Example: Finding Limiting Reactant, Theoretical Yield, and Percent Yield

Given: 28.6 kg C, 88.2 kg TiO2, 42.8 kg Ti produced Reaction: TiO2(s) + 2 C(s) → Ti(s) + 2 CO(g)

• Step 1: Convert masses to moles using molar masses.

• Step 2: Use stoichiometry to find which reactant produces less Ti (limiting reactant).

• Step 3: Calculate theoretical yield (maximum Ti from limiting reactant).

• Step 4: Calculate percent yield using actual and theoretical yields.

Periodic Table of the Elements

The periodic table organizes elements by atomic number and properties, essential for identifying reactants and products in chemical equations.

Summary Table: Key Stoichiometry Terms

Term	Definition
Physical Change	Change in state or appearance, not composition
Chemical Change	Change in composition; new substances formed
Reactant	Substance consumed in a reaction
Product	Substance formed in a reaction
Stoichiometry	Numerical relationships in chemical reactions
Limiting Reactant	Reactant that limits product formation
Excess Reactant	Reactant not completely consumed
Theoretical Yield	Maximum possible product from limiting reactant
Actual Yield	Measured product obtained
Percent Yield	Efficiency of product recovery

Additional info: These notes cover the essential concepts of chemical reactions, balancing equations, stoichiometry, limiting reactants, and yield calculations, as presented in a college-level General Chemistry course.