Definition of Chemistry

The Scientific Method

Chemistry is the study of matter, its properties, and the changes it undergoes. The scientific method is a systematic approach used in scientific study to ensure objective and repeatable results.

• Observation: Gathering information using the senses or instruments.

• Hypothesis: A tentative explanation for observations, which can be tested.

• Law: A statement that summarizes a vast number of experimental observations; for example, the Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction.

• Theory: A well-substantiated explanation of some aspect of the natural world that can incorporate laws, hypotheses, and facts.

Example: The Law of Conservation of Mass: In a closed system, the total mass before and after a chemical reaction remains constant.

Measurement and Problem Solving

Scientific Notation and Significant Figures

Measurements in chemistry require precision and accuracy. Scientific notation expresses numbers as a product of a coefficient and a power of ten, making it easier to handle very large or small numbers.

• Significant Figures: Digits in a measurement that are known with certainty plus one estimated digit. They reflect the precision of a measurement.

• Writing Numbers to Reflect Precision: The number of significant figures indicates the reliability of a measurement.

• Measuring Instruments: The precision of a measurement depends on the instrument used (e.g., ruler, balance, graduated cylinder).

Significant Figures in Calculations

• Multiplication/Division: The result should have as many significant figures as the measurement with the fewest significant figures.

• Addition/Subtraction: The result should have as many decimal places as the measurement with the fewest decimal places.

Units of Measurement

• SI Units: The International System of Units includes the meter (m), kilogram (kg), second (s), mole (mol), kelvin (K), ampere (A), and candela (cd).

• SI Prefix Multipliers: Prefixes such as kilo- (103), centi- (10-2), and milli- (10-3) are used to express multiples or fractions of units.

Derived Units and Density

• Derived Units: Units that are combinations of SI base units, such as density (kg/m3 or g/cm3).

• Density Formula:

Conversion Units

• Unit conversions use conversion factors to change from one unit to another.

• Example: Converting 10 cm to meters:

Matter and Energy

Classifying Matter

• By State: Solid (fixed shape and volume), liquid (fixed volume, variable shape), gas (variable shape and volume).

• By Composition: Pure substances (elements and compounds) and mixtures (homogeneous and heterogeneous).

Physical and Chemical Changes

• Physical Change: Alters the state or appearance but not composition (e.g., melting ice).

• Chemical Change: Alters the composition, producing new substances (e.g., burning wood).

Physical and Chemical Properties

• Physical Properties: Can be observed without changing the substance (e.g., color, melting point).

• Chemical Properties: Describe the ability to undergo chemical changes (e.g., flammability).

Energy, Temperature, and Heat

• Energy: The capacity to do work or transfer heat. Measured in joules (J) or calories (cal).

• Temperature Scales: Celsius (°C), Kelvin (K), Fahrenheit (°F).

• Heat: The transfer of energy due to temperature difference.

• Specific Heat Capacity: The amount of heat required to raise the temperature of 1 gram of a substance by 1°C.

• Where = heat (J), = mass (g), = specific heat (J/g·°C), = change in temperature (°C).

Atoms and Elements

Structure of the Atom

• Subatomic Particles: Protons (positive, in nucleus), neutrons (neutral, in nucleus), electrons (negative, outside nucleus).

• Atomic Number (Z): Number of protons in the nucleus.

• Mass Number (A): Total number of protons and neutrons.

• Isotopes: Atoms of the same element with different numbers of neutrons.

• Atomic Mass: Weighted average mass of all isotopes of an element.

Periodic Table

• Organizes elements by increasing atomic number and similar properties.

• Groups: Columns with similar chemical properties.

• Periods: Rows indicating energy levels.

Ions

• Cations: Positively charged ions (loss of electrons).

• Anions: Negatively charged ions (gain of electrons).

Molecules and Compounds

Molecular View and Polyatomic Ions

• Molecules: Two or more atoms bonded together.

• Compounds: Substances composed of two or more elements in fixed ratios.

• Polyatomic Ions: Ions composed of multiple atoms (e.g., , ).

Writing and Naming Compounds

• Formulas for Ionic Compounds: Combine cations and anions in ratios that yield a neutral compound.

• Naming Ionic Compounds: Type I (fixed charge metals), Type II (variable charge metals), and those containing polyatomic ions.

• Naming Molecular Compounds: Use prefixes (mono-, di-, tri-, etc.) to indicate number of atoms.

• Naming Acids: Binary acids (hydrogen + nonmetal), oxyacids (hydrogen + polyatomic ion containing oxygen).

Formula Mass

• The sum of the atomic masses of all atoms in a chemical formula.

Chemical Composition

Mole Concept and Avogadro's Number

• Mole: Amount of substance containing entities (Avogadro's number).

• Conversions: Atoms ↔ Moles ↔ Mass; Molecules ↔ Moles ↔ Mass.

Chemical Formulas as Conversion Factors

• Use subscripts in formulas to relate moles of elements to moles of compounds.

Mass Percent Composition

• Percentage by mass of each element in a compound.

Empirical Formulas

• Lowest whole-number ratio of elements in a compound.

• Can be determined from percent composition.

Chemical Reactions

Balancing Chemical Equations

• Ensure the same number of each atom on both sides of the equation.

Solubility

• Describes how much of a substance can dissolve in a solvent at a given temperature.

Quantities in Chemical Reactions

Stoichiometry

• Relates quantities of reactants and products using balanced equations.

• Mole-to-mole conversions: Use coefficients from balanced equations.

• Mass-to-mass conversions: Convert mass to moles, use stoichiometry, then convert back to mass.

Limiting Reactant and Yields

• Limiting Reactant: The reactant that is completely consumed first, limiting the amount of product formed.

• Theoretical Yield: Maximum amount of product possible from given reactants.

• Actual Yield: Amount of product actually obtained.

• Percent Yield:

Enthalpy and Reaction Stoichiometry

• Enthalpy Change (): Heat absorbed or released during a chemical reaction at constant pressure.

Electrons in Atoms and the Periodic Table

Light and Electromagnetic Radiation

• Wavelength (): Distance between two consecutive peaks.

• Frequency (): Number of waves passing a point per second.

• Energy (): where is Planck's constant.

Quantum Numbers and Electron Configuration

• Principal Quantum Number (n): Indicates energy level.

• Subshells: s, p, d, f (types of orbitals).

• Electron Configuration: Distribution of electrons among orbitals.

Chemical Bonding

Lewis Structures and Molecular Shapes

• Lewis Structures: Diagrams showing valence electrons and bonding in molecules and ions.

• Predicting Shapes: Use VSEPR theory to predict molecular geometry.

• Electronegativity: Tendency of an atom to attract electrons in a bond.

• Polarity: Distribution of electrical charge over atoms joined by a bond.

Gases

Gas Laws

• Boyle's Law: (at constant T and n)

• Charles's Law: (at constant P and n)

• Avogadro's Law: (at constant P and T)

• Combined Gas Law:

• Ideal Gas Law:

• Molar Mass of a Gas:

Solutions

Concentration Units

• Mass Percent:

• Molarity (M):

• Solution Dilution:

Example: To prepare 250 mL of 0.50 M NaCl from a 2.0 M stock solution: ;