Chemistry Chapters 6, 7, and 8
Chemistry Chapters 6, 7, and 8 CHM 160 001
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Chapter 6: Thermochemistry ● Thermochemistry is the study of the relationships between matter and energy. ○ Energy is the capacity to do work. ■ Heat is the source of much of the work done on our planet. ● Heat is energy in transit. ■ Temperature is not the same as heat. ○ Energy may be classified as either Kinetic orPotential Energy. ■ Kinetic (KE) energy due to motion. ■ Potential (IE) energy due to position. ■ When a ball is released, the potential energy is transformed into kinetic energy. ○ Thermal Energy the energy associated with the temperature of an object (a form of kinetic energy). ○ Energy cannot be created or destroyed, only rearranged. ● Kinetic Theory states that atoms, molecules and ions are in constant, random motion. ● The most common energy unit is the Joule. ○ Calories © are another common energy unit. ■ One calorie is equal to 4.184 J. ○ “Calories” © are used to measure the energy content of food. ■ The energy is released when the food is metabolized. ● Energy units measure the amount of energy given off, or taken in, during a chemical reaction. ○ Energy changes that occur during a chemical reaction are due to the making and breaking of chemical bonds. ● Fossil fuels are sunshine in the Gas, Liquid and Solid form. ○ One of the main reactions responsible for metabolism in humans produces energy. ○ When wood is burned cellulose, a polymer of sugar, is consumed. ○ When plants (and animals) die and are exposed to air, water and other organisms they decompose. ■ Over millions of years, plants that captured the rays of our young sun were transformed into coal and petroleum. ○ Petroleum is pumped to the surface from its natural, underground reservoirs and can then be transported via pipelines to its place of use. ■ Petroleum also produces more energy than a comparable amount of coal. ○ Petroleum is an incredibly complex mixture and gasoline is only one part of it. ■ Distillation towers are used to process petroleum and separate out the different chemicals. ■ Over 87% of each barrel of petroleum is used for transportation and heating. ○ Octane (C₈ H₁ ₆), the main component of gasoline, is the most useful form of petroleum and is used to fuel vehicles. ■ If more energy is produced than is consumed during a chemical reaction, it can be used for other purposes. ● Thermodynamics energy transfers and conversions in chemical reactions. ○ The First Law of Thermodynamics states that the total energy of the universe is constant. ■ In other words, the energy has to come from someplace we cannot simply create it. ■ When chemical bonds are formed, energy is released. ■ When chemical bonds are broken, energy is consumed. ● Chemists often divide the universe into a system and its surroundings. ○ Heat energy will transfer from the hotter to the colder region until they reach the same temperature. ■ Molecules in the surroundings slow down (cool off), while molecules in the system speed up (heat up). ○ Heat, when it enters a system, produces an increase in the average motion with which the particles of the system move. ■ Total energy before and after the process is the same (energy is conserved!). ○ If the system loses energy, the surroundings gain energy and vice versa. ■ ΔEsys = ΔEsurr ○ If energy is gained by a system = + sign. ○ If energy is lost by a system = sign. ■ Fuels produce a ΔEsys. ■ CH₃ CH₂ OH + 3O₂ → 2CO₂ + 3H₂ O + 1275 kJ given off → ΔEsys = 1275 kJ/mol ● Enthalpy is the heat exchanged between the system and surroundings. ○ The energy change in the system equals the change in energy of the gas plus the energy change of the piston. ■ ΔEtotal = ΔEsys + ΔEsurr = 0 ΔEsys = ΔEgas + ΔEpiston ΔEgas = q (the sum of heat transferred) ● Brick heats gas → the pressure increases → the piston moves. ○ The change in energy of the piston is equal to the work done to or by the system. ■ ΔEpiston = mgΔh = work = w m = mass g = acceleration due to gravity Δh = height piston has been raised. ○ The change in energy of the system is the sum of the heat transferred and the work done to or by the system. ● Bomb calorimeters can be used to determine reaction enthalpy. ○ 2Na + Cl₂ → 2NaCl ΔE = 411 ● Atmospheric pressure acts like a piston. ● Enthalpy is the heat exchanged with with the surroundings under constant pressure. ○ Enthalpy change values indicate the amount of energy consumed or produced in chemical reactions. ■ CH₄ + 2O₂ → CO₂ + 2HO ΔHrxn = 802 kJ/mol ■ 2C₂ H₆ + 7O₂ → 4CO₂ + 6H₂O ΔHrxn = 1560 kJ/mol ○ Sum of bond energies of formation greater than the sum of the bond energies broken. ● Exothermic reactions have a negative ΔH (the vessel becomes hot). ○ 2H₂ + O → 2H₂ O ΔH = 438.64 kJ/mol ● Endothermic reactions have a positive ΔH (the vessel becomes cold). ○ 2H₂ O → 2H₂ + O₂ ΔH = 438.64 kJ/mol ● ΔHrxn and Molrxn represent the balanced chemical equation as a whole unit. ○ If the amount of reactants is doubled, the enthalpy is doubled. ■ 2H₂ + O₂ → 2H₂O ΔH = 438.64 kJ/mol ■ 4H₂ + 2O₂ → 4H₂O ΔH = 877.28 kJ/mol ○ ΔHrxn depends on the reaction. ● Standard state enthalpies (ΔH°) are measured at 1 atm and 25°C. ○ Enthalpy is a state function, meaning it depends only on the initial and final states, not the path used to get there. ● Reactions produce or consume the same amount of energy. ○ Decomposition ■ CH₄ → C + 2H₂ ΔH° = 74.6 kJ/mol ○ Formation ■ C + 2H₂ → CH₄ ΔH° = 74.6 kJ/mol Chapter 7: The Quantum Mechanical Model of the Atom ● Quantum mechanics is a model that explains where electrons exist in atoms and how the electrons determine the chemical and physical properties of elements and compounds. ○ Electromagnetic radiation has been used to determine how the electrons are distributed in atoms. ○ Light is electromagnetic radiation, a type of energy with oscillating electric and magnetic fields. ■ Electromagnetic radiation can be described as a wave composed of oscillating electric and magnetic fields. The fields oscillate in perpendicular planes. ● A wave is a continuously repeating change or oscillation in matter or a physical field. ○ Visible light, Xrays and radio waves are all forms of electromagnetic radiation. ■ Waves can be described by wavelength , amplitude and frequency. ● The amplitude of a wave is the vertical height of a crest (or depth of a trough). ● The wavelength is the distance between any adjacent identical points on a wave. ○ When light passes through a prism, it splits into its different wavelengths. ■ Frequency is the number of wavelengths of a wave that pass a fixed point in one unit of time (usually a second). ● Frequency and wavelength are inversely related (lower frequency = longer wavelength). ■ The greater the wavelength, the smaller the frequency for waves traveling the same speed. ● λ × v = speed of wave ● The range of frequencies or wavelengths of electromagnetic radiation stretch from gamma to radio waves. ○ R O Y G B I V ○ Humanity uses or interacts with most forms of electromagnetic radiation daily. ● Waves, including electromagnetic waves, interact with each other in a characteristic way called interference ○ If waves align with overlapping crests when they interact with one another a wave with twice the amplitude results called constructive interference. ○ If waves align so that the crest from one source overlaps the trough from the other source the waves cancel called destructive interference. ● When light encounters an obstacle that is comparable in size to its wavelength, it bends around it or diffracts. ○ When a wave passes through a small opening, it spreads out. Particles, by contrast, do not diffract; they simply pass through the opening. ● When light passes through two slits, constructive and destructive interference are observed. ○ Light displays both particlelike and wavelike properties. ○ The photoelectric effect was the observation that many metals emit electrons when light shines on them. ● Max Planck found that the color emitted (given off) from a hot solid indicated the temperature of the solid. ○ Only certain energies of light were allowed. ○ E = ɳ hv n = 1, 2, 3, … ■ h is Planck’s constant → 6.63 × 10⁻ ³⁴ ■ ɳ are quantum numbers. ■ The energy, E, is said to be quantized (limited to certain values). ● Einstein postulated in 1905 that light exists as quanta (called photons), or particles of electromagnetic energy, with E proportional to the observed frequency of light. ○ Electrons are ejected only if the energy of the light (hv) is higher than a certain value (characteristic of the material). ○ Einstein postulated that an electron is ejected when struck by a single photon (packet of light energy). ■ This photon must have enough energy to remove the electron from attractive forces within the metal. ■ E = hv & hc/λ ■ In essence, the photon is absorbed by the electron and gains energy. ○ Einstein’s equation, E = hv, can be used to calculate the energy, wavelength and frequency of light. ■ E = hv & c = λv ● Light interacting with atoms in results in the movement of electrons. ○ Passing electricity through hydrogen gas results in the emission of light. ● Bohr used the work of Planck, Einstein and others to formulate a model of electron distribution. ○ Bohr assumed that the negatively charged electron and the positively charged proton were held together by attractive forces. ■ The attractive forces cannot be strong enough so that the electron joins the nucleus ○ Postulate 1 says that an electron can only have specific energies. ○ Postulate 2 says that an electron can only change energy by going from one energy level to another. ■ Transitions between energy levels occurs when the electrons gain or lose energy. ● Energy emitted (hv) = E initial E final ■ Since electrons move between levels this indicates that there is more than one ‘shell’ or energy level around the nucleus. ■ The four lines in the hydrogen spectrum are four different transitions between energy levels or shells. ● The theory of quantum mechanics applies to particles of matter such as electrons as well as chemical molecules. ○ Particles such as electrons have wavelike properties. ● You cannot simultaneously observe both the wave nature and the particle nature of the electron. ○ The introduction of a tool to observe the electrons as they pass through the two slits changes how they interact with one another. ○ Complementary properties are properties that exclude each other. ● The more accurately you know the position of the electron, the less accurately you know its velocity and vice versa. ● When dealing with subatomic particles, we must think terms of probabilities. ● Schrodinger’s formulation of the way electrons interacted with the nucleus allows us to calculate the energies and orbitals where the electrons reside. ○ Quantum numbers describe the orbitals in which electrons can be found. ■ The principal quantum number (n = 1, 2, 3, …) determines the overall size and energy of an orbital. ■ The angular momentum quantum number (l) determines the shape of an orbital and can be any number between 0 and n1. ■ The magnetic quantum number (m₁ ) determines the orientation of an orbital and can be any integer between 1 and +1. ○ To summarize, only certain combinations of quantum numbers are allowed. ■ The three quantum numbers (n, l, and m₁ ) are all integers. ■ The principle of quantum number cannot be zero. (n = 1, 2, 3, …) ■ The angular momentum quantum number can be any number between 0 and n1. ■ The magnetic quantum number can be any integer between 1 and +1. ● Atomic spectroscopy involves the movement of electrons among orbitals. ● The shapes of orbitals are important because chemical bonds depend on the sharing or movement of electrons between atoms. ○ Bonding is the movement or sharing of electrons between orbitals. ○ The shape of the orbital is determined by the angular momentum quantum number. ■ ɫ = 0 s ■ ɫ = 1 p ■ ɫ = 2 d ■ ɫ = 3 f ○ Orbitals are probability distributions. ● To get a better idea of where an electron is most likely to be found, we use a radial distribution function. ○ Each principal level with n = 2 or higher has three porbitals (m₁ = 1, 0, +1). ○ Each principal level with n = 3 or higher has five dorbitals (m₁ = 2, 1, 0, +1, +2). ○ Each principal level with n = 4 or higher has seven forbitals (m₁ = 3, 2, 1, 0, +1, +2, +3). ● We do not ‘lose’ orbitals as we add others. We simply have more orbitals with a larger value of n. ● Atoms are drawn as spherical since all the orbitals together make up a roughly spherical shape. Chapter 8: Periodic Properties of the Elements ● Mendeleev organized the elements based on periodic law: when elements are arranged in order of increasing mass, their properties recur periodically. ○ Mendeleev’s ordering allows predictions to be made. ● Bohr postulated that electrons exist in orbitals (or shells) around the nucleus. ○ Electrons can only change energy by going from one energy level to another. ■ Ionization completely removes an electron from an atom. ○ The closer the electron is to the nucleus, the harder it is to remove the electron. ■ The nucleus is positively charged so the closer an electron is to the nucleus, the stronger it is attracted by the atom. ● The first ionization energy is the minimum energy required to completely remove an electron from a ground state atom in the gas phase. ○ Ionization energy increases across the table and decreases as you go down the table. ● The outermost electrons are known as the valence electrons ● Photoelectron spectroscopy (PES) uses photons to knock electrons out of atoms from both valence and innermost shells. ● Shells contain subshells. ○ An ssubshell can hold 2electrons; a psubshell can hold 6electrons. ● There are four subshells that we will focus on; s, p, d and f. ○ Ionization energies and subshells correlate with positions on the periodic table. ● An electron configuration for an atom shows the particular subshells that are occupied for that atom. ○ This is a predictable relationship between the periodic table and electron configuration. ○ Outer shell (or valence) electrons are often the only ones shown, but inner shell (or core) electrons are still present. ● Schrodinger’s formulation of the way electrons interacted with the nucleus led to the same results derived from photoelectron spectroscopy. ● Quantum numbers describe the orbitals in which electrons can be found. ○ n = principal quantum number (size of the orbital) ○ l = angular momentum quantum number (shape of the orbital) ○ ml = magnetic quantum number (orientation in space) ● The spin of the electrons within atoms creates a magnetic field that interacts with the external magnetic field. ○ Hydrogen atoms are paramagnetic and interact with the magnetic field. ○ Helium atoms are diamagnetic and do not interact with the magnetic field. ● An orbital is a region of space where only electrons of opposite spin can reside. ● The Pauli Exclusion principle states that electrons in an orbital must have opposite spin. ● Hund’s rule states that every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin. ○ Experimental evidence from magnetic studies indicate that the electrons fill in degenerate orbitals one at a time with parallel spins, then start to pair up. ● As you move down a column, the number of electrons in the outermost principle energy level remains the same. ○ The valence electrons are those in the outermost principle energy level. ○ If d or f shells are full, these do not count as valence electrons. ○ Elements that have the same number of valence electrons have similar properties. ● The chemical properties of the elements are largely determined by the number of valence electrons they contain. ● Combining a metal and a nonmetal produces a ‘salt’. ● Elements with a full valence shell are not reactive. ● The size of an atom or ion influences many of the chemical and physical properties of the atom or ion. ○ The radius is equal to onehalf the distance between nuclei of adjacent atoms in a solid. ■ Atoms become larger as we go down a column in the periodic table. ■ Atoms become smaller as we go from left to right across a row in the periodic table. ○ Core electrons shield outer electrons from the full force of the nucleus, while electrons in the valence shell do not shield each other. ○ The number of protons in the nucleus increases as we go across a row, so the core charge increases and the force of attraction between the nucleus and electrons also increase. ● The electron configuration for an ion is determined by adding or removing electrons from the electron configuration of the neutral atom. ○ Cations are much smaller than their corresponding atoms. ○ Anions are much larger than their corresponding atoms. ● Ionization energies also help explain why elements on the left side of the table are more likely than those on the right to form cations. ● Electron Affinity is a measure of how easily an atom will accept an addition electron. ○ EAs “tend to” become more positive as we move down as column, and are more negative as we move across a row. ● Metallic character decreases from left to right, and decreases from bottom to top in the periodic table. ίħ(▯/▯t)ψ(r,t) = (ħ²/2m)Δ²ψ(r,t) + V(r,t)ψ(r,t) With i being the imaginary number, √1 and h being Planck’s constant divided by 2π: 1.05459 ×10⁻ ³⁴ and ψ(r,t) acting a wave function defined over space and time. Determine the potential energy influencing the particle and the probability of the electron’s location in a region of time and space.
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