Chem 221 - midterm 3 study guide questions and answers fully solved

Do you understand the relation between ∆E and ∆H and why work (w) is important to this? (23) - Correct Answers∆H = ∆E + P×∆V, where ∆E is the internal energy change and ∆H is the enthalpy change. If P and ∆V are constant, then we can say that ∆H is equal to ∆E. Work is equal to -P×∆V. ∆E = q - P×∆V which is also ∆E = q + w. ∆H = q + w + P×∆V. We can rewrite our first equation as ∆H = ∆E - w. So if work is constant, then again ∆H = ∆E. Do you understand the relation between ∆Hrxn and the heat of a reaction including the distinction between exothermic and endothermic processes? (23) - Correct Answers∆Hrxn = heat/enthalpy of the reaction. ∆Hrxn is the difference between enthalpy of products and reactants. If ∆H is negative, or ∆H 0, then the reaction releases heat and is exothermic. If ∆H is positive, or ∆H 0, then the reaction absorbs heat and is endothermic. Do you understand why the first law predicts that the sum of all of the heat transfers in an ideal calorimetry experiment is 0? (24) - Correct AnswersThe law of conservation of energy states that the total heat energy cannot change and thus heat is conserved in the experiment. Therefore, the heat transfers would all add up to 0 because no heat was ultimately lost or gained. Do you understand the relation between ∆Hrxn and amount of substance produced or consumed in a reaction? In other words, how is the ∆Hrxn given in a thermochemical equation related to the moles of reactants or products consumed or produced in a chemical reaction? (25) - Correct AnswersIn a thermochemical equation, the chemical equation is also given a ΔH value. This ΔH value is equivalent(aka: ⇔) to the coefficients shown in the equation. For example, in the thermochemical equation: 2H2(g) + O2(g) - 2H2O(l) ; ΔH = -570 kJ. This means: 2 moles of H2 ⇔ 1 mol O2 ⇔ 2 mol H2O ⇔ -570 kJ Can you relate the heat transferred in a reaction to the amounts of substances changing? In other words, can you use ∆Hrxn to solve a stoichiometry like problem to determine the heat evolved/consumed from reacting or forming a certain mass or moles of a reactant or product? (25) - Correct AnswersThe heat transferred in a reaction is ΔHrxn. Transferred heat depends on ΔT, mass, and phase of substance. If phase of substance is standard (aka: 0, naught), then: "The change in temperature of a substance when a certain amount of heat is transferred to it is directly proportional to its mass." Remembering that ΔH ⇔ mole coefficients of the reaction, we can use the given ΔH and equation coefficients to solve problems. [WILL REVISIT THIS ANSWER, NOT SURE IF I ANSWERED IT COMPLETELY] Can you use Hess's law to calculate ∆Hrxn values, and can you demonstrate why the process works through summing chemical equations? (26) - Correct AnswersHess's Law states that the total enthalpy change of a reaction is the sum of all changes (which also makes enthalpy a state function). This can be shown by this equation: ΔH(overall) = ΔH(1) + ΔH(2) + ΔH(3) ... . To calculate ∆Hrxn of a reaction, we use this equation: ΔHrxn0 = Σ(m × Hf0 products ) - Σ(n × Hf0 reactants) To calculate ∆H of multiple reactions, we follow these steps: 1) Multiply reactions to get products/reactants on proper side AND with same coefficients as in target reaction 2) Add resulting reactions /OR/ add reactions without explicitly rewriting them Do you understand how a reaction can be viewed as the decomposition of reactants followed by the formation of products, and what this has to do with the utility of ∆H0f? (27) - Correct Answers Do you understand the interrelations of frequency, wavelength, and speed for a light wave as well as the energy of a photon (∆E = hν)? (28) - Correct AnswersThe equation for speed of light (c) in a vacuum (c = 2.9979×10^8 m/s) is: c = frequency × wavelength; where c is a constant Frequency and wavelength are inversely proportional. So, if frequency increases then wavelength will decrease, and vise versa. Now in relating frequency and wavelength to energy of a photon: ∆E = hν; where h is Planck's constant. Short wavelengths + High frequency = High energy (vise versa) Wavelength and energy are inversely proportional. Energy and frequency are directly proportional. Do you understand the meaning of wave amplitude? (28) - Correct AnswersThe amplitude of a wave is defined as the height of a wave from the zero point, or more specifically the distance between a wave's crest or trough from the zero point/midline. Amplitude affects brightness/intensity. As seen in the photoelectric effect, increasing amplitude alone cannot always emit an electron. (It more relies on frequency.) Do you understand that the phenomena of refraction, diffraction, and interference provide evidence for the wave nature of light? (28) - Correct AnswersRefraction "The changing of a light ray's direction (loosely called bending) when it passes a boundary between materials of different composition" like a wave, the light changes speed when going from one material to another best visualized when putting a straw or pencil into a clear cup of water. Diffraction "The bending of a wave around the edges of an opening or an obstacle" best visualized as 'straight' waves from the open ocean passing through an opening, where the waves will become curved/round Interference "The identifying behavior of a wave" "a phenomenon in which two waves superimpose to form a resultant wave of greater or lesser amplitude" [MAY COME BACK AND REVISE "INTERFERENCE] Can you describe how the photon theory explains the photoelectric effect and how this provides evidence for the particle-like properties of light? (28) - Correct AnswersThe photon theory is basically just the concept of the photon. "The photoelectric effect proves that light has particle-like activity. The photoelectric effect happens when photons are shone on metal and electrons are ejected from the surface of that metal. The electrons that are ejected are determined by the wavelength of light which determines the energy of photons." Do you understand quantization of energy and how an atom changes its energy by emitting or absorbing quanta of radiation? (28) - Correct AnswersThe quantization of energy is defined as: "An electron can radiate or absorb energy as radiations only in limited amounts or bundles called quanta." An atom can change its energy in two different ways 1) Emission a photon is emitted when an electron moves from a higher energy level to a lower energy level 2) Absorption a photon loses its energy to an electron which here, an electron goes from ground state to an excited state Can you describe how Bohr's theory explained the line spectra of the H atom? (29) - Correct AnswersBohr's theory claims "the electron moves around the nucleus in circular orbits that can have only certain allowed radii." [NOT COMPLETE] Do you know why the Bohr theory is wrong, and which ideas we retain? (29) - Correct AnswersBohr's model of the hydrogen atom depicts electrons occupying planetary-like orbits around the nucleus. However, his ideas weren't completely true for all other atoms. What is wrong: there are no "orbits" but there are orbitals we don't know the exact location of an electron, so instead we say where there is a high probability of finding an electron since hydrogen only has 1 electron, the model doesn't apply to bigger atoms because it doesn't account for interactions between electrons What we retained: electrons orbit the nucleus at specific levels orbitals closest to the nucleus have the lowest energy state as an electron jumps from different energy states, it gives off radiation/a photon electromagnetic radiation given off is like a light wave that delivers light in packets of energy Can you find the energy change and the wavelength of the photon absorbed or emitted when an H atom changes its energy level? (29) - Correct Answers Can you describe the uncertainty principle and how uncertainty limits our knowledge of electron properties? (30/31) - Correct AnswersThe Uncertainty Principle states that you can't know the position of an electron and its velocity/momentum at the same time. It limits our knowledge of electron properties because the more we know about the location, the less we can know about the velocity (and vise versa). Do you understand the distinction between ψ (wave function, or atomic orbital) and ψ^2 (probability density)? (30/31) - Correct Answers"Although the wave function [ψ] itself does indeed tell us the probability amplitude of the wave produced, it usually is given by some complex value with imaginary numbers and therefore does not hold any true physically measurable meaning. To rid the wave function of its imaginary numbers, we need to take the square of the absolute value [ψ^2] which does in fact produce a measurable and meaningful number" [NOT COMPLETE] Do you understand how radial and angular probability distribution plots and constant probability contours depict the electron's location within the atom? (30/31) - Correct AnswersRadial shows the probability of being a distance "r" from center the peak on the plot shows the most probable distance of an electron from a nucleus Angular a "polar plot" that shows the probability of being anywhere along "r" at a particular θ distance is the probability of being at that angle Probability Contours each contour is the constant probability replaces "z" with color constant probability contour with 90% probability to be inside contour Can you describe the hierarchy of quantum numbers that describe the size and energy (n), shape(l),and orientation (ml) of an orbital? (30/31) - Correct AnswersEnergy (n) - The Principal Quantum Number describes the size and identifies the energy level Shape (l) - The Angular Momentum Quantum Number describes the shape divides the levels into sublevels (s, p, d, f) Orientation (ml) - Magnetic Quantum Num