Chem. 1B – 9/24 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) –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’m working to schedule a help session Tuesday (9/29) sometime between 3:00 and 5:30 PM –Will have some review of exam topics on Tuesday

Announcements II Exam 1 – cont. –Example exams posted + key will be posted on Mr. Sparks website next week Lab –Quiz 4 on Monday/Tuesday (Experiment 2 + lecture topics) –Pre-labs for Experiment 2 needed Today’s Lecture – Chapter 15/16 Topics –Relating Acid Strength to Molecular Structure –Chapter 16 – Section 16.2: Buffer Solutions –Titrations (if time + not on Exam 1)

Chem 1B – Aqueous Chemistry Molecular Structure – Acidity Relationship Acid strength depends on ability for H bond to break and on stability of conjugate base formed More stable conjugate bases means stronger acid For example, what makes ethanol (C 2 H 5 OH) neutral while acetic acid (CH 3 CO 2 H) is acidic? acetate: stabilized by delocalized electrons ethanol anion (not very stable)

Chem 1B – Aqueous Chemistry Molecular Structure – Acidity Relationship General Rules for Simpler Structures: –Binary Acids: e.g. HCl more electronegative element makes for stronger acid longer (and weaker bond) makes for stronger acid (HCl is stronger than HF due to bond strength) –Oxyacids: e.g. HClO 2 more oxygens make acid stronger (HClO 4 is a strong acid, HClO is a very weak acid)

Chem 1B – Aqueous Chemistry Buffers (Chapter 16) We have discussed some mixtures briefly (e.g. strong acid + weak acid) One particular type of mixture: weak acid + conjugate base (or weak base + conjugate acid) makes a solution called a buffer Buffers are desirable because they keep the pH nearly constant even if an acid or base is added Buffers are very important in Biology because many enzymes (protein catalysts) will only work over a narrow pH range

Chem 1B – Aqueous Chemistry Buffers (Chapter 16) Example: Determine pH of a mix of M HCHO 2 and M Na + CHO 2 - solution

Chem 1B – Aqueous Chemistry Buffers (Chapter 16) Buffer Solutions: –Question: Was the ICE Problem set up needed? –Answer: No. The assumption of x << [HA], [A - ] is valid for all “traditional” buffers –Traditional Buffer Weak acid (3 < pK a < 11) Ratio of weak acid to conjugate base in range 0.1 to 10 mM+ concentration range

Chem 1B – Aqueous Chemistry Buffers (Chapter 16) Buffer Solutions: –Since ICE not needed, can just use K a equation –K a = [H + ][A - ]/[HA] = [H + ][A - ] o /[HA] o (always valid) (valid for traditional buffer) –But log version more common –pH = pK a + log([A - ]/[HA]) –Also known as Henderson-Hasselbalch Equation

Chem 1B – Aqueous Chemistry Buffers (Chapter 16) Addition of small amounts of acid to a buffer: –Example: let’s say we have a buffer made to be M NH M NH 4 Cl in 1.00 L –Calculate the pH –Now lets add moles of HCl. What is the new pH?

Chem 1B – Aqueous Chemistry Buffers (Chapter 16) Change of pH and Buffer Capacity –A buffer is designed to minimize the change in pH from addition of base or acid –What is the most effective pH? (show next slide) –Can we add too much acid or base to a buffer? –Does the absolute concentration of acid/base affect pH? Why is a higher concentration better? –Buffer Capacity is the ability of the buffer to absorb acid or base without the pH changing significantly

Chem 1B – Aqueous Chemistry Buffers (Chapter 16) Most Effective pH Range –NH 3 + NH 4 Cl in 1.00 L Example [NH 3 ] + [NH 4 + ] = M –Addition of moles HCl [NH 3 ] (M)[NH 4 + ] (M)pH  pH Best region (equal moles of WA and CB

Chem 1B – Aqueous Chemistry Buffers (Chapter 16) A Second Way to Make Buffers (not on Exam 1) –To make a traditional buffer, we need both an acid and its conjugate base, but this also can be “made” through other combinations such as: –A weak acid and a strong base or a weak base and a strong acid –This is not covered in the text for buffers, but is needed for titrations (section 16.4) –Example: What is the pH when we add 10.0 mL of 0.50 M KOH with 34.1 mL of 0.19 M HC 2 H 3 O 2 ?

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?

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)

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?