Chemical Reactions and Reaction Stoichiometry: Study Notes

Chemical Reactions and Reaction Stoichiometry

Chemical Equations

Chemical equations are the standard way chemists represent chemical reactions on paper. They provide a concise method to describe the transformation of reactants into products.

• 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 an element that appears in only one reactant and one product.

• Balance by changing coefficients, not subscripts.

• Use trial and error, moving to other elements as needed.

• Check totals for all elements 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, preserving the identity of the substance. Changing subscripts alters the chemical identity and properties of the compound.

• For example, $H_2O$ (water) and $H_2O_2$ (hydrogen peroxide) have different properties due to different subscripts.

• Balanced equations must reflect the correct chemical formulas.

Example:

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

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

Required Symbols in Chemical Equations

States of matter are indicated in parentheses to the right of each 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. Three broad classes are commonly encountered:

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

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

• Combustion reactions: Rapid reactions that produce a flame, usually involving oxygen as a reactant.

Example Table: Types of Chemical Reactions

Type	General Form	Example
Combination	$A + B \rightarrow AB$	$2H_2 + O_2 \rightarrow 2H_2O$
Decomposition	$AB \rightarrow A + B$	$2H_2O \rightarrow 2H_2 + O_2$
Combustion	Hydrocarbon + $O_2 \rightarrow CO_2 + H_2O$	$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

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