2. 1 - Strong vs. Weak Acids - Indicators Mr. ShieldsRegents Chemistry U15 L04

3. 2 Acid strength We all know there must be a difference between Different acids. For instance Vinegar is acetic acid and we all Put vinegar on our food from time to time. People used to chew willow bark to help reduce fever. In doing so they were swallowing salicylic acid (an Organic Acid). But would anyone try putting sulfuric acid (car battery acid) instead of vinegar on their food and eat it??? I HOPE NOT !!!

4. 3 Strong vs. Weak acids/bases Obviously some acids are stronger than others. But what makes one acid strong and the other Weak? A Strong acid is one that is mostly dissociated (separated into ions) when dissolved in Water. The same can be said for bases. Strong bases are Also highly dissociated in water.

5. 4 -Do not confuse strong acid vs. weak acids & bases with concentration. - 6M Acetic acid (CH 3 COOH) is just as concentrated as 6M H 2 SO 4 or 6M HCl But it is a much weaker acid NOTE:

6. 5 Strong & Weak Acids Example of a Strong Acid: HCl    H + + Cl - Lots of H + & Cl -. Very little undissociated HCl in Sol’n. Example of a Strong Base: KOH    K+ + OH- Lots of K + & OH -. Very little undissociated KOH in Sol’n.

7. 6 Strong vs. Weak acids/bases So if strong acids and bases are highly dissociated In solution what defines a weak acid or base? Weak acids are NOT highly dissociated into their respective ions in solution The same can be said for weak bases. They are Also NOT highly dissociated in solution.

8. 7 Weak Acids and bases Weak acids/bases don’t ionize very much in solution. Instead they tend to reach an equilbrium that is mostly the undissociated acid or base:Weak acids/bases don’t ionize very much in solution. Instead they tend to reach an equilbrium that is mostly the undissociated acid or base:  C 2 H 4 O 2  H + + C 2 H 3 O 2 - (4% dissociated) Lots of C 2 H 4 O 2. Very little H +, C 2 H 3 O 2 -. (CH 3 COOH) (CH 3 COO - )  NH 4 OH  NH 4 + + OH - (in equilibrium) Lot’s of NH 4 OH and very little NH 4 +, OH - “Vinegar”

9. 8 The greater the dissociation of the acid (or base) The Stronger the acid (or base) Strength of acid/base 1%5%40% 90% 100% Degree of dissociation Metals react more vigorously with strong acids than weak acids Mg + HCl  MgCl 2 + H 2 Strong acid/ Fast reaction Mg + CH 3 COOH  CH 3 COOMg + H 2 Weak Acid/ Slow reaction

10. 9 Mg & Weaker acid Mg & Strong acid Like H 2 SO 4 Like Acetic Acid H2

11. 10 Strong Acids HCl HBr H 2 SO 4 Sulfate HNO 3 Nitrate Strong Bases NaOH KOH Ca(OH) 2 Mg(OH) 2 Weak Acids H 2 C 2 H 3 O 2 (acetic) HCN HF HNO 2 (Nitrite) or H 2 SO 3 (Sulfite) H 2 CO 3 Weak Bases NH 3

12. 11 Indicators Remember that pH is a measure of how much H + is in solution. Let’s look at how we can measure pH. There are two ways we might do this: 1) Measure pH directly using electronic instruments 2) Use INDICATORS Indicators are substances that change color Based on the pH of the solution.

13. 12 Indicators Look at Table M in your Reference Table Booklet. The table lists the color changes of specific indicators at specific pH’s It also lists The useful pH Range of each Indicator. Using these Indicators we Can determine The pH of an unknown.

14. 13 Color range of a few indicators

15. 14 Solving Problems w/ Indicators Problem: An Unknown sol’n turns methyl orange Yellow & turns bromocresol green…green. What is the pH of the solution? 2) Methy orangeif pH > 4.4 it’s yellow 1) Check Methyl orange in table M: 3) Check Bromocresol green <3.8 yellow >5.4 blue 4) pH must be >4.4 but < 5.4 and a green color must be the midpoint of 3.8 and 5.4 or about 4.6

16. 15 Solving Problems w/ Indicators Problem: An Unknown sol’n causes Bromothymol blue to turn blue and phenolphthalein To turn colorless. What is the approximate pH of The solution? 2) Bromthymol Blue if pH is >7.6 it’s blue 1) Check Bromothymol blue pH range in table M: 3) Check Phenolphthalein <8.2 colorless 4) pH is between 7.6 and 8.2