Composition+of+substances+and+solutions

Composition of Substances and Solutions

Elements and the Periodic Table

Key topics include understanding elements, their properties, and how they are organized in the periodic table. Emphasis on interactive and applied knowledge through case studies.

Learning Objectives

• Properties of Physical States: Describe properties of solids, liquids, and gases, including characteristics such as volume, shape, and compressibility.

• Domains of Matter: Illustrate macroscopic, microscopic, and symbolic domains, with examples of how each perspective contributes to the understanding of matter.

• Classification of Matter: Classify matter as an element, compound, homogeneous mixture, or heterogeneous mixture based on physical state and composition, with examples like water as a compound and air as a homogeneous mixture.

• Atoms and Molecules: Define atoms and molecules with examples, exploring the difference between the two through further definitions and common substances.

• Properties and Changes: Identify properties and changes in matter as physical or chemical, detailing how these transformations can be observed in various chemical reactions.

Composition of Matter: Atoms

• Description: All things consist of atoms, which are perpetually moving particles, demonstrating kinetic energy even in solid forms.

• Quote: Richard Feynman described atoms as particles that attract when at a distance and repel when close, illustrating fundamental forces.

• Historical Context: The idea of atoms dates back to Greek philosophers like Leucippus and Democritus, later supported by John Dalton in the 19th century, emphasizing the historical development of scientific thought.

Dalton's Atomic Theory

1. All matter comprises extremely small particles known as atoms.

2. Atoms are indivisible; they cannot be created or destroyed in chemical reactions.

3. Atoms of the same element are identical in size, mass, and properties.

4. Different elements have distinct atoms sizes and weights.

5. Atoms combine in simple, whole-number ratios to form compounds.

6. Atoms are rearranged in chemical reactions.

Models of the Atom

Historical Models

• Solid Sphere Model (Dalton): Atoms are indivisible, resembling solid spheres.

• Plum Pudding Model (Thomson): Atoms contain electrons within a positive charge matrix, leading to the discovery of subatomic particles.

• Nuclear Model (Rutherford): Atoms have a small, dense nucleus with protons and neutrons, refining the understanding of atomic structure.

• Planetary Model (Bohr): Electrons orbit the nucleus in fixed energy levels, introducing quantum concepts.

• Quantum Model (Schrödinger): Electrons exist in 'clouds of probability', highlighting the uncertainty principle of quantum mechanics.

Structure of the Atom

Component Properties

• Nucleus: Contains most of the atom’s mass and is comprised of protons and neutrons, fundamentally defining the identity of the element.

• Electrons: Negatively charged particles occupying most of the atom’s volume; they possess a negligibly small mass relative to protons and neutrons, influencing chemical behavior.

Atomic Mass Units

• Definition: Atomic mass unit (amu) is a unit of mass used to express atomic and molecular weights.

• Calculation: 1 amu = 1.6605 × 10⁻²⁴ g;

• Example: Carbon-12 has a defined mass of exactly 12 amu, providing a standard reference for atomic mass calculations.

• Moles: Understanding moles connects mass with the number of particles through Avogadro’s number, facilitating stoichiometric calculations in chemistry.

Subatomic Particle Properties

Atomic Number and Mass Number

• Atomic Number (Z): Number of protons in an atom, equals the number of electrons in a neutral atom, defining the element's identity.

• Mass Number (A): Total number of protons and neutrons in an atom, relevant for understanding isotopes.

Chemical Elements Arrangement in Periodic Table

• Elements are organized in columns (groups) and rows (periods). Groups exhibit similar chemical properties based on valence electron configuration; periods show a trend in properties related to increasing atomic number.

• Ion is a charged atom. Atoms (and molecules) typically acquire charging by gaining or losing electrons.

• Molecules consist of 2 or more atoms of different elements

The law of definite proportions/law of constant composition

• All samples of a pure compound contain the same elements in the same proportion by mass.

• When two elements react to for, more than one compound, a fixed mass of one element will react with masses of the other element in a ratio of small, whole numbers.

Chemical Formulae

• Molecular Formula is a representation of a molecule that uses chemical symbols to indicate the type of atoms followed by subscripts to show the number of atoms of each type in that molecule.

• Structural Formula for a compound gives the same information as it molecular formula but also shows how the atoms are connected in the molecule

• Empirical Formula shows the types of atoms present and the simplest whole number ratio of the number of atoms (or ions) in the compound.

Deprivation of molecular formulae

• A compounds molecular formula can be determined from its empirical formula and its molecular or molar mass.

• (molecular or molar mass/ empirical formula mass) = n formula unit/molecules

• The molecular formula is then obtained by multiplying each subscript in the empirical formula by n.

Isotopes

Atoms sharing the same element with different atomic masses; these have the same number of protons but different neutrons, affecting their stability and reactions. Frederick Soddy received a Nobel Prize for his work on isotopes in 1921, which advanced nuclear chemistry.

Key Concepts of the Periodic Table

• Chemical Reactivity: Elements in the same group display similar reactions, providing insights into predicting chemical behavior.

• Influence of Position: Placement in the periodic table affects chemical behavior, including electronegativity, ionization energy, and atomic radius.

Composition of Solutions and Molarity

Case Study: Contamination

• Example Event: A sulphuric acid spill affecting waterways, underscoring the urgency of chemical management and cleanup regarding environmental safety and human health.

Formula Mass

• Definition: Total of average atomic masses in a compound.

• Example: Chloroform (CHCl3) has a formula mass of 119.37 amu, used in various applications and demonstrating molecular weight calculations.

Mole Concept

• Definition: The mole is a measurement representing a specific number (6.022 × 10²³) of discrete entities such as atoms or molecules, forming the basis for quantitative chemical analysis.

Molar Mass

• Definition: Mass of one mole expressed in grams per mole (g/mol), essential for converting between grams and moles in stoichiometric calculations.

Molarity

• Definition: Molarity (M) is defined as the number of moles of solute in 1 liter of solution, a crucial concentration measure in chemical reactions and solution chemistry.

• M = mol solute / L solution

Solutions

• Solvents: component with a concentration that is significantly greater than that of all other components

• Solute: component that is typically present at much lower concentration than the solvent

• A solution in which water is the solvent is called an aqueous solution

Dilution Process

• Definition: Reducing concentration by adding more solvent.

• The molar amount of solute (n) in a solution is equal to the product of the solutions molar its and its volume in litres - n=ML

• Equation: M1L1 = M2L2 (before and after dilution), facilitating calculations for preparing solutions of desired concentrations.

Concentration Units

Various ways to express concentration include:

• Mass Percentage: Mass of component / Mass of solution × 100%.

• Volume Percentage: Volume of solute / Volume of solution × 100%.

• Parts per Million/Billion: Ver low solute concentration are lofted expressed using appropriately small units such as parts per million (ppm) or parts per billion (ppm)

• Ppm = mass of solute / mass of solution x 10^6

• Ppb = mass of solute / mass of solution x 10^9

Isomers and Their Properties

• Isomers: Compounds with the same formula but different structural arrangements (e.g., acetic acid vs. methyl formate), affecting their chemical behavior and properties.

• Spatial Isomers: Differences in three-dimensional orientations in space, significantly influencing the physical and chemical properties of compounds.

Avogadro's Number: Avogadro's number is defined as 6.022 × 10²³, which represents the number of constituent particles (usually atoms or molecules) in one mole of a substance. This fundamental constant is key in bridging the gap between the atomic scale and everyday quantities, allowing to perform calculations involving the amounts of substances in chemical reactions.