Fundamental Definitions, Changes, and Energy in Chemistry
Fundamental Definitions of Matter and Chemistry
Chemistry: The scientific study of matter and its properties, the changes that matter undergoes, and the energy associated with those changes.
Matter: The physical material of the universe; anything that possesses mass and volume (e.g., air, glass, planets, students).
Composition: The types and amounts of simpler substances that make up a sample of matter.
Substance: A specific type of matter that has a defined, fixed composition.
The Physical States of Matter
Overview: Matter commonly exists in three distinct physical forms known as states: solid, liquid, and gas.
Macroscopic Characteristics:
Solid: Possesses a fixed shape that does not conform to the shape of its container. Solids are not defined by hardness or rigidity:
Solid iron is rigid and hard.
Solid lead is flexible.
Solid wax is soft.
Liquid: Possesses a varying shape that conforms to the container shape, but only to the extent of the liquid's volume; a liquid forms an upper surface.
Gas: Possesses a varying shape that conforms to the container shape and expands to fill the entire container; a gas does not form a free surface.
Atomic-Scale Characteristics:
Solid State: Particles lie directly next to each other in a regular, three-dimensional array or repeating pattern.
Liquid State: Particles lie close together but move randomly around one another.
Gas State: Particles are separated by large distances and move randomly throughout the entire volume of the container.
Physical and Chemical Properties
Properties: Unique characteristics that provide a substance with its distinct identity, analogous to identifying features in humans (e.g., height, weight, hair color, eye color, fingerprints, DNA patterns).
Physical Properties:
Characteristics shown by a substance by itself, without changing into or interacting with another substance.
Examples include color, melting point, electrical conductivity, and density.
Chemical Properties:
Characteristics shown by a substance as it transforms into or interacts with another substance or group of substances.
Examples include flammability, corrosiveness, and reactivity with acids.
Characteristic Properties of Copper:
Physical Properties:
Malleability: Easily hammered into thin sheets.
Ductility: Easily drawn into wires.
Thermal/Phase behavior: Can be melted and mixed with zinc to produce brass.
Chemical Properties:
Slowly forms a blue-green carbonate layer when exposed to moist air.
Reacts directly with nitric acid or sulfuric acid.
Slowly forms a deep-blue solution when placed in aqueous ammonia.
Physical and Chemical Changes
Physical Change:
Occurs when a substance alters its physical properties without altering its chemical composition.
The chemical composition remains identical before and after the change.
All state changes (phase transitions) are physical changes.
Example: The melting of ice to liquid water alters hardness, density, and flowability, but the chemical particles remain identical water molecules. In ice, particles sit in a solid array; in liquid water, particles are jumbled, but their intrinsic chemical structure is unchanged.
Chemical Change:
Occurs when one or more original substances are converted into one or more new substances possessing different compositions and unique properties.
Also referred to as a chemical reaction.
Example: Passing an electric current through liquid water decomposes it into two distinct substances, gaseous hydrogen and gaseous oxygen. The final sample is no longer water.
Thermal Reversibility:
Physical changes induced by heating can generally be reversed by cooling (e.g., heating solid iron in a steel mill forms molten liquid iron, which returns to solid iron upon cooling).
Chemical changes induced by heating generally cannot be reversed simply by cooling (e.g., heating iron in moist air produces brown, crumbly rust; cooling rust does not recreate iron, requiring a separate sequence of chemical reactions).
Sample Problems: Classifying and Visualizing Changes
Sample Problem 1.1: Visualizing Change on the Atomic Scale:
Problem Statement: Evaluate atomic-scale depictions of sample converting to sample (left) and sample (right) to determine whether physical or chemical changes occurred.
Initial State (Sample A): Particles each consist of one blue sphere bonded to two red spheres.
Transformation to Sample B: The original particles separate into two new particle types—one composed of one red sphere bonded to one blue sphere, and another composed of two red spheres bonded together. Because particle composition changes, transformation is a chemical change.
Transformation to Sample C: Particles remain composed of one blue sphere bonded to two red spheres, but are packed closer together in an ordered, regular pattern. Because particle composition is unaltered, transformation is a physical change.
Sample Problem 1.2: Distinguishing Between Physical and Chemical Changes:
(a) Frost formation on a humid winter night: Physical change. Water vapor in humid air condenses into solid ice crystals as temperature drops (change of state).
(b) Cornstalk growth from a watered and fertilized seed: Chemical change. The seed incorporates water, atmospheric components, soil minerals, fertilizer, and solar energy to synthesize complex chemical compounds.
(c) Match ignition forming ash and gases: Chemical change. Combustible substances within the match head chemically convert into distinct combustion products (ash and gaseous products).
(d) Perspiration evaporation during rest: Physical change. Liquid water in sweat undergoes a phase transition to gaseous water vapor without composition change.
(e) Tarnishing of a silver fork in air: Chemical change. Elemental silver reacts with atmospheric sulfur-containing compounds to yield silver sulfide.
Follow-Up Problem Set 1.2A:
(a) Purple iodine vapor appearing when solid iodine is warmed: Physical change (sublimation / state change).
(b) Gasoline fumes ignited by a spark in an engine cylinder: Chemical change (combustion).
(c) Boiling sugar to form caramel: Chemical change (thermal decomposition / caramelization).
Follow-Up Problem Set 1.2B:
(a) Cloud formation in the sky: Physical change (condensation of water vapor).
(b) Sour milk formation: Chemical change (biochemical fermentation / spoilage).
(c) Melting butter for popcorn: Physical change (phase change from solid to liquid).
The Central Theme in Chemistry
Core Theme Statement: Macroscopic-scale properties and behavior (phenomena that are directly observable) are the direct result of atomic-scale properties and behavior (phenomena occurring among particles that cannot be directly observed).
Macroscopic vs. Submicroscopic Connection:
Macroscopic composition studies depend fundamentally upon particle structure at the atomic scale.
Macroscopic states of matter arise directly from the spatial distribution and motion of microscopic particles.
Primary Chemical Objective: Observable physical and chemical changes in matter are investigated to uncover and understand their underlying, unobservable submicroscopic causes.
Energy in the Study of Matter
Fundamental Definitions:
Energy: The capacity or ability to do work.
Work: The displacement of an object against an opposing force (moving an object).
Energy Transfer: An object performing work transfers a portion of its stored energy to the object undergoing displacement.
Forms of Energy:
Potential Energy (): Energy resulting from the position of an object relative to other objects.
Kinetic Energy (): Energy resulting from the motion of an object.
Total Energy: The comprehensive sum of potential and kinetic energy in a system:
Governing Principles of Energy:
Conservation of Energy: When energy converts between different forms, total energy is strictly conserved; energy cannot be created or destroyed.
Energy and Stability: Systems possessing lower potential energy are inherently more stable and thermodynamically favored over systems possessing higher potential energy.
Illustrative Energy Systems:
Gravitational System (Elevated Weight):
Lifting a weight against gravitational force increases its potential energy.
Releasing the elevated weight causes potential energy to convert into kinetic energy as it falls.
The high potential energy state (elevated position) is less stable; the fallen state possesses lower potential energy and higher stability.
Mechanical System (Two Balls Connected by a Spring):
Stretching a relaxed spring requires energy input, elevating system potential energy.
Releasing the stretched spring converts stored potential energy into kinetic energy as the balls move together.
The stretched state represents higher potential energy (less stable) than the relaxed spring state (lower potential energy, more stable).
Electrostatic System (Charged Particles):
Electrostatic forces dictate that opposite electrical charges attract, whereas like electrical charges repel.
Separating oppositely charged particles requires energy, raising the potential energy of the system.
Releasing separated opposite charges converts stored potential energy to kinetic energy as attraction pulls them together.
Forcing like charges closer together increases potential energy; releasing them converts potential energy into kinetic energy as repulsive forces push them apart.
Chemical System (Fuel and Combustion Products):
Matter consists of subatomic particles bearing positive and negative charges.
Chemical Potential Energy: Results from the relative spatial positions of particles and the net attractive and repulsive forces acting among them.
Reactants such as gasoline and oxygen possess higher chemical potential energy than their combustion products (exhaust gases).
When fuel burns, the difference in chemical potential energy converts into kinetic energy, propelling a vehicle, supplying thermal energy, and operating electrical components.
Nutritional fuels (food and oxygen) possess higher chemical potential energy than excreted metabolic wastes; this potential energy difference converts into kinetic energy enabling bodily movement, physiological growth, body temperature regulation, and cognitive activity.