Lab Exam 2 Study Guide: Chemical Reactions, Lewis Structures, and Molar Mass Determination

Chemical Reactions

Types of Chemical Reactions

Chemical reactions involve the transformation of substances through the breaking and forming of chemical bonds. Understanding the main types of reactions is essential for predicting products and balancing equations.

• Combustion Reaction: A reaction in which a substance combines with oxygen, releasing energy in the form of heat and light. Typically, hydrocarbons react with O2 to produce CO2 and H2O. Example: $\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}$

• Decomposition Reaction: A single compound breaks down into two or more simpler substances. Example: $\mathrm{2H_2O_2 \rightarrow 2H_2O + O_2}$

• Combination (Synthesis) Reaction: Two or more substances combine to form a single product. Example: $\mathrm{2H_2 + O_2 \rightarrow 2H_2O}$

• Single Displacement Reaction: An element replaces another element in a compound. Example: $\mathrm{Zn + 2HCl \rightarrow ZnCl_2 + H_2}$

Balancing Chemical Equations: To satisfy the law of conservation of mass, the number of atoms of each element must be the same on both sides of the equation.

• Identify reactants and products.

• Write the unbalanced equation.

• Adjust coefficients to balance atoms.

Word Equations: These describe reactions using words instead of chemical formulas. For example, "hydrogen reacts with oxygen to form water."

Lewis Structures and Molecular Shapes

Drawing Lewis Structures

Lewis structures represent the arrangement of valence electrons in molecules and ions. They help predict molecular shape and reactivity.

• Valence Electrons: Electrons in the outermost shell of an atom, involved in bonding.

• Steps to Draw Lewis Structures:

  1. Count total valence electrons for all atoms.

  2. Arrange atoms (central atom is usually the least electronegative).

  3. Connect atoms with single bonds (pairs of electrons).

  4. Distribute remaining electrons to complete octets (or duets for H).

  5. Use double or triple bonds if necessary to satisfy octet rule.

• Example: For $\mathrm{CO_2}$:

  • Carbon: 4 valence electrons

  • Oxygen: 6 valence electrons each

  • Total: $4 + 6 \times 2 = 16$ electrons

  • Lewis structure: $\mathrm{O=C=O}$

Molecular Shapes (VSEPR Theory)

The shape of a molecule is determined by the number of electron pairs around the central atom, according to the Valence Shell Electron Pair Repulsion (VSEPR) theory.

• Linear: 2 electron groups, bond angle 180° (e.g., CO2)

• Trigonal Planar: 3 electron groups, bond angle 120° (e.g., BF3)

• Tetrahedral: 4 electron groups, bond angle 109.5° (e.g., CH4)

• Bent: 2 bonds + 1 or 2 lone pairs (e.g., H2O)

Example: Water ($\mathrm{H_2O}$) has a bent shape due to two lone pairs on oxygen.

Determination of Molar Mass

Calculating Molar Mass

Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). It is calculated by summing the atomic masses of all atoms in a formula.

• Formula: $\mathrm{Molar\ Mass = \sum (atomic\ mass\ of\ each\ element \times number\ of\ atoms)}$

• Example: For $\mathrm{H_2O}$:

  • H: 1.008 g/mol × 2 = 2.016 g/mol

  • O: 16.00 g/mol × 1 = 16.00 g/mol

  • Total molar mass = 18.016 g/mol

Experimental Determination: Molar mass can be determined by measuring the mass and amount (moles) of a substance produced or consumed in a chemical reaction.

Summary Table: Types of Chemical Reactions

Reaction Type	General Form	Example
Combustion	$\mathrm{Fuel + O_2 \rightarrow CO_2 + H_2O}$	$\mathrm{C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O}$
Decomposition	$\mathrm{AB \rightarrow A + B}$	$\mathrm{2KClO_3 \rightarrow 2KCl + 3O_2}$
Combination	$\mathrm{A + B \rightarrow AB}$	$\mathrm{2Na + Cl_2 \rightarrow 2NaCl}$
Single Displacement	$\mathrm{A + BC \rightarrow AC + B}$	$\mathrm{Zn + 2HCl \rightarrow ZnCl_2 + H_2}$

Additional info: The above notes expand on the exam topics and provide academic context for each, including definitions, examples, and formulas relevant to Introduction to Chemistry.