1. Module 5 Intro

1.19. Page 4

Lesson 3

Module 5—Acids and Bases

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Strong and Weak Acids

 

While acids generally demonstrate similar properties, there can be noticeable differences. In the Comparing Two Acids lab, you probably noticed some differences between the behaviour of ethanoic acid and hydrochloric acid. Possibly, the most noticeable difference between the two acid solutions was the intensity of their reaction to magnesium. While magnesium reacts with both acids, the reaction with hydrochloric acid produces bubbles quickly compared to its reaction with ethanoic acid. You may have also noticed that hydrochloric acid is more conductive and has a lower pH compared to ethanoic acid.

 

Maybe these differences could be due to the hydrochloric acid solution being more concentrated; but by examining the list of materials in the lab, both solutions had identical concentrations. So, what could be the difference between these solutions?

 

The results from diagnostic tests, such as conductivity and pH, along with the difference in speed of reaction, differentiate acids into two groups—strong acids and weak acids. Refer to the following table. You may need to review the previous lessons or consult your textbook to refresh your memory about the scientific principles involved with these tests.

 

Test

Strong Acid*

Weak Acid*

conductivity

Observation: highly conductive

Interpretation: high concentration of ions [H3O+(aq) and anions] in solution

Observation: slightly conductive

Interpretation: low concentration of ions [H3O+(aq) and anions] in solution

pH

Observation: low pH

Interpretation: higher concentration of H3O+(aq); more extensive reaction between solute and water to form H3O+(aq)

Observation: low pH

Interpretation: lower concentration of H3O+(aq); less extensive reaction between solute and water to form H3O+(aq)

reaction with magnesium

Observation: quick, vigorous bubbling

Interpretation: faster rate of reaction; higher concentration of H3O+(aq) in the solution

Observation: slower, less vigorous bubbling

Interpretation: slower rate of reaction; lower concentration of H3O+(aq) in the solution

* Observations and interpretation are based on tests of solutions containing equal concentrations of strong and weak acids.

 

While completing the Comparing Two Acids lab, you may have developed a hypothesis relating the differences between strong and weak acids to the extent to which the acids ionize.

 

Read “Strong and Weak Acids” on pages 254 and 255 of your textbook. As you read this section, summarize what you have learned by writing the definitions for a strong acid and a weak acid. You may wish to compare your summary with the following points.

 

Summary Point

Definitions

Effects

There is a difference between strength and concentration of an acid or base.

Strength refers to extent of the reaction of the acid or base with water to form hydronium ions, or hydroxide ions, respectively.

 

Concentration refers to the quantity of acid or base particles dissolved in solution.  It does not account for how many of these particles actually react with water to form hydronium ions, or hydroxide ions, respectively.

A concentrated solution of a weak acid can have many ionized particles.

 

A dilute solution of a strong acid may contain few ionized particles.