Chemical Reactions and Reaction Stoichiometry (CHEM 200, Chapter 3)

Chemical Reactions and Chemical Equations

Introduction to Chemical Reactions

Chemical reactions are processes in which one or more substances (reactants) are transformed into one or more new substances (products). Chemists use chemical equations to represent these changes on paper, using chemical symbols and formulas to show what happens during a reaction.

• Reactants: The starting materials in a chemical reaction, written on the left side of the equation.

• Products: The substances formed as a result of the reaction, written on the right side of the equation.

• Chemical Equation: A representation using chemical symbols and formulas to show the reactants and products.

• Example: $2H_2 + O_2 \rightarrow 2H_2O$

Balancing Chemical Equations

Law of Conservation of Mass

Balancing chemical equations ensures that the same number of each type of atom appears on both sides of the equation, reflecting the law of conservation of mass.

• Identify each element that appears in the reactants and products.

• Balance by changing coefficients (the numbers in front of formulas), not subscripts (the numbers within formulas).

• Check to make sure the same number of each atom is present on both sides.

• Example: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

Why Add Coefficients Instead of Changing Subscripts?

Coefficients change the amount of a substance, while subscripts change the identity and properties of the substance. Changing subscripts alters the chemical compound itself, which is not allowed when balancing equations.

• Example: $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$ (water)

• Incorrect: $O_2(g) \rightarrow H_2O_2(l)$ (hydrogen peroxide)

• Do not change the formula to balance; only adjust coefficients.

Other Symbols in Chemical Equations

Physical States and Reaction Conditions

Chemical equations often include symbols to indicate the physical state of reactants and products, as well as reaction conditions.

• (s): solid

• (l): liquid

• (g): gas

• (aq): aqueous (dissolved in water)

• The symbol $\Delta$ over the reaction arrow indicates that heat is required for the reaction to take place.

• Example: $CH_4(g) + 2O_2(g) \xrightarrow{\Delta} CO_2(g) + 2H_2O(g)$

How to "Read" Chemical Equations

Interpreting Chemical Equations

Chemical equations can be interpreted in terms of atoms, molecules, moles, and mass.

• Atoms: 2 atoms Mg + 1 molecule O2 → 2 formula units MgO

• Moles: 2 moles Mg + 1 mole O2 → 2 moles MgO

• Mass: 48.6 g Mg + 32.0 g O2 → 80.6 g MgO

• Incorrect: 2 g Mg + 1 g O2 → 2 g MgO (does not reflect correct molar masses)

Stoichiometry: Amounts of Reactants and Products

Introduction to Stoichiometry

Stoichiometry is the quantitative study of reactants and products in a chemical reaction. It allows chemists to predict the amounts of substances consumed and produced.

• Stoichiometric coefficients in a balanced equation indicate the number of moles of each substance.

• If the quantity of a reactant or product is known, it can be used to calculate the quantity of any other substance in the reaction.

• Use moles to calculate the amount of reactants or products.

Stoichiometric Calculations: The Method

1. Write a balanced chemical equation.

2. Convert quantities of known substances into moles.

3. Use coefficients in the balanced equation to calculate the number of moles of the sought quantity.

4. Convert moles of the sought quantity into desired units (mass, volume, etc.).

Summary Table: Physical State Symbols in Chemical Equations

Symbol	Meaning
(s)	Solid
(l)	Liquid
(g)	Gas
(aq)	Aqueous (dissolved in water)
$\Delta$	Heat is required for the reaction

Additional info: These notes are based on CHEM 200, Chapter 3, and cover foundational concepts in chemical reactions and stoichiometry, suitable for General Chemistry college students.