Fundamental Concepts in Chemistry

Chemistry Fundamentals

Physical Changes

Affect only physical properties like physical state. They do not alter the nature of the substance. Example: Frozen and liquid water have the same composition; nothing changes chemically.

Chemical Changes

Occur when there is a change in the composition of matter. Example: Paper converting into a gas (combustion).

Indicators of Chemical Change

• A gas is produced (bubbles are formed).
• There is a change in temperature due to heat exchange between the environment and reacting substances.
• A precipitate is formed (an insoluble solid that appears as a result of the reaction between solutions).
• The color changes.

Related Concepts

Heat: The interchange of thermal energy between objects or substances having different temperatures.

Temperature: A measure of the movement of particles.

Chemical Equation

A representation of a chemical reaction.

Example: H₂(g) + O₂(g) → H₂O(l)

Chemical Reaction

A process in which one or more pure substances are converted into one or more different pure substances. A loss or gain of matter never takes place.

Collision Theory

Particles are in movement to a greater or lesser degree, collide with kinetic energy, and break their bonds. (Catalan note: bonds of reactants break upon collision of reactant particles)

Why is Energy Needed for a Reaction?

If there isn't enough kinetic energy, the bonds of reactants do not break upon collision, and the reaction doesn't start.

Related Terms

• Yield, Go to, Produce: Terms often used to describe the formation of products.
• Reactants: Substances on the left side of a chemical equation.
• Subscript: Indicates the number of atoms of an element in a molecule.
• Physical State: Indicated by symbols like (g) for gas, (l) for liquid, (s) for solid, (aq) for aqueous solution.

Energy for Product Bonds

The energy needed to form the new bonds of the products comes from the breaking of the bonds of the reactants.

Stoichiometric Coefficients

Stoichiometric coefficients indicate the proportion of reactants and products involved in a reaction.

Balancing Chemical Equations

Balancing a chemical equation means ensuring the same number of atoms of each element are present on both the reactant and product sides.

Law of Conservation of Mass

The mass of reactants must equal the mass of products. All atoms present on the reactant side of an equation are also present on the product side; atoms are neither lost nor gained.

Exothermic Reactions

Energy is released into the surroundings. This occurs when the energy released during the formation of product bonds is greater than the energy required to break reactant bonds.

Endothermic Reactions

Energy is absorbed from the surroundings. This occurs when the energy required to break reactant bonds is greater than the energy released during the formation of product bonds.

Factors Affecting Reaction Rate

The speed of a reaction can be changed by modifying certain conditions:

• Temperature: Reaction rate increases when temperature increases.
• Concentration: Higher concentration means more reactant particles per unit volume, leading to more collisions.
• Pressure of Gaseous Reactants: Increasing pressure brings gas particles closer, resulting in more collisions.
• Particle Size of Reactants: Smaller particle size means a larger surface area exposed for reaction.
• Use of Catalysts: A catalyst is a substance that speeds up a reaction without being consumed in the process.
• Solution pH: pH can affect reaction rate, especially in reactions involving acids or bases.

Why Temperature Affects Rate

Increasing temperature increases the kinetic energy of particles, leading to more frequent and energetic collisions among reactants.