Comprehensive Overview of Chemical Reactions

Introduction to Chemical Reactions

In this unit, you will learn:

That substances undergo various chemical changes.
To classify and distinguish these chemical changes.
That matter is neither created nor destroyed in chemical changes.
That energy is exchanged during chemical reactions.
To read, write, and utilize chemical equations.
To design experiments to control chemical reactions.
That chemical reactions are fundamental to life.
That chemical reactions have many industrial applications.

Detecting Chemical Reactions

Methods of Detection

Chemical reactions often change the properties of the substances involved:

Odor: Many organic substances react with oxygen producing detectable aromas.
Temperature: For instance, when sodium bicarbonate (NaHCO₃) reacts with acetic acid (C₂H₄O₂), it produces sodium acetate (C₂H₃O₂Na) and carbon dioxide (CO₂), leading to a noticeable temperature drop.
Color: A reaction between potassium permanganate (KMnO₄) and sugar (C₆H₁₂O₆) in the presence of sodium hydroxide (NaOH) results in a sequence of color changes from purple to green to brown as new substances are formed.
Physical State: A reaction between lead(II) nitrate (Pb(NO₃)₂) and potassium iodide (KI) produces solid yellow lead(II) iodide (PbI₂) from two colorless liquids.

Understanding Chemical Changes

Definition

A chemical change or reaction is defined as a process whereby substances transform into different substances, altering their composition and chemical bonds. The initial reactants have distinct physical and chemical properties compared to the products formed.

Visible vs. Invisible Changes

Visible changes can include alterations in odor, color, state, or temperature.
Invisible changes involve properties such as electrical conductivity or melting point that cannot be detected with the naked eye.
Experiments designed to test reactivity can help reveal these invisible changes.

Law of Conservation of Mass

The total quantity of matter remains unchanged during any chemical or physical change. In a closed system, matter cannot be exchanged with the surroundings, maintaining constant mass. For example:

Open Systems: Can exchange matter with their environment.
Closed Systems: Do not exchange matter with their environment, keeping the mass constant.

Atomic Model and Chemical Reactions

During a chemical change, the total mass must be conserved. This is because:

The number of atoms of each element is unchanged before and after the reaction.
Bonds holding atoms together in molecules are broken.
New bonds form to produce the molecules of the products.

Chemical Equations

A chemical equation symbolically represents a chemical reaction. Left side exhibits reactants, right side shows products, connected by an arrow indicating the reaction direction. Conservation of mass implies:

No atoms are lost or created; coefficients indicate molar ratios.
If substances are gases, coefficients indicate volumetric proportions.

Collision Theory

Effective chemical reactions occur when reactant molecules collide with sufficient energy.

Activation Energy: Minimum energy required for a reaction to occur.
Example: Iron (Fe) reacts with hydrochloric acid (HCl) requiring sufficient energy for the reaction to proceed.

Classification of Chemical Reactions

Chemical reactions can be classified based on different criteria:

By Atom Reorganization

Synthesis Reactions: Two or more reactants combine to form one product.
Decomposition Reactions: A single reactant breaks down into two or more products.
Substitution Reactions: Two or more reactants form multiple different products.

By Subatomic Particle Exchange

Acid-Base Reactions: Involve proton transfer.
Redox Reactions: Involve electron transfer.

By Energy Exchange

Exothermic Reactions: Release energy to surroundings.
Endothermic Reactions: Absorb energy from surroundings.

Acid-Base Reactions

A reaction where one reactant donates protons (acids) and the other accepts them (bases). Typical products include salts and water. The pH scale measures acidity:

pH < 7: Acidic
pH = 7: Neutral
pH > 7: Basic

Redox Reactions

Redox reactions involve the transfer of electrons between substances:

The substance gaining electrons is called the oxidizing agent.
The substance losing electrons is known as the reducing agent.

Energy Exchange in Reactions

Energy can be exchanged as heat in chemical reactions:

Exothermic Reactions: More energy is released than absorbed, thus overall energy is negative.
Endothermic Reactions: Less energy is released than absorbed, thus overall energy is positive.

Reaction Rates

The speed of a chemical reaction is influenced by collision frequency among reactant molecules:

Temperature: Higher temperatures increase molecular movement and collision rates, enhancing reaction speed.
Particle Size: Smaller particles increase surface area, leading to faster reactions.
Concentration: Increased concentration enhances the likelihood of effective collisions and, in turn, reaction velocity.

Catalysts

Catalysts speed up reactions by lowering activation energy. They remain unchanged and are used in small amounts:

Example: Potassium iodide (KI) accelerates the decomposition of hydrogen peroxide (H₂O₂).

Chemical Reactions in Living Organisms and Industry

Chemical reactions are vital for life processes, including:

Photosynthesis: Plants convert carbon dioxide and water to glucose and oxygen using sunlight.
Cellular Respiration: Cells convert glucose and oxygen to energy, producing carbon dioxide and water.
Fermentation: Organisms like yeast convert glucose to different products without oxygen.

In industrial contexts, chemical reactions are employed to produce energy and various products:

Combustion: Fossil fuels release energy during combustion with oxygen, albeit emitting carbon dioxide.
Electrolysis: Uses electric energy to break down compounds, producing elements like hydrogen and chlorine.
Fermentation: Controlled fermentation processes yield lactic acid and ethanol, crucial for food production.

Study Guide on Chemical Reactions

Study Guide on Chemical Reactions