What’s really happening in an aqueous acid or base solution?

2nd Semester General Chemistry

The Story so Far…

And now…

We will now see if we can apply our model to one of the most significant classes of aqueous reaction systems: acid and base solutions. Can we better understand acid and base substances by exploring their behavior in water? And since these solutions are “closed” systems, how might the model of equilibrium apply to acid and base solutions?

In closed systems, we have observed how reversible reactions reach an equilibrium state, where the forward and reverse rates of change are equal. The equilibrium state is usually described by the equilibrium constant, which is equal to the value of Q when the reaction has reached equilibrium:

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Figure 1: Pictogram from the Globally Harmonized System of Classification and Labeling of Chemicals

List any ideas that come to mind when you hear the terms “acid” and “base”. Be prepared to share your ideas (and questions!) with your group, and the rest of the class.

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Figure 2: Data Table for substances to be tested

If testing solutions yourself:

• Wear goggles.
• Your instructor will demonstrate the use of the conductivity probe. Be sure to rinse the conductivity probe with distilled water between each test.
• Use a small piece of fresh litmus paper for each test.

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Group Conversation Starter:

Based on your observations of conductivity, do you think a solution of distilled water contains mainly molecules or mainly ions? What about the other solutions tested? Discuss and explain your reasoning!

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Group Conversation Starter:

Now consider the structures of an H₂O molecule and an HCl molecule. What ions might result in the solution when HCl is mixed with water? Develop a model that could explain how these ions result.

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Group Conversation Starter:

Now consider the structures of an H₂O molecule and an HC₂H₃O₂ molecule. What ions might result in the solution when HC₂H₃O₂ is mixed with water? Develop a model that could explain how these ions result.

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Group Conversation Starter:

Next, consider the structures of an H₂O molecule and an NH₃ molecule. What ions might result in the solution when NH₃ is mixed with water? Develop a model that might explain how these ions result.

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Figure 3: Reaction of NH₃(aq) and HC₂H₃O₂(aq)

Now, using your understanding of base strength so far, predict the conductivity of a 50-50 mixture of HC₂H₃O₂ and NH₃. After reporting your predictions, the instructor will demonstrate this mixture.

Using the observed conductivity results, rank the strengths of the two bases in this reaction

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Figure 4: Rank ordering base strength

Finally, you now have enough information to rank-order the strengths of H₂O(l), Cl^1– (aq) , C₂H₃O₂^1–(aq), OH^1–(aq) and NH₃(aq). Show your ranking from best base to worst base, as illustrated below:

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Best Base → → → → Worst Base

Figure 5: Predicting equilibrium distributions using base strength

In reactions 1–3* below, identify the competing bases in each reaction and make a comparison of Base Strength. Then predict the extent of reaction (quantitative / limited OR mostly reactants / mostly products).

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Figure 6: Models for Acids and Bases

Many before you have studied and attempted to define compounds as acids and bases. Two early models, named after the scientists who proposed them, are summarized below:

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Descriptions of Ionization

In your groups, label the competing bases, then write the appropriate equilibrium expression for each reaction:

HA(aq) + H₂O(l) ⇄ H₃O¹⁺(aq) + A^1–(aq) K =

​

B(aq) + H₂O(l) ⇄ BH¹⁺(aq) + OH^1–(aq) K =

​

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Figure 7: Data Tables for some acids and bases

Group Conversation Starters:

What do you notice about the acids and bases in the tables?

How does this relate to our discussions of Base Strength, the idea of H¹⁺ thievery / competition, and the terms “strong” and “weak”?

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Examine the data presented in Tables 1-4

Group Discussion Questions:

Discuss the questions on Pages 7 & 8 with your group. Make sure to provide appropriate evidence and justification for your claims!

Whiteboard and/or present your ideas as instructed!

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“Big Ideas” / Ticket out the door

What are the two-three most important ideas you’ll take away from the tests you did today and the conversations you had about acid and base solutions?

What’s one question you still have?

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