Chemical Reactions and Changes

Physical vs. Chemical Changes

Understanding the difference between physical and chemical changes is fundamental in chemistry. These changes affect substances in different ways:

• Physical Change: A change that alters a substance without changing its chemical identity.

  • Examples: Boiling, melting, vaporizing, grinding.

• Chemical Change: A process where a substance is transformed into a different substance through a chemical reaction.

  • Examples: Burning, oxidizing, rotting, fermenting.

Chemical Reaction: The process by which substances (reactants) interact to form new substances (products) through the breaking and forming of chemical bonds.

Collision Theory

Collision theory explains how and why chemical reactions occur:

• Reacting particles must collide for a reaction to occur.

• Collisions must have sufficient energy to break bonds and form new ones.

• Not all collisions result in a reaction; only those with enough energy (activation energy) do.

• The rate of reaction depends on the frequency and energy of collisions.

Evidence of Chemical Reactions

Chemical changes are often indicated by observable signs:

• Energy change: Release or absorption of heat, light, or a sudden temperature change.

• Color change: A new color appears unexpectedly.

• Odor change: A new smell is produced.

• Gas release: Bubbles or fumes are observed.

• Formation of a precipitate: A solid forms from a solution.

Precipitate: The solid that forms and separates from a solution during a chemical reaction.

Law of Conservation of Mass

Definition and Application

The Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction; it can only change forms.

• The mass of reactants equals the mass of products.

• The number of atoms of each element is conserved.

Example: If 25 g of reactants are used, 25 g of products must be formed. If there are 2 oxygen atoms in the reactants, there must be 2 oxygen atoms in the products.

This principle is the basis for balancing chemical equations.

Writing and Interpreting Chemical Equations

Notation and Structure

Chemical equations represent chemical reactions using symbols and formulas:

• Word equations: Describe the reaction in words. Example: Hydrogen gas and oxygen gas react to form liquid water.

• Formula equations: Use chemical formulas and symbols. Example:

Symbols used in equations:

• + : Separates multiple reactants or products.

• \rightarrow : Indicates 'yields' or 'produces' (sometimes shown as an arrow).

• (s) : Solid state

• (l) : Liquid state

• (g) : Gaseous state

• (aq) : Aqueous (dissolved in water)

Reactants: Starting substances (ingredients). Products: Substances formed (results).

Coefficients and Subscripts

• Coefficient: Number in front of a formula indicating the number of molecules or moles. Coefficients can be changed to balance equations.

• Subscript: Number within a formula indicating the number of atoms in a molecule. Subscripts cannot be changed when balancing equations.

• If only one molecule is present, the coefficient '1' is not written.

Example: In , the ratio of sodium to chlorine is 2:1.

States of Matter in Equations

• (s): Solid

• (l): Liquid

• (g): Gas

• (aq): Aqueous (dissolved in water)

Examples of Writing Equations

• Word to Formula: Sodium chloride reacts with silver nitrate to produce silver chloride and sodium nitrate.

• Formula to Words: Lithium chloride and oxygen gas react to make lithium chlorate.

• State Notation Example: Aqueous aluminum bromide reacts with aqueous potassium sulfate to produce solid potassium bromide and aqueous aluminum sulfate.

Balancing Chemical Equations

Why Balance Equations?

Balancing equations ensures the Law of Conservation of Mass is not violated. Only coefficients are changed to balance equations; subscripts must remain unchanged.

Steps to Balance a Chemical Equation

1. Write the unbalanced equation.

2. List each element (or polyatomic ion) under reactants and products.

3. Count the number of atoms of each element on both sides.

4. Change coefficients to balance the number of atoms for each element.

5. Reduce coefficients to the lowest whole number ratio.

Balancing Examples

Example 1:

Balanced equation:

Example 2:

Balanced equation:

Summary Table: Key Notation in Chemical Equations

Key Points to Remember

• Only coefficients can be changed to balance equations.

• Subscripts represent the chemical identity and must not be altered.

• Balanced equations reflect the conservation of both mass and atoms.

Additional info: In practice, polyatomic ions that appear unchanged on both sides of the equation can be balanced as single units to simplify the process.