Week 8 homework 3parts
CHE105 – Week 8 Formative Assessment: Acids and Bases Instructions: • • • • • Each student will work on this assignment alone. Assignments with identical answers will earn grades of zero. If you have questions, you are to ask me, your instructor, not your peers. The assessment will be printed out, completed, scanned, or photographed and uploaded to the upload area. Only assignments uploaded to the class will be graded. You must show ALL work. Submissions with answers only will not be evaluated. Formats: The following formats will be evaluated: • Microsoft Word Note: if you cannot scan your work before submitting, you are welcomed and encouraged to do the following: • • • • Take an image of each page of your work with your phone. Copy/paste those images to a Word document. Make sure the image size is the same as the page. Submit one image per page. Save the file as a Word document. Submitted files in the following formats will NOT be evaluated: • Pages • Lone Jpeg images • Any other file formats Please work through and complete the following pages regarding this week’s topic: balancing chemical equations. Two Definitions of Acids and Bases: Arrhenius Definitions • An acid is a substance that produces hydronium ions, H30+(aq), when it is added to water. • A base is a substance that produces hydroxide ions, OH-(aq), when it is added to water. Bronsted-Lowry Definitions • An acid is a substance that donates a proton, H+, to another species. • A base is a substance that accepts a proton, W, from another species. • Acid-base reactions are one of the most important types of chemical reactions. Evaluate/think about the following reactions and apply your understanding of the definitions: Table 1. Equilibrium constants for some acid-base reactions. Reaction HCl(g) + H2O(l) H3O+(aq) + Cl-(aq) NH3(aq) + H2O(l) NH4+(aq) + OH(aq) HCN(aq) + H2O(l) H3O+(aq) + CN(aq) Kc 2 104 3.3 10-7 1 2 1.1 10-11 3 The current table shows the equilibrium constants for three acid-base reactions. The first is the dissociation of HCl in water, the second is the dissociation of ammonia, NH3, in water, and the third is the dissociation of HCN in water. Questions a) What chemical species are the Arrhenius acids in each of the forward reactions? 1) 2) 3) b) What chemical species are the Arrhenius bases in each of the forward reactions? 1) 2) 3) c) What chemical species are the Bronsted-Lowry acids in each of the forward reactions? 1) 2) 3) d) What chemical species are the Bronsted-Lowry bases in each of the forward reactions? 1) 2) 3) Complete the following table of conjugate acids and bases: Acid Base H2S S2NO2H3PO4 OCl+ H3O pH Is Defined as – log[H3Q+] The water-dissociation equilibrium constant, Kw, is (at 25 0C) Several definitions have been found to be useful: pOH = -log [OH-] pKw = -log Kw In general, pX = -log X For pX expressions involving concentrations, the concentration units are always mole/liter, but they are omitted in the calculation. Thus, for example, values for pH are unitless. Questions: 1. Show that the pH reading in the model is correct (given that the hydronium ion concentration is 5.0 x 10-4 M). 2. Consider a neutral aqueous solution: a) What is the pH of a neutral aqueous solution? b) What is the pOH of a neutral aqueous solution? 3. What values of pH characterize: a) an acidic solution? b) a basic solution? 4. What is the numerical value of pKw? 5. Recall that log (A x B) = log A + log B. What is the relationship between pH, pOH, and pKw? Please load the PhET WebLab: https://phet.colorado.edu/en/simulation/acid-base-solutions Click to begin. Part 1: Procedure 1. The lab has 2 tools that allow you to test for pH values: A probe , and pH paper . Use each one by dipping it into the solution to be tested. You can determine pH value by comparing the color of the pH paper to the scale. Try all the given types of solutions and fill in the Data Chart with the pH value 0-14. 2. The circuit with a battery and bulb as shown: is the tool used to test for conduction of a solution. By dipping the wire leads into the solution, the bulb with either remains unlit, be dimly lit, be somewhat bright or very bright. Test each solution and record your observation for the brightness of the bulb in the chart below. Data pH Value from Probe Color & pH Value Observations from Circuit Tool from pH Paper Describe the brightness Water (H2O) Strong Acid (HA) Weak Acid (A) Strong Base (MOH) Weak Base (B) Analysis 1. What pH value range is observed: a. for acids? ______________b. for bases?____________ 2. Why are some solutions better conductors of electricity? Part 2 Procedure, Data & Analysis: Recall: The amount of ionization or dissociation of ions determines the strength of an acid or base. The concentration of [H3O+], hydronium ion and [OH-], hydroxide ion, can be used to calculate pH and pOH as shown in the diagram here: Note: we use [H3O+] and [H+] interchangeably. 1. Click on Water Solution, Graph View, Probe Tool. Insert the probe in the water. Notice that the initial concentration of the solution is given before any ionization or dissociation takes place. 2. Fill in the missing concentration values for the hydronium and hydroxide ions on the chart here: Use the concentration value for [H3O⍅] to calculate the pH. Show work: 3. Use the concentration value for [OH-] to calculate the pOH. Show work: 4. Did your answer to #2 match the pH given in the simulation? _________________________ 5. Is the answer to #3 equal to: (14 – pH)? ___________ Show work: _____________________ 6. Is the solution an acid, a base or neutral, based upon the calculated pH?_________________ Part 3. Procedure, Analysis, Conclusion: My Solution Across the bottom of the screen, click the button. The default setting shows a weak acid with a concentration of 0.010 M. Insert the pH probe to show an initial pH of 4.50. The beaker is shown below: 1. Slide the initial concentration bar to the right to increase the number of solute molecules and then slide it to the left. What effect does changing the concentration (Moles of HA/Liters of Soln’) have on the pH value? (Be specific without giving values) 2. Return to your default setting and insert the probe. Now slide the strength to the right to make the acid stronger. a. As you increase the strength, describe the change in the number of blue A- ions, orange H3O⍅ ions, and the original HA acid molecule: b. As you increase the strength, describe the change in the concentrations of both ions in the solution? Hint: Click to see how the concentrations rise and fall. 3. Yes or No? Does the pH seem to depend upon the concentration of [H3O+] ions? 4. We always assume that strong acids will 100% ionize in water. Click reset and move the slider to strength: strong. Insert the probe. Record pH. Observe the number of ions in the beaker and click to observe the concentrations. a. pH Value = _________________ b. YES or NO? Did the beaker contain particles that now has 0% concentration? If so, what particle seems missing? ______________________________. Why is it likely missing? 5. Click reset and change to a base. Repeat 1-4 above and answer the questions. #1: What effect does changing the concentration of the base has on the pH? Be specific. #2: a. How do the # of OH- and BH⍅ and B change as you increase strength? Be specific. b. How does the concentration of OH- and BH+ change as you increase strength? Be specific. #3: Yes or No? Does the pH seem to depend on the concentration of [OH-]? Explain & Be specific: #4: We always assume that strong bases will 100% ionize in water. Click reset and move the slider to strength: strong. Insert the probe. Record pH. Observe the number of ions in the beaker and click to observe the concentrations. a.pH = __________ b. Is there a particle missing? __________ If so, what is it? ________________ Conclusions: If the answer is no, explain why not. 6. YES or NO? Can a weak acid be concentrated? 7. YES or NO? Can a strong acid be dilute? 8. YES or NO? For acids, can increase the initial concentration increase the pH? 9. YES or NO? For Bases, can increase the initial concentration increase the pH? Adapted from Richard S. Moog and John J. Farrell, Chemistry: A Guided Inquiry, 5th Edition, John Wiley & Sonds, Inc., 2011, and PhET Interactive Simulations, University of Colorado Boulder, Introduction to Ionic & Covalent Bonding, Published 7/16/2013, https://phet.colorado.edu/en/simulation/acid-basesolutions
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