Chem. 1B – 9/29 Lecture

Announcements I Exam 1 –On Oct. 1 – here except for Sect. 7 students (in Sequoia 426) –Will Need Scantron Form SC982-E (blue form – show example) –Test format: Main Section: Multiple Choice Secondary Section: 1 Problem – do on back side of Scantron + show work there (Print out exam will not be collected, but you can use as scratch paper) –I will review topics today

Announcements II Exam 1 – cont. –Help Session – Today 3:30 to 4:30 on Sequoia 452 –Remaining Review Sessions (lead by PAL): today, 5:30 to ? Sequoia 102 or 452 Mastering Assignments – due today (easy) and Thurs. (on buffers) Today’s Lecture –Finish buffers (last example of 2 nd method to make buffers – not on Exam 1) –Exam 1 Review (topics on Exam 1) –Titrations (Chapter 16 material not on Exam 1)

Chem 1B – Aqueous Chemistry Buffers (Chapter 16) One Final Question: –How many mL of 1.00 M HCl must be added to mL of M NH 3 to make a pH buffer? K b (NH 3 ) = 1.8 x 10 -5

Exam 1 Review General Advice –Multiple choice questions: Read full question and all answers before selecting (some answers may “sound right” until you see the actual right answer) If it is taking time to calculate or if you are uncertain how to proceed, skip that problem and get back to later (circle question so you know you still need to do it) –General Question (12 pts): Will be multiple parts and involve calculations (e.g. like determination of equilibrium concentrations on quiz) Need to show work (don’t just punch numbers into your calculator and write answer) I will give partial credit for parts done correctly

Exam 1 Review Topics – Chapter 14 –Understand how equilibrium relates to kinetics –Understand how to get equilibrium equations from equilibrium reactions (chemical equations) –Understand what the K value tells you about equilibrium conditions –Know the difference and be able to convert between K P and K C (for given reaction with R and T given) –Know how equilibrium reaction “manipulation” affects K values (e.g. reversing reaction leads to K new = 1/K old )

Exam 1 Review Topics – Chapter 14 – cont. –Be able to determine K from all equilibrium concentrations or from initial and equilibrium conditions –Be able to determine equilibrium concentrations from “at equilibrium” conditions and K values or from initial conditions and K values –Be able to determine whether reaction will proceed to products or reactants from K and initial concentrations

Exam 1 Review Topics – Chapter 14 – cont. –Know how to apply Le Châtelier’s principle to systems initially at equilibrium with the following changes: reactant/product addition/subtration increase/decrease in volume (including dilutions) changes in T (with  H known) Chapter 15 –Know all three definitions of acids and bases and be able to identify them in examples –Know differences between weak and strong acids and bases and the meaning of K a and K b

Exam 1 Review Chapter 15 – cont. –Know what autoionization is and how to determine pH and pOH –Be able to calculate the pH when a strong or weak acid is added to water –Be able to calculate the pH when a strong or weak base is added to water –Be able to calculate K a from K b (or visa versa) –Be able to calculate % ionization of weak acids –Know differences between weak and strong acids and bases and the meaning of K a and K b

Exam 1 Review Chapter 15 – cont. –Be able to predict if ions or salts are acidic, neutral or basic –Understand how polyprotic acids react and how to determine the pH and related species concentrations –Be able to calculate pH of buffer solutions (prepared or modified by addition of acid or base) –Know why buffer solutions are useful –Understand useful range of buffers and buffer capacity

Chem 1B – Aqueous Chemistry Titrations (Chapter 16) Review from 4.8 –Titrations are a way to accurately tell when we have reached a stoichiometric point in a reaction –Generic reaction: aA + bB → products –The equivalence point is defined where (moles A)/(moles B) = a/b –mL of titrant (either A or B) are carefully measured (usually to determine the concentration of B or A) –The end point is where the equivalence point is observed (color change from indicator or inflection point in measured titration curve)

Chem 1B – Aqueous Chemistry Titrations (Chapter 16) Review from 4.8 –Example question: An unknown H 2 SO 4 solution is pipeted (25.00 mL) into a flask. It is titrated with KOH until reaching an endpoint (where the equivalence point is observed). It requires 39.1 mL of M KOH. What is the concentration of H 2 SO 4 in the unknown solution?

Chem 1B – Aqueous Chemistry Titrations (Chapter 16) Titrations – pH behavior –Review material covered how we can use titrations, but not the exact behavior during the titration –In Chapter 16, we cover, through calculations, the pH of the full titration curve –The most valuable titrations are accurate (observed end point gives the equivalent point) and precise (reproduceable) –This occurs when change in pH in a titration is rapid

Chem 1B – Aqueous Chemistry Titrations (Chapter 16) Strong Acid – Strong Base Titration –How does pH change as NaOH is added? –3 regions to titrations (different calculations in each region): before equivalence point at equivalence point after equivalence point –Show pH at 5 mL, 12.5 mL, and 15 mL M NaOH M HCl, 25 mL

Chem 1B – Aqueous Chemistry Titrations (Chapter 16) M NaOH M HCl, 25 mL Titration Plot