🧪AP Chemistry AP Cram Sessions 2020

Key Concepts and Theories

• Matter consists of atoms, the fundamental building blocks of all substances
• Elements are pure substances composed of only one type of atom, while compounds are made up of two or more elements chemically combined in a specific ratio
• Atoms are composed of protons (positively charged), neutrons (neutral), and electrons (negatively charged)
• The atomic number represents the number of protons in an atom, which determines its identity as a specific element
• Isotopes are atoms of the same element with different numbers of neutrons, resulting in varying atomic masses
• The mole is a unit of measurement used to express amounts of a substance, equal to 6.02 × 10^23 particles (Avogadro's number)
  • Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol)
• Chemical reactions involve the rearrangement of atoms to form new substances, with the total number of atoms conserved throughout the reaction

Atomic Structure and Periodic Trends

• Atoms consist of a dense, positively charged nucleus containing protons and neutrons, surrounded by negatively charged electrons in specific energy levels (shells)
• The electron configuration of an atom describes the arrangement of electrons in its orbitals, following the Aufbau principle, Hund's rule, and the Pauli exclusion principle
  • Valence electrons, those in the outermost shell, determine an atom's chemical properties and bonding behavior
• The periodic table organizes elements based on their atomic number and electron configuration, revealing trends in properties such as atomic radius, ionization energy, and electronegativity
  • Atomic radius generally decreases from left to right across a period and increases down a group
  • Ionization energy, the energy required to remove an electron from an atom, generally increases from left to right and decreases down a group
• Electronegativity, the ability of an atom to attract electrons in a chemical bond, increases from left to right and decreases down a group
• Metals, located on the left side of the periodic table, tend to have low ionization energies and electronegativities, forming cations (positively charged ions) when bonding
• Nonmetals, located on the right side, have high ionization energies and electronegativities, forming anions (negatively charged ions) when bonding

Chemical Bonding and Molecular Geometry

• Chemical bonds form when atoms share or transfer electrons to achieve a more stable electron configuration, typically that of a noble gas
• Ionic bonds involve the transfer of electrons from a metal to a nonmetal, resulting in the formation of oppositely charged ions attracted to each other (sodium chloride, NaCl)
• Covalent bonds involve the sharing of electrons between nonmetals, forming molecules or networked structures (water, H2O; diamond, C)
  • Single, double, and triple bonds represent the sharing of one, two, or three pairs of electrons, respectively
• Metallic bonds occur between metal atoms, characterized by a sea of delocalized electrons surrounding positively charged metal ions
• The valence shell electron pair repulsion (VSEPR) theory predicts the geometry of molecules based on the number of electron pairs (bonding and lone) around the central atom
  • Molecular geometries include linear (CO2), trigonal planar (BF3), tetrahedral (CH4), trigonal bipyramidal (PCl5), and octahedral (SF6)
• Polarity in molecules arises from unequal sharing of electrons in covalent bonds (due to electronegativity differences) and asymmetric molecular geometries
  • Polar molecules have a net dipole moment, while nonpolar molecules have a balanced distribution of charge (water vs. carbon dioxide)

Stoichiometry and Reactions

• Stoichiometry involves the quantitative relationships between reactants and products in a chemical reaction, based on the law of conservation of mass
• Balanced chemical equations represent the relative numbers of reactant and product molecules, with the total number of each type of atom conserved
  • Coefficients indicate the relative numbers of molecules or moles of each substance involved in the reaction
• Mole ratios, derived from the coefficients in a balanced equation, allow for the calculation of reactant and product quantities
• Limiting reactants determine the maximum amount of product that can be formed in a reaction, while excess reactants remain unconsumed
• Theoretical yield is the maximum amount of product that can be obtained based on the limiting reactant, while actual yield is the experimentally obtained amount
  • Percent yield is calculated as (actual yield / theoretical yield) × 100%
• Types of reactions include synthesis (combination), decomposition, single displacement, double displacement, and combustion
• Oxidation-reduction (redox) reactions involve the transfer of electrons between species, with oxidation representing a loss of electrons and reduction representing a gain of electrons

Thermodynamics and Kinetics

• Thermodynamics is the study of heat and its relationship to chemical reactions and physical changes
• Enthalpy (H) is a measure of the total heat content of a system, with exothermic reactions releasing heat (negative ΔH) and endothermic reactions absorbing heat (positive ΔH)
  • Hess's law states that the overall enthalpy change of a reaction is independent of the pathway and can be calculated by summing the enthalpy changes of individual steps
• Entropy (S) is a measure of the disorder or randomness of a system, with spontaneous processes increasing the total entropy of the universe
• Gibbs free energy (G) determines the spontaneity of a process, considering both enthalpy and entropy changes (ΔG = ΔH - TΔS)
  • Processes with negative ΔG are spontaneous, while those with positive ΔG are nonspontaneous
• Kinetics is the study of reaction rates and the factors that influence them, such as temperature, concentration, pressure, and the presence of catalysts
• The rate law expresses the relationship between the reaction rate and the concentrations of reactants, with the order of the reaction determined experimentally
  • Zero-order reactions have rates independent of reactant concentrations, while first-order and second-order reactions have rates proportional to the concentration of one or two reactants, respectively
• Activation energy is the minimum energy required for reactants to overcome and proceed to products, with catalysts lowering this energy barrier without being consumed in the reaction

Equilibrium and Le Chatelier's Principle

• Chemical equilibrium is a dynamic state in which the rates of forward and reverse reactions are equal, resulting in constant concentrations of reactants and products over time
• The equilibrium constant (K) is a quantitative measure of the position of equilibrium, expressed as the ratio of product concentrations to reactant concentrations, each raised to their stoichiometric coefficients
  • Large K values indicate a product-favored equilibrium, while small K values indicate a reactant-favored equilibrium
• Le Chatelier's principle states that when a system at equilibrium is subjected to a stress (change in concentration, pressure, volume, or temperature), the equilibrium shifts to counteract the stress and re-establish a new equilibrium
  • Adding reactants or removing products shifts the equilibrium towards the products, while adding products or removing reactants shifts it towards the reactants
  • Increasing pressure (decreasing volume) favors the side with fewer moles of gas, while decreasing pressure (increasing volume) favors the side with more moles of gas
• Increasing temperature shifts the equilibrium in the endothermic direction (towards the products for endothermic reactions, towards the reactants for exothermic reactions), while decreasing temperature shifts it in the exothermic direction
• Catalysts do not affect the position of equilibrium but rather accelerate the rate at which equilibrium is reached by lowering the activation energy of both forward and reverse reactions

Acids, Bases, and Buffers

• Acids are proton (H+) donors, while bases are proton acceptors
  • Arrhenius acids dissociate in water to produce H+ ions (HCl), while Arrhenius bases produce OH- ions (NaOH)
  • Brønsted-Lowry acids donate protons to bases (HCl + H2O -> H3O+ + Cl-), while Brønsted-Lowry bases accept protons from acids (NH3 + H2O -> NH4+ + OH-)
• The pH scale measures the acidity or basicity of a solution, with pH = -log[H+]
  • Acidic solutions have pH < 7, basic solutions have pH > 7, and neutral solutions have pH = 7
• The strength of an acid or base depends on its degree of ionization in water
  • Strong acids and bases ionize completely (HCl, NaOH), while weak acids and bases ionize partially (CH3COOH, NH3)
• The acid dissociation constant (Ka) and base dissociation constant (Kb) quantify the strength of weak acids and bases, respectively
  • Larger Ka or Kb values indicate stronger acids or bases
• Buffers are solutions that resist changes in pH when small amounts of acid or base are added, consisting of a weak acid and its conjugate base (or a weak base and its conjugate acid)
  • The Henderson-Hasselbalch equation relates the pH of a buffer solution to the pKa of the acid and the concentrations of the acid and its conjugate base: pH = pKa + log([A-]/[HA])
• Titration is a technique used to determine the concentration of an unknown acid or base by reacting it with a known concentration of a standard solution
  • The equivalence point is reached when the moles of acid and base are equal, and the endpoint is the point at which the indicator changes color, signaling the completion of the titration

Electrochemistry and Redox Reactions

• Electrochemistry is the study of the interconversion of electrical and chemical energy through oxidation-reduction (redox) reactions
• Oxidation is the loss of electrons, while reduction is the gain of electrons
  • Oxidation numbers (states) are assigned to atoms in compounds to track electron transfer, with increases in oxidation number indicating oxidation and decreases indicating reduction
• Redox reactions can be balanced using the half-reaction method, separating the oxidation and reduction processes and balancing atoms and charges
• Electrochemical cells convert chemical energy into electrical energy through spontaneous redox reactions
  • Voltaic (galvanic) cells consist of two half-cells connected by a salt bridge, with the anode undergoing oxidation and the cathode undergoing reduction
  • The standard cell potential (E°cell) is the voltage of the cell under standard conditions (1 M concentrations, 1 atm pressure, 25°C) and can be calculated using standard reduction potentials (E°cell = E°cathode - E°anode)
• Electrolysis is the process of using electrical energy to drive nonspontaneous redox reactions
  • Electrolytic cells consist of an electrolyte solution and two electrodes connected to an external power source, with oxidation occurring at the anode and reduction at the cathode
• Faraday's laws of electrolysis relate the amount of substance produced or consumed in an electrolytic cell to the quantity of electricity passed through the cell
  • The mass of a substance produced or consumed is proportional to the amount of charge passed and the molar mass of the substance

Lab Techniques and Data Analysis

• Proper lab safety protocols include wearing personal protective equipment (goggles, lab coats, gloves), handling chemicals and equipment with care, and disposing of waste properly
• Accurate and precise measurements are essential for reliable data collection
  • Accuracy refers to the closeness of a measured value to the true value, while precision refers to the closeness of repeated measurements to each other
  • Significant figures reflect the precision of a measurement and should be considered in calculations and reporting results
• Separation techniques are used to isolate components of a mixture
  • Filtration separates a solid from a liquid using a porous material (filter paper)
  • Distillation separates liquids based on differences in boiling points
  • Chromatography separates components based on their interactions with a stationary phase and a mobile phase (paper, thin-layer, column)
• Spectroscopy techniques use the interaction of electromagnetic radiation with matter to identify and quantify substances
  • Atomic absorption spectroscopy (AAS) measures the absorption of light by atoms, while atomic emission spectroscopy (AES) measures the emission of light by excited atoms
  • Infrared (IR) spectroscopy detects the vibrations of chemical bonds, while ultraviolet-visible (UV-Vis) spectroscopy measures electronic transitions
• Data analysis involves organizing, interpreting, and drawing conclusions from experimental results
  • Graphical representations (line graphs, bar graphs, pie charts) can help visualize trends and relationships
  • Statistical methods (mean, median, mode, standard deviation) provide information about the central tendency and spread of data
  • Error analysis identifies sources of uncertainty (random and systematic errors) and their impact on results

Exam Strategies and Practice Questions

• Read each question carefully and identify the key concepts being tested
  • Highlight or underline important information in the question stem and answer choices
  • Eliminate clearly incorrect answer choices to narrow down the options
• Manage your time effectively by budgeting a certain amount of time for each question or section
  • If you encounter a difficult question, make an educated guess and move on, returning to it later if time allows
• For calculation problems, show your work clearly and include units in your final answer
  • Double-check your calculations and ensure that your answer makes sense in the context of the problem
• For conceptual questions, think critically about the relationships between key concepts and apply your understanding to the given scenario
  • Use process of elimination to rule out answer choices that contradict your knowledge or the information provided
• Practice regularly with a variety of question types (multiple-choice, free-response) to familiarize yourself with the exam format and content
  • Analyze your performance on practice questions to identify areas of strength and weakness, focusing your study efforts accordingly
• Develop a study schedule leading up to the exam, allocating time for reviewing content, practicing problems, and taking full-length practice exams
  • Collaborate with classmates or a study group to discuss challenging concepts and share study strategies
• During the exam, stay calm and focused, taking deep breaths and maintaining a positive attitude
  • If you feel overwhelmed or anxious, take a brief mental break before returning to the question at hand