1 Acids and Bases (courtesy of L. Scheffler, Lincoln High School, 2010)

2 Acids React with certain metals to produce hydrogen gas. React with carbonates and bicarbonates to produce carbon dioxide gas Have a bitter taste Feel slippery. Many soaps contain bases. Bases

3 Properties of Acids þ Produce H + (as H 3 O + ) ions in water (the hydronium ion is a hydrogen ion attached to a water molecule) þ Taste sour þ Corrode metals þ Good Electrolytes þ React with bases to form a salt and water þ pH is less than 7 þ Turns blue litmus paper to red “Blue to Red A-CID”

Some Common Acids HC 2 H 3 O 2 acetic acidin vinegar HClhydrochloric acidstomach acid H 3 C 6 H 5 O 7 citric acidfruits H 2 CO 2 carbonic acidsoft drinks H3 2 PO 4 phosphoric acidsoft drinks

5 Properties of Bases  Generally produce OH - ions in water  Taste bitter, chalky  Are electrolytes  Feel soapy, slippery  React with acids to form salts and water  pH greater than 7  Turns red litmus paper to blue “Basic Blue”

Some Common Bases NaOHsodium hydroxidelye KOHpotassium hydroxideliquid soap Ba(OH) 2 barium hydroxidestabilizer for plastics Mg(OH) 2 magnesium hydroxide“MOM” Milk of magnesia Al(OH) 3 aluminum hydroxideMaalox (antacid) Al(OH) 3 aluminum hydroxideMaalox (antacid)

7 Arrhenius Definition Arrhenius Acid - Substances in water that increase the concentration of hydrogen ions ((H + or hydronium ions H 3 O + ). Base - Substances in water that increase concentration of hydroxide ions (OH - ). Categorical definition – easy to sort substances into acids and bases Problem – many bases do not actually contain hydroxides

8 Bronsted-Lowry Definition Acid- substance that donates a proton. Base- substance that accepts a proton. HA + B  HB + + A - Ex HCl + H 2 O  H 3 O + + Cl - Acid Base Conj Acid Conj Base A “proton” is really just a hydrogen atom that has lost it’s electron! The classification depends on how the substance behaves in a chemical reaction

9 Conjugate Base-The species remaining after an acid has transferred its proton. Conjugate Acid-The species produced after base has accepted a proton. HA & A - -conjugate acid/base pair A - -conjugate base of acid HA B & HB + -conjugate acid/base pair HB + -conjugate acid of base :B Conjugate Acid Base Pairs

A Brønsted-Lowry acid is a proton donor A Brønsted-Lowry base is a proton acceptor acid conjugate base base conjugate acid

11 Note: Water can act as acid or base AcidBase Conjugate Acid Conjugate Base HCl+ H 2 O  H 3 O + +Cl - H 2 PO H 2 O   H 3 O + + HPO 4 2- NH H 2 O   H 3 O + +NH 3 Examples of Bronsted- Lowry Acid Base Systems

12 Lewis Acid-an electron pair acceptor Base-an electron pair donor Lewis Definition

Brønsted-Lowry vs. Lewis All B/L bases are Lewis bases BUT, by definition, a B/L base cannot donate its electrons to anything but a proton (H+) While B/L is most useful for our purposes, Lewis allows us to treat a wider variety of reactions (even if no H+ transfer occurs) as A/B reactions

14 Acid Strength Strong Acid-Transfers all of its protons to water; - Completely ionized; - Strong electrolyte; - The conjugate base is weaker and has a negligible tendency to be protonated. Weak Acid-Transfers only a fraction of its protons to water; - Partly ionized; - Weak electrolyte; - The conjugate base is stronger, readily accepting protons from water  As acid strength decreases, base strength increases.  The stronger the acid, the weaker its conjugate base  The weaker the acid, the stronger its conjugate base

15 Acid Dissociation Constants Dissociation constants for some weak acids

16 Base Strength Strong Base-all molecules accept a proton; - completely ionizes; - strong electrolyte; - conjugate acid is very weak, negligible tendency to donate protons. Weak Base-fraction of molecules accept proton; - partly ionized; - weak electrolyte; - the conjugate acid is stronger. It more readily donates protons.  As base strength decreases, acid strength increases.  The stronger the base, the weaker its conjugate acid.  The weaker the base the stronger its conjugate acid.

17 Common Strong Acids/Bases Strong Bases Sodium Hydroxide Potassium Hydroxide *Barium Hydroxide *Calcium Hydroxide *While strong bases they are not very soluble Strong Acids Hydrochloric Acid Nitric Acid Sulfuric Acid Perchloric Acid

A/B Behavior & Chemical Structure 1. Binary Acids Hydrogen and another element 2. Polyprotic Acids Have more than 1 Hydrogen to give away 3. Oxyacids have O in compound 4. Carboxylic Acids have –COOH in compound

Wait, water can go both ways? amphoteric substances can behave as either an acid or base depending on what they react with. water and anions with protons (H + ) attached are the most common amphoteric substances

Autoionization of Water H 2 O + H 2 O OH - + H 3 O 25 o C the concentrations for both [H 3 O + ] and [OH - ] = 1.00 x and [H 3 O + ] [OH - ] = 1.00 x = K w

[H 3 O + ] [OH - ] = 1.00 x = K w Since [H 3 O + ] [OH - ] = 1.00 x = K w [H 3 O + ]=[OH - ] when [H 3 O + ]=[OH - ] the solution is neutral [H 3 O + ]>[OH - ] when [H 3 O + ]>[OH - ] the solution is acidic [H 3 O + ]<[OH - ] when [H 3 O + ]<[OH - ] the solution is basic

The pH scale is a way of expressing the strength of acids and bases. Instead of using very small numbers, we just use the NEGATIVE power of 10 on the Molarity of the H + (or OH - ) ion. Under 7 = acid 7 = neutral Over 7 = base

pH of Common Substances

pH calculations – Solving for H+ If the pH of Coke is 3.12, [H + ] = ??? Because pH = - log [H + ] then - pH = log [H + ] Take antilog (10 x ) of both sides and get 10 -pH = [H + ] [H + ] = = 7.6 x M *** to find antilog on your calculator, look for “Shift” or “2 nd function” and then the log button

Calculating the pH pH = - log [H+] (Remember that the [ ] mean Molarity) Example: If [H + ] = 1 X pH = - log 1 X pH = - (- 10) pH = 10 Example: If [H + ] = 1.8 X pH = - log 1.8 X pH = - (- 4.74) pH = 4.74

pH calculations – Solving for H+ A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the solution? pH = - log [H + ] 8.5 = - log [H + ] -8.5 = log [H + ] Antilog -8.5 = antilog (log [H + ]) = [H + ] 3 X = [H + ] pH = - log [H + ] 8.5 = - log [H + ] -8.5 = log [H + ] Antilog -8.5 = antilog (log [H + ]) = [H + ] 3 X = [H + ]

pOH Since acids and bases are opposites, pH and pOH are opposites! pOH does not really exist, but it is useful for changing bases to pH. pOH looks at the perspective of a base pOH = - log [OH - ] Since pH and pOH are on opposite ends pH + pOH = 14

28 The pH Scale pH [H 3 O + ] [OH- ] pOH

pH testing There are several ways to test pH Blue litmus paper (red = acid) Red litmus paper (blue = basic) pH paper (multi-colored) pH meter (7 is neutral, 7 base) Universal indicator (multi-colored) Indicators like phenolphthalein Natural indicators like red cabbage, radishes

pH indicators Indicators are dyes that can be added that will change color in the presence of an acid or base. Some indicators only work in a specific range of pH Once the drops are added, the sample is ruined Some dyes are natural, like radish skin or red cabbage

31 Indicators

32 pH and acidity The pH values of several common substances are shown at the right. Many common foods are weak acids Some medicines and many household cleaners are bases.

34 Titration & Titration Curves Titration: the adding of one solution of an known concentration into another solution standard solution: a solution with a known concentration Titration curve: a graph showing pH vs volume of acid or base added The pH shows a sudden change near the equivalence point The Equivalence point (a.k.a. stoichiometric point) is the point at which the moles of OH- are equal to the moles of H 3 O +

35 Strong acid-strong base Titration Curve At equivalence point, V eq : Moles of H 3 O + = Moles of OH - There is a sharp rise in the pH as one approaches the equivalence point With a strong acid and a strong base, the equivalence point is at pH =7 Neither the conjugate base or conjugate acid is strong enough to affect the pH pH cm 3 base added

36 Weak acid-strong base Titration Curve The increase in pH is more gradual as one approaches the equivalence point With a weak acid and a strong base, the equivalence point is higher than pH = 7 The strength of the conjugate base of the weak acid is strong enough to affect the pH of the equivalence point

37 Buffered Weak Acid-Strong Base Titration Curve The initial pH is higher than the unbuffered acid As with a weak acid and a strong base, the equivalence point for a buffered weak acid is higher than pH =7 The conjugate base is strong enough to affect the pH

38 Neutralization An acid will neutralize a base, giving a salt and water as products Examples Acid Base Salt water HCl + NaOH  NaCl + H 2 O H 2 SO NaOH  Na 2 SO H 2 O H 3 PO KOH  K 3 PO H 2 O 2 HCl + Ca(OH) 2  CaCl H 2 O A salt is an ionic compound that is formed from the positive ion (cation) of the base and the negative ion (anion) of the acid

39 Neutralization Problems The volume of solution in L multiplied by concentration in moles/L will yield moles. If an acid and a base combine in a 1 to 1 ratio, the moles of acid will equal the moles of base. Therefore the volume of the acid multiplied by the concentration of the acid is equal to the volume of the base multiplied by the concentration of the base. M acid V acid = M base V base If any three of the variables are known, it is possible to determine the fourth.

40 Neutralization Problems Example 1: Hydrochloric acid reacts with potassium hydroxide according to the following reaction: HCl + KOH  KCl + H 2 O If mL of M HCl exactly neutralizes mL of KOH solution, what is the concentration of the KOH solution? Solution: M acid V acid = M base V base (0.500 M) (15.00 mL) = M base (24.00 mL ) M base = (15.00 mL )(0.500 M) (24.00 mL ) M base = M

41 Neutralization Problems Whenever an acid and a base do not combine in a 1 to 1 ratio, a mole factor must be added to the neutralization equation n M acid V acid = M base V base The mole factor (n) is the number of times the moles the acid side of the above equation must be multiplied so as to equal the base side. (or vice versa) Example H 2 SO NaOH  Na 2 SO H 2 O The mole factor is 2 and goes on the acid side of the equation. The number of moles of H 2 SO 4 is one half that of NaOH. Therefore the moles of H 2 SO 4 are multiplied by 2 to equal the moles of NaOH.

42 Neutralization Problems Example 2: Sulfuric acid reacts with sodium hydroxide according to the following reaction: H 2 SO NaOH  Na 2 SO H 2 O If mL of M H 2 SO 4 exactly neutralizes mL of NaOH solution, what is the concentration of the NaOH solution? Solution: In this case the mole factor is 2 and it goes on the acid side, since the mole ratio of acid to base is 1 to 2. Therefore 2 M acid V acid = M base V base 2 (0.400 M) (20.00 mL ) = M base (32.00 mL ) M base = (2) (20.00 mL )(0.400 M) (32.00 mL ) M base = M

43 Neutralization Problems Example 3: Phosphoric acid reacts with potassium hydroxide according to the following reaction: H 3 PO KOH  K 3 PO H 2 O If cm 3 of M KOH exactly neutralizes cm 3 of H 3 PO 4 solution, what is the concentration of the H 3 PO 4 solution? Solution: In this case the mole factor is 3 and it goes on the acid side, since the mole ratio of acid to base is 1 to 2. Therefore 3 M acid V acid = M base V base 3 ( M acid ) (15.00 mL) = (0.300 M) (30.00 mL) M acid = (30.00 mL)(0.300 M) (3) (15.00 mL) M acid = M

44 Neutralization Problems Example 4: Hydrochloric acid reacts with calcium hydroxide according to the following reaction: 2 HCl + Ca(OH) 2  CaCl H 2 O If cm 3 of M HCl exactly neutralizes cm 3 of Ca(OH) 2 solution, what is the concentration of the Ca(OH) 2 solution? Solution: In this case the mole factor is 2 and it goes on the base side, since the mole ratio of acid to base is 2 to 1. Therefore M acid V acid = 2 M base V base ( 0.400M ) (25.00 mL) = (2) ( M base ) (20.00 mL) C base = (25.00 mL) (0.400 M) (2) (20.00 mL) C base = M

45 Buffer Solutions - Characteristics A solution that resists a change in pH. It is pH stable. A weak acid and its conjugate base form an acid buffer. A weak base and its conjugate acid form a base buffer. We can make a buffer of any pH by varying the concentrations of the acid/base and its conjugate.