How is the Strength of an Acid Determined?

Palm Ng

Bayside High School, Bayside, Queens

Summer Research Program for Science Teachers

August 2009

Aim: How is the strength of an Acid defined?

Materials:

Electric Pickle set up:  2 stands, 2 claps, to corks with iron nails ( 3-4”) in each Cork, and wire with plug at one end. Dill Pickles

Conductance set up: light blub with electrodes and wire plug. 6 beakers, HCl(aq), CH3COOH(aq), NaCl, Sugar, ethanol, NaOH, lemon, lime, O.J.

Objectives:

SWBAT:

1)   Describe how conductance can be measured?

2)   Distinguish among the properties of non electrolyte, strong electrolyte, and weak       

       electrolyte

3)   Define: ionization

4)   Identify & compare the dissociation of ionic and molecular compounds in                               

      aqueous solutions

5)   Describe what determines a strong acid/base and a weak acid/base

Do Now:    (1) complete and balance the following equation

            HNO₃(aq)  +  KOH (aq)  ---------à   ___________  +  ___________

                   (2) using Reference Tables, give the names of all the compounds above

Motivation:

Each and everyone WATCH  Demo: Electric Pickle

1. Q Why does it light up?  Complete circuit (flow of electrons)

2. Q What do we need to conduct electricity? (flow of e’s, moving charges, an electrolyte)

3. Q What is in the pickle which aids the flow of e’s?  (water, salt, vinegar)

4. Q How are pickles made? Recipe for pickles? Label? (water,salt,vinegar,dill,garlic)

5. Q Does NaCl (s) have charges? What charges do they have? YES, Na+, Cl^-

6. Q Will it conduct electricity? No why? (charges can not move in solids)

7. Q Will NaCl (aq) conduct electricity? Yes Why? (b/c charges can flow)

         Remember: What did you learn about vinegar?

8. Q Does vinegar have charges? Yes, why? Acid = Electrolyte

            CH₃COOH(aq)  --------à  CH₃COO^-(aq)  +  H⁺(aq)

When solutions contain charged particles, they can carry e’s (electricity) so the light bulb lights up.

9. Q What do you think the word  CONDUCTANCE means?

Conductance- a measure of a solution’s ability to conduct.

Electrolytes- any substance when dissolved in solution will break up(dissociate/ionize) and conduct an electric current.

            Ex:  NaCl(aq)  ----------à  Na⁺(aq)  +  Cl^-(aq)   

Dissociates- ionic compounds break apart giving ions(charged particles)

10.Q What do you think the word non electrolyte means?

Nonelectrolyte – any substance when dissolved in solution will NOT conduct electricity

                        Usually covalent compounds, with exception of acids

            Ex: Sugar(s)   C₆H₁₂O₆(aq)  -------à

Handout Electrolytes Worksheet

Ask a volunteer read the directions out loud.

Q Does anyone have questions?

Student will do:

Q1  NaCl

Q4 HCl

Q5 C₆H₁₂O₆

Q6 NaOH

Q7 C₂H₅OH ethanol

Ask volunteers to give answers from the handout.

Q. OJ, lemonade, soda are acids, yet we drink them. Why won’t we drink HCl(aq), it is in our

stomachs already? 

Q. Why do we wear gloves when handling HCl(aq)? And not OJ?

Demo:  Conductance

Write the following on the board:                                                      

                                                                    Observation light intensity

HCl(aq) -----à  H⁺ (aq)    +  Cl^-(aq)                        (bright)               (strong Acid  - st electrolyte)

CH₃COOH (aq)  ------à   H⁺(aq)   +   CH₃COO^-                  (dim)      (weak Acid  - weak electrolyte)

11. Q Which is the stronger Acid?   Why?         (Brighter)

S pair/share(1 min)

12. Q How is the strength of an acid defined using the data I just gave you?

13 Q What does the strength of an acid /base depend on?

Student answer:

Strength of an acid/base depends on the degree of ionization/dissociation in water

Strong acid (st electrolyte) most  of the substance breaks apart( ionizes/dissociates) into ions

                                           Many ions in solution, ionizes completely

Weak acid (wk electrolyte)  very few molecules break apart to form ions in solutions

                                                Most of the molecules remain intact.

                                                Have very few ions in soln.

Summary assessment

Using short but complete sentences and using four of the new vocabulary words you learnt in class answer the following question.  (hand in at end of class as they leave).

Q  Why is it safe to drink Orange Juice and not Hydrochloric acid?

If time available : Share out one or two student summaries.

Homework: finish Electrolytes Handout,

         R P 605-611 P.611Q24,28,29

         Prentice Hall, Chemistry  textbook.

NYSPS (New York State Performance Standards)

S1c – Demonstrates an understanding of chemical reactions

S5b – Use concepts from Science Standards 1 to 4 to explain a variety of observations and phenomena

S5f - works individually and in teams to collect and share information and ideas

S6b – Records and stores data using a variety of formats