Chemical Reactions and Reaction Stoichiometry: Study Notes

Chapter 3: Chemical Reactions and Reaction Stoichiometry

Chemical Equations

Chemical equations are the symbolic representation of chemical reactions, showing the transformation of reactants into products. They are essential for communicating chemical changes and for quantitative calculations in chemistry.

• Reactants: The starting materials, written on the left side of the equation.

• Products: The substances formed, written on the right side.

• Arrow (→): Separates reactants from products, indicating the direction of the reaction.

• Plus sign (+): Separates multiple reactants or products.

Example:

• $2H_2 + O_2 \rightarrow 2H_2O$

Balancing Chemical Equations

Balancing equations ensures the Law of Conservation of Mass is obeyed, meaning the number of atoms of each element is the same on both sides of the equation.

• Start with elements that appear in only one reactant and one product.

• Balance by changing coefficients, not subscripts.

• Use trial and error, adjusting coefficients until all elements are balanced.

• Check totals for each element at the end.

Example:

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

Why Use Coefficients Instead of Changing Subscripts?

Changing coefficients adjusts the number of molecules, while changing subscripts alters the identity of the substance. For example, water ($H_2O$) and hydrogen peroxide ($H_2O_2$) have different properties and compositions.

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

• $H_2(g) + O_2(g) \rightarrow H_2O_2(l)$

Required Symbols in Chemical Equations

States of matter are indicated in parentheses after each chemical formula:

• (g): gas

• (l): liquid

• (s): solid

• (aq): aqueous (dissolved in water)

Example:

• $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$

Simple Patterns of Chemical Reactivity

Chemical reactions can be classified into several types based on their patterns. Three broad classes are commonly encountered:

• Combination reactions: Two or more substances combine to form one product.

• Decomposition reactions: One substance breaks down into two or more products.

• Combustion reactions: Rapid reactions with oxygen that produce a flame.

Example of Combination Reaction:

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

Example of Decomposition Reaction:

• $2H_2O_2(l) \rightarrow 2H_2O(l) + O_2(g)$

Example of Combustion Reaction:

• $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$

Additional info: These notes are based on textbook slides and cover foundational concepts in chemical equations and reaction types, suitable for General Chemistry students.