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