Understanding Acids, Bases, and Their Properties, pH Measurement, Titrations, and Buffers

1. Unit 16 Section 1 – Properties of Acids and Bases Section 2 – Determining the Acidity of a Solution Section 3 – Titrations and Buffers

2. Objectives Section 1 – Properties of Acids and Bases • To learn about two models of acids and bases • To understand the relationship of conjugate acid-base pairs • To understand the concept of acid strength • To understand the relationship between acid strength and the strength of the conjugate base • To learn about the ionization of water

3. Acids and Bases • The Arrhenius Model • Acid – produces hydrogen ions in aqueous solution • Base – produces hydroxide ions in aqueous solution

4. Acids and Bases • The Bronsted-Lowry Model • Acid – proton donor • Base – proton acceptor • The general reaction for an acid dissolving in water is

5. Acids and Bases • The Bronsted-Lowry Model • Conjugate acid-basepair

6. Acids and Bases • The Bronsted-Lowry Model • Water acts as a base accepting a proton from the acid. • Forms hydronium ion (H3O+)

7. Acid Strength • Strong acid – completely ionized or completely dissociated

8. Acid Strength • Weak acid – most of the acid molecules remain intact

9. Acid Strength • A strong acid contains a relatively weak conjugate base.

10. Acid Strength • Common strong acids are • Sulfuric acid, H2SO4 • Hydrochloric acid, HCl • Nitric acid, HNO3 • Perchloric acid, HClO4

11. Acid Strength • Oxyacid – acidic proton is attached to an oxygen atom • Organic acid – have a carbon atom backbone and commonly contain the carboxyl group • Typically a weak acid

12. Acid Strength

13. Water as an Acid and a Base • Water is amphoteric – it can behave as either an acid or as a base • Ionization of water • Concentration of hydronium and hydroxide are equal

14. Water as an Acid and a Base • Product of [H3O+] and [OH] is always constant.

15. Water as an Acid and a Base

16. Objectives Section 2 – Determining the Acidity of a Solution • To understand pH and pOH • To learn to find pH and pOH for various solutions • To use a calculator to find pH • To learn methods for measuring pH of a solution • To learn to calculate the pH of strong acids

17. The pH Scale • The “p scale” is used to express small numbers. • pH = log [H+]

18. The pH Scale

19. The pH Scale • Because the pH scale is a log scale based on 10, the pH changes by 1 for every power of 10 change in the [H+].

20. The pH Scale

21. The pH Scale • pOH scale • pOH = log [OH] • pH + pOH = 14.00

22. The pH Scale

23. Measuring pH • Indicators – substances that exhibit different colors in acidic and basic solutions • In an acid solution the indicator will be in the HIn form. • In a basic solution the indicator will be in the In form.

24. Measuring pH • Other methods • Indicator paper • pH meter

25. C. Calculating the pH of Strong Acid Solutions • Determine the [H+]. • pH = log[H+]

26. Objectives Section 3 – Titrations and Buffers • To learn about acid-base titrations • To understand the general characteristics of buffered solutions

27. Acid-Base Titrations • Titration – delivering a measured volume of a solution of known concentration into the solution being analyzed • Titrant – a standard solution • Buret – device used for accurate measurement of the delivery of a liquid • Stoichiometric point (equivalence point) – when just enough titrant has been added to react with all of the solution being analyzed

28. Acid-Base Titrations • Titration curve (pH curve) – plot of the data (pH vs volume) for a titration

29. Buffered Solutions • Buffered solution – resists a change in its pH when either and acid or a base has been added • Presence of a weak acid and its conjugate base buffers the solution

30. Buffered Solutions