Unit D Outcome Analysis
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Vista College**We aren't endorsed by this school
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CHE 30
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Chemistry
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Dec 19, 2024
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21
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Chemistry 30: Unit DChemical Equilibrium Focusing on Acid-Base SystemsYou were given some “I can” statements and other questions as part of “Questions You Should Be Able toAnswer For This Unit” document. These statements are the Outcomes you’re supposed to learn in this course.To answer questions like this, it's important to focus on understanding the basic idea behind energy release infusion. Fusion is the process where light atomic nuclei (like deuterium and tritium) combine to form a heaviernucleus, and in this process, some mass is lost. This lost mass is converted into energy, which is the reason whyfusion releases energy. The question asks why energy is released in a fusion reaction. The key idea here is thatthe mass of the products (the particles formed after the fusion) is less than the mass of the particles beforefusion. This difference in mass, called the "mass defect," is what gets turned into energy. This is similar to theidea behind fission, where energy is also released when a heavy nucleus splits into lighter nuclei, but fusion isdifferent because it involves the combination of smaller nuclei.Your task for this assignment is to●Find an example diploma exam question that requires the information in each Outcome, alternativelyyou can use the diploma question provided.●Answer the diploma question, showing your work●Explain how to generally answer these types of questions – This is your main taskYou can find example diploma exams at the links provided next to this file in Unit A on Moodle.Some diploma exam questions might be usable for multiple Outcomes in this document. While you can use thesame question more than once, you will need to re-explain how the diploma exam question answers thataddresses that Outcome.Remember, this assignment is worth a considerable amount of your grade for this unit, so put a considerableamount of effort into this assignment.You will be graded with the following rubric:Excellent (4points)The responses accurately answer the diploma questions andprovidea thorough and in-depth explanation of the concepts used to answerit. The explanation demonstrates a clear understanding of thematerial.Proficient (3points)The responses accurately answer the diploma question andprovide agood explanation of the concepts used. The explanationdemonstrates a solid understanding of the material.Adequate (2points)The responses accurately answer the diploma question butprovide alimited explanation of the concepts used. The explanationdemonstrates some understanding of the material.Limited (1point)The responses provide an inaccurate or incomplete answer to thediploma questions andprovide a limited or weak explanation of theconcepts used. The explanation demonstrates a minimalunderstanding of the material.
1. What is the criteria needed for a chemical system to be in equilibrium? (D1.1k)Diploma question:My answer, including all work: AA: Correct: Mass by itself does not provide definitive evidence that the forward and reverse reactionrates are equal, which is the key characteristic of equilibrium.B: Incorrect: A constant pressure indicates equilibrium in gaseous systemsC: Incorrect: A constant temperature is a valid indicator of equilibrium because no net heat transferoccurs in the system at equilibriumD: Incorrect: A constant number of gaseous molecules is consistent with equilibrium becauseequilibrium ensures no net change in the amounts of reactants and products over timeAnswer: AMy explanation of how to answer these types of questions, and how this relates to the outcomeconcepts:To answer these types of questions, it’s important to identify answers that directly indicate achemical system is at equilibrium. Equilibrium is achieved when the forward and reverse reactionrates are equal, which results in no net change in reactant or product concentrations. Things likeconstant temperature, pressure, and concentrations are used to identify equilibrium. However, aconstant mass has nothing to do with the equilibrium. In this question, option A is correct because aconstant mass is not a specific indicator of equilibrium. While the other options likeconstant
pressure, constant temperature, and constant numbers of gaseous molecules are direct effects of thesystem reaching equilibrium.This question relates to the outcome “What is the criteria needed for a chemical system to be inequilibrium? (D1.1k)” because the question requires us to understand the criteria needed for achemical system to be in equilibrium in order to understand it. In order for a chemical system to be inequilibrium, there must be constant observable macroscopic properties, like temperature, pressure,and concentration. Understanding this allows us to answer the question and eliminate options thatdo affect equilibrium and choose the right answer.
2. What is Le Chatelier’s principle? (D1.3k)Diploma question:My answer, including all work: 1561: Correct. Adding H2(g) increases the concentration of a product, shifting the equilibrium towardthe reactants.2: Incorrect. Adding CO2(g) increases the concentration of a reactant, which would shift theequilibrium toward the products.3: Incorrect. Adding a catalyst does not shift the equilibrium; it only increases the rate of both theforward and reverse reactions equally.4: Incorrect. Removing CO(g), a product, decreases its concentration, causing the equilibrium to shiftto the right to the products5: Correct. Removing CH4(g), a reactant, decreases its concentration, causing the equilibrium to shifttoward the left to increase it.
6: Correct. Decreasing the volume increases the pressure, favoring the side of the reaction withfewer moles of gas. The shift will occur toward the reactants.Answer: 156My explanation of how to answer these types of questions, and how this relates to the outcomeconcepts:To answer these types of questions, we must apply Le Chatelier’s principle, which explains how asystem at equilibrium responds to external stresses. First, we need to understand that when a systemis at equilibrium, any change in concentration, temperature, or pressure will cause the system toadjust itself in such a way as to counteract the stress that is applied. This results in a shift of theequilibrium position, either toward the reactants or products, depending on what the stress is. In thisquestion, the stresses are changes in the concentration of reactants or products, the addition of acatalyst, and a change in pressure/volume. For concentration changes, adding a reactant or removinga product will shift the equilibrium toward the products, while adding a product or removing areactant will shift it toward the reactants. A catalyst only speeds up the reaction, allowingequilibrium to be reached more quickly, but it has no effect on equilibrium. For pressure changes, ifthe volume is decreased (increasing the pressure), the system will shift toward the side with fewermoles of gas. In this case, since the reactants have fewer gas moles than the products, a decrease involume will favor the reactants.This question relates to “What is Le Chatelier’s principle? (D1.3k)” because it requires recognizinghow systems at equilibrium respond to changes in their environment. By applying Le Chatelier'sprinciple, we can predict the direction of the equilibrium shift based on the type of stress applied. Inorder to answer these types of questions we need to understand what Le Chatelier’s principle is,which is that the position of equilibrium shifts to counteract the change to reestablish anequilibrium. By knowing this, we can eliminate incorrect options and select the right ones.
3. How do you predict the extent of a chemical reaction and write corresponding equilibrium-lawexpressions? (D1.4k)Diploma question:My answer, including all work: DA: Incorrect: Equilibrium is not established here.B: Incorrect: Equilibrium is also not established here.C: Incorrect: This does not best represent the progress of the system as equilibrium is established.D: Correct: Initially, the reactants are consumed, and the products are formed.Answer: DMy explanation of how to answer these types of questions, and how this relates to the outcomeconcepts:To answer questions about equilibrium and the progress of a chemical reaction, it is important tounderstand the behavior of the system as it moves toward equilibrium. When a reaction is at
equilibrium, the concentrations of the reactants and products remain constant over time. Initially, asthe system begins, the reactants are consumed and the products are formed. This leads to changes inthe concentration of reactants and products, but over time, these changes slow down and eventuallystabilize. The key principle in these types of questions is to understand how the concentrations ofreactants and products evolve as the system reaches equilibrium. As reactants are converted toproducts, the concentration of reactants decreases, while the concentration of products increases. Atequilibrium, the rate of the forward reaction equals the rate of the reverse reaction, so theconcentrations of all species remain constant.This relates to the outcome “How do you predict the extent of a chemical reaction and writecorresponding equilibrium-law expressions? (D1.4k)” because it directly involves understanding theprogress of a system toward equilibrium and the way equilibrium concentrations are established. Inthis case, recognizing that the concentrations of both reactants and products change initially andthen stabilize is key to selecting the correct graph. The equilibrium-law expression for the reactioncan also be written as: K=[CO][H2]^2/[CH4][CO2] This expression reflects the relationship betweenthe concentrations of reactants and products at equilibrium and helps predict how the system willbehave over time.
4. How do you write BrØnsted-Lowry equations for acid-base equilibrium reactions for bothmonoprotic and polyprotic acids and bases? (D1.6k)Diploma question:My answer, including all work: CThe correct answer is C because propionic acid (C₂H₅COOH) is weaker than the other acids listed, andwhen a weaker acid is added to the system, it will shift the equilibrium to favor the formation ofreactants (the sorbate ion and water), because the weaker acid doesn't contribute as many protonsto the system.Answer: CMy explanation of how to answer these types of questions, and how this relates to the outcomeconcepts:To answer these types of questions, it's important to apply the concept of acid strength and its effecton equilibrium. In an acid-base equilibrium, the strength of the acid involved determines how it shiftsthe equilibrium position. Stronger acids dissociate more readily, and when they are added to asystem, they will donate protons, which can shift the equilibrium toward the reactants if they arestronger than the acid that is already present. In this case, the sorbate ion is a weak base. When weintroduce another acid, the equilibrium will shift to reduce the effect of the added stronger acid. Theweaker the acid, the less it will interfere with the equilibrium, and the equilibrium will shift towardthe reactants.
This relates to the outcome “How do you write Bronsted-Lowry equations for acid-base equilibriumreactions for both monoprotic and polyprotic acids and bases? (D1.6k)” because it involvesunderstanding the interaction between acids and bases in equilibrium systems. WritingBronsted-Lowry equilibrium reactions requires us to recognize how acids donate protons and howbases accept them, and also understanding how equilibrium constants (Kaor Kb) influence thesereactions. In the case of monoprotic acids, such as those given in the question, the dissociationinvolves a single proton transfer. For polyprotic acids, each dissociation step has its own Kavalue, andthe equilibrium for each step must be done separately. Recognizing the difference betweenmonoprotic and polyprotic acids is so important for predicting equilibrium positions and writing theircorresponding reactions.
5. What is the equilibrium constant and how do you calculate it? (D1.4k)Diploma question:My answer, including all work: 1536Bitartate system doesnt have a Kw value so that is autmotaically removed. Liquid water isnt includedin the equilibrium law so that is also removed. There is no OH-(aq) so that is also removed.Ka = [produces]/[reactants]Ka=[H3O+(aq)][C4H4O62-(aq)]/[C4H5O6-(aq)]1536My explanation of how to answer these types of questions, and how this relates to the outcomeconcepts:To answer these types of questions, we must first understand how equilibrium constants are writtenand calculated for a given reaction. The equilibrium constant (Ka) for an acid dissociation reaction
expresses the ratio of the concentrations of the products to the reactants at equilibrium. It is derivedfrom the balanced chemical equation for the system. For acids, this typically involves theconcentration of the conjugate base, the hydronium ion (H3O+(aq)), and the undissociated acid.Most importantly, pure solids and liquids (like H2O(l) in this case) are not included in the equilibriumlaw expression because their concentrations do not change during the reaction. Using this, we canremove incorrect answers and choose the correct one.This relates to the outcome“What is the equilibrium constant, and how do you calculate it? (D1.4k)”because constructing and understanding the equilibrium constant expression is fundamental tosolving equilibrium problems. It requires recognizing which species appear in the expression,understanding their roles in the reaction, and excluding species that are not involved in theexpression (e.g., pure liquids or solids). To calculate it, we need to take the equilibrium concentrationof the products, and divide that by the concentration of reactants. It is important to note that anycoefficients must be exponents, so we must raise the concentrations to that power.
6. What is a buffer? (D1.8k)Diploma question:My answer, including all work: 23122. Polyprotic because numerous COOH and it can act as a base because it can accept a proton butalso act as an acid since it can donate one3. It is only differs from the other species by a proton, therefore it is its conjugate1. Has the highest Ka which means its the strongest acid so it has the weakest conjugate base2. This species will form a buffer when mixed with an equal amount of the other species2312My explanation of how to answer these types of questions, and how this relates to the outcomeconcepts:To answer these types of questions, it is important to look at the properties of the species involved inacid-base equilibrium. A polyprotic species has more than one ionizable proton and participates inmultiple stages of proton donation. An amphiprotic species can act as both a proton donor (acid) anda proton acceptor (base), which is often observed in intermediate stages of polyprotic acids or bases.Understanding these concepts allows for proper identification of such species. The conjugate base of
an acid is formed when the acid donates a proton. By removing a proton from a given species, wecan identify its conjugate base. Conversely, by adding a proton to a base, we find its conjugate acid.To identify the species with the weakest conjugate base, we have to compare the acid strengths ofthe parent molecules. A stronger acid has a larger Kavalue, and its conjugate base will be weakerbecause strong acids donate protons readily, leaving behind less reactive conjugate bases. Finally,buffers are solutions that resist changes in pH upon the addition of small amounts of acid or base.They consist of a weak acid and its conjugate base (or a weak base and its conjugate acid). To form abuffer, we pair an acid with its conjugate base in significant proportions.This question relates to the outcome “What is a buffer? (D1.8k)” because understanding buffersinvolves recognizing weak acid-conjugate base pairs and how they maintain equilibrium to resist pHchanges. Buffers play an important role and identifying their components requires us to know howKa values and conjugates work. This question also directly requires us to know what a buffer is inorder to answer it.
7. How can you use Ka, Kband Kwto determine pH, pOH, [H3O+] and [OH-] for both acidic and basicsolutions? (D2.2k)Diploma question:My answer, including all work: 2519pOH = -log[OH-(aq)][OH-(aq)] = 10^-pOHpH + pOH = 1414 - pH = pOH14 - 5.400 = 8.6[OH-(aq)] = 10^-8.6[OH-(aq)] = 2.511 x 10^-9 mol/L[OH-(aq)] = 2.51 x 10^-9 mol/LAnswer: 2519My explanation of how to answer these types of questions, and how this relates to the outcomeconcepts:To solve these types of questions, it is important to understand how Kw, pH, and pOH work together.The equilibrium constant for water (Kw) is 1.0×10^-14 at 25°C (SATP), which allows us to connect theconcentrations of [H3O+] and [OH−] through [H3O+][OH−]=Kw. This relationship helps us determineeither [H3O+] or [OH−] if one is known. The pH and pOH scales are logarithmic representations of[H3O+] and [OH−]. The formula pH+pOH=14 is important for determining one value from the other.So if you have one value, like in the question for example, you can use that to find the missing valye.Once pOH is found, the concentration of hydroxide ions ([OH−]) is calculated using 10^-pOH.
This relates to the outcome“How can you use Ka, Kband Kwto determine pH, pOH, [H3O+] and [OH-]for both acidic and basic solutions? (D2.2k)”because we need to apply these to quantify the acidityor basicity of a solution. For example, if we had the Ka, we can use the relationship (Ka)(Kb)=Kw tofind the Kb, and then find the concentration of hydroxide ions in solution. After that, we could evendetermine the pOH or anything else. All of these values correlate to one another, and they can all beused to identify missing variables as mentioned above.
8. How do you calculate equilibrium constants and concentrations for both homogeneous systemsand BrØnsted-Lowry acids and bases when…●concentrations at equilibrium are known? (D2.3k)Diploma question:My answer, including all work: CKc = [CH3OH(g)]/[CO(g)][H2(g)]^2Kc = [2.75 mol/L]/[0.75 mol/L][1.25 mol/L)]^2Kc = 2.346Kc = 2.3Answer: CMy explanation of how to answer these types of questions, and how this relates to the outcomeconcepts:To solve these types of questions, we need start by writing the equilibrium constant expression fromthe balanced chemical equation. The equilibrium constant, Kc, relates the concentrations ofreactants and products at equilibrium. For the given system, Kc is calculated using the formula:Kc=concentration of products raised to their stoichiometric coefficients / concentration of reactants
raised to their stoichiometric coefficient. Once the correct equilibrium expression is written,substitute the known equilibrium concentrations of each species. After that we simply solve for Kc.This relates to the outcome “How do you calculate equilibrium constants and concentrations for bothhomogeneous systems and Bronsted-Lowry acids and bases when concentrations at equilibrium areknown? (D2.3k)” because it demonstrates the process of calculating an equilibrium constant for ahomogeneous system. Calculating Kcprovides insight into the position of equilibrium and the relativequantities of reactants and products. Furthermore, we are only able to use the equilibrium values todetermine the Kc.
9. How do you calculate equilibrium constants and concentrations for both homogeneous systemsand BrØnsted-Lowry acids and bases when…●initial concentrations and one equilibrium concentration are known? (D2.3k)Diploma question:My answer, including all work: CConcentrationsNH3(g)O2(g)NO(g)H2O(g)Initial (mol/L)0.200.5000Change (mol/L)-x-x+x+xEquilibrium(mol/L)0.20-x0.50-xxxC=n/vInitial:[NH3(g)] = 10mol/50L = 0.20 mol/L[O2(g)] = 25mol/50L = 0.5- mol/LAnswer: CMy explanation of how to answer these types of questions, and how this relates to the outcomeconcepts:To solve questions involving equilibrium concentrations when initial concentrations are provided, weneed to set up an ICE table. This step ensures we understand the stoichiometric relationshipsbetween the reactants and products. The stoichiometry determines how much each species changesas the system approaches equilibrium. Next, we define the change in concentrationas x and expressthe equilibrium concentrations of all species in terms of these variables. We can use thestoichiometric coefficients from the balanced equation to relate changes in reactant concentrations
to changes in product concentrations if we need to. In this problem, the key is to identify whichoption matches the initial concentrations and their stoichiometric changes as equilibrium isestablished, and to also match the ICE table.This question relates to the outcome “How do you calculate equilibrium constants andconcentrations for both homogeneous systems and Brønsted-Lowry acids and bases when initialconcentrations and one equilibrium concentration are known? (D2.3k)” because it involves setting upexpressions for equilibrium concentrations using known initial concentrations and changes based onstoichiometry. We use the initial concentrations to set up an icetable, and then continue from thereto fill it up. If we had the equilibrium concentration, we would simply work backwards but it involvesthe same concept.
10. How do you calculate equilibrium constants and concentrations for both homogeneous systemsand BrØnsted-Lowry acids and bases when…●The equilibrium constant and one equilibrium concentration are known? (D2.3k)Diploma question:My answer, including all work: DConcentrationsAl(H2O)63+ (aq)Al(OH)(H2O)52+(aq)H3O+(aq)Initial (mol/L)0.58500Change (mol/L)-x+x+xEquilibrium (mol/L)0.585-xxxKc = [Al(OH)(H2O)52+(aq)][H3O+(aq)] / [Al(H2O)63+ (aq)]Kc = [x][x] / [0.585-x]1.4 x 10^-5 = x^2/0.58(1.4 x 10^-5)(0.58) = x^2√(1.4 x 10^-5)(0.58) = xx = 2.86×10^-3x = 2.86×10^-3 mol/L[H3O+(aq)] = x[H3O+(aq)] = 2.0=9×10^-3 mol/LAnswer: D
My explanation of how to answer these types of questions, and how this relates to the outcomeconcepts:To solve questions like this, we start by writing the equilibrium constant expression based on thebalanced chemical equation. We need to identify the initial concentrations and use the stoichiometryof the reaction to create an ice table so we can find the equilibrium concentrations. After that wesubstitute these expressions into the equilibrium constant equation, and solve for x, the change inconcentration. If Kcis very small, the assumption that x is negligible compared to the initialconcentration simplifies the calculation significantly. This approximation is valid when theequilibrium constant indicates the reaction favors the reactants. Once x is calculated, we caninterpret it in the context of the problem to determine the desired concentration which is [H3O+] inthis case.This relates to the outcome“How do you calculate equilibrium constants and concentrations for bothhomogeneous systems and Bronsted-Lowry acids and bases when the equilibrium constant and oneequilibrium concentration are known? (D2.3k)” because it demonstrates how to set up and solveequilibrium problems where initial conditions and Kcare provided. Using the Kc value, we are able todetermine the equilibrium concentration for the hydroniom ions. If we had one equilibriumconcentration we could work our way backwards to determine the equilibrium of all other species.