1. Chapter 16 Acids and Bases

2. 16.1 Defining Acids and Bases Since the 17 th century, chemists have known about acids and bases… however, it took a while before they were understood and could be defined.

3. Properties of Acids and Bases An acid looks the same as a base in water. However, there are many other ways to tell them apart. Taste : – Acid- has a sour or tart taste – Base- has a bitter taste Touch: – Acid- if dilute, feels like water. If not, has a sting – Base- feels smooth or slippery Reactions with metals: – Acid- reacts VIGEROUSLY with metals – Base- does not react

4. Electrical conductivity: – Both conduct electricity Indicators: a substance that changes color in an acid but a different color in a base. – Acid- has many indicators but most common is LITMUS PAPER… turns Blue to Red – Base- has many indicators but most common is LITMUS PAPER… turns Red to Blue Neutralization: a reaction between an acid and a base… the acid neutralizes the base and vice versa. What is formed is an ionic compound or a SALT.

5. The Arrhenius Definition Swedish Chemist, Svante Arrhenius, defined acids and bases in 1884. Acid- substance that dissociates in water to produce hydrogen ions (H + ) Base- substance that dissociates in water to produce hydroxide ions (OH - )

6. Common Arrhenius Acids & Bases AcidFormulaBaseFormula Hydrochloric acidHClSodium hydroxideNaOH Nitric acidHNO 3 Potassium hydroxide KOH Acetic acidHC 2 H 3 O 2 Magnesium hydroxide Mg(OH) 2 Sulfuric acidH 2 SO 4 Calcium hydroxideCa(OH) 2 Carbonic acidH 2 CO 3 Barium hydroxideBa(OH) 2 Phosphoric acidH 3 PO 4

7. Arrhenius acids and bases, in a acid-base neutralization reaction, will always produce water and a salt. Hydrochloric acid + sodium hydroxide → water + sodium chloride (table salt) – HCl + NaOH → H 2 O + NaCl (double replacement reaction) What would be the product of HNO 3 and KOH?

8. The Brønsted-Lowry Definition In 1923, two chemists independently developed a new definition of acids and bases. Danish Chemist, Johannes Brønsted and English Chemist, Thomas Martin Lowry said: An acid is any substance that can donate H + ions A base is any substance that can accept H + ions

9. Brønsted-Lowry expands the definition of Arrhenius acids/bases. 1.Defines acids/bases independently of how they behave in water. 2.Focuses only on the H + ions and ignores the OH - ions.

10. The Hydronium Ion Because H + will combine with water… a Hydronium ion is actually created. Water acts as an acid and a base… substances that do so are called amphoteric.

11. Conjugate Acid-Base Pairs Conjugate means… joined together or coupled. An acid and a base which differ by a proton are said to form a conjugate acid base pair or the pairs of substances which can be formed from one another by the gain or loss of protons are known as conjugate acid base pairs.

12. Determining the Strengths of Acids & Bases 1M Acetic acid Is a weak acid. We can eat it and use it to clean. 1M Hydrochloric acid Is a strong acid. Will eat through your clothes and burn your skin. CAUTION!

13. Strong & Weak Acids Strong acids disassociate 100%. Ex. HCl – HCl + H 2 O → H 3 O + + Cl - 100% – Excellent conductors Weak Acids only disassociate a small amount. Ex. HC 2 H 3 O 2 – HC 2 H 3 O 2 + H 2 O ↔ H 3 O + + C 2 H 3 O 2 - 0.4% – Poor conductors

14. Strong & Weak Bases Strong bases have the strongest affinity for H + meaning they react 100% with water. Ex. CaO – O 2- + H 2 O → 2OH - Weak bases only partially react with water. Ex. CO 3 2- – CO 3 2- + H 2 O ↔ HCO 3 - + OH -

15. Strength of Conjugate Acid-Base Pairs The stronger the acid the weaker the conjugate base and vice versa.

16. Acid-Base Properties of Salts When salts disassociate in water, they break down into cations and anions. – NaCl + H 2 O → Na + + Cl - Many of these ions are weak Brønsted-Lowry acids or bases. Salt hydrolysis reactions- rxn of ions from salts to form H 3 0 + or OH -

17. We can predict if the salt soln. will be acidic (formed H 3 0 + ) or basic (formed OH - ) Salts of: – STRONG acids + STRONG bases = neutral soln. – STRONG acids + weak bases = acidic soln. formed H 3 0 + – Weak acids + STRONG bases = basic soln. formed OH - – Weak acids + weak bases = neutral soln.

18. 16.2 The Self-ionization of Water and pH Pure water does not have H 2 O molecules only. It also contains H 3 O + ions and OH - ions. Water is amphoteric, can act as an acid or a base with other chemicals. Self-ionization-

19. Mathematical Definition “In pure water at 25°C, both H 3 O + and OH - ions are found at concentrations of 1.0 X 10 -7 M.” K w = [H 3 O + ] [OH - ] K w = 1.0 X 10 -14 – [H 3 O + ] = 1.0 X 10 -7 – [OH - ] = 1.0 X 10 -7 1.0 X 10 -7 [H 3 O + ] > 1.0 X 10 -7 > [OH - ] Acidic Soln 1.0 X 10 -7 both = 1.0 X 10 -7 Neutral soln 1.0 X 10 -7 [OH - ] > 1.0 X 10 -7 > [H 3 O + ] Basic Soln

20. Practice Problems If the concentration of H 3 O + in blood is 4.0x10 -8 M, the blood is acidic, basic or neutral? – What is the concentration of OH - ions? What is the concentration of OH - ions in chocolate milk if [H 3 O + ] = 4.5x10 -7 M? – Is it acidic, basic, or neutral? What is the concentration of H 3 O + ions in black coffee if [OH - ] = 1.3x10 -9 M. – Is it acidic, basic, or neutral?

21. The pH Scale In 1909, Søren Sørensen developed the pH scale. – 0-6.9: acid – 7.1-14: base Its based on logarithms: pH= -log [H 3 O + ] Using a calculator, try: -log(8.7 x 10 -4 ) What is the pH of a soln with [H 3 O + ] = 7.3x10 -5 M? [H 3 O + ] = 6.23x10 -4 M? [OH-] = 5.0x10 -2 M? (14=pH + pOH)

22. Measuring pH 2 Common methods 1.Use an indicator, (litmus paper) – can use a combination of indicators to get a more precise pH reading 2.Use an pH meter.

23. Some Acid-Base Indicators

24. Acid-Base Titration Acid-base titration- a carefully controlled neutralization reaction. To test an unknown solution you must use a standard solution to compare against. Also need an indicator to determine the pH range.

25. Begin the titration by adding the standard solution to the unknown… eventually enough of the standard solution is added to neutralized the acid/base of the unknown. This is the equivalence point. End point- the point at which the indicator changes.

26. Calculating Concentration You can calculate the concentration of the acid or base using the data from the titration with this formula. total moles of H + @ end point = total moles of OH - @ end point

27. Choosing the Proper Indicator Picking the indicator depends on the strength of the acid and base. 3 combinations: – Strong acid/Strong base neutral – Weak acid/Strong base basic – Weak base/Strong acid acidic

30. 16.3 Buffers There are times when you want to control the pH of a solution. – Ex: your blood has to stay with in a pH of 7.35-7.45 or you can become very sick and could die! We can use a buffer to control the pH – BUFFER- a mixture that is able to release or absorb H+ ions, which keeps the pH level constant.

31. Most buffer solns are made from a weak acid and their conjugate base (or their salt). All buffers have a limited capacity to neutralize acids and bases. – BUFFER CAPACITY – amount of acid or base a buffer can neutralize. – http://www.youtube.com/watch?v=g_ZK2ABUjvA http://www.youtube.com/watch?v=g_ZK2ABUjvA