Activity 7

Activity 7: Acids and Bases

 

Water is everywhere!  So, lets spend one more activity learning about one of the key aspects of water.  Water has the ability to dissociate (break apart from HOH (or H₂O) into H+ ions and OH- ions).  We refer to solutions with lots of H+ ions as acids and solutions with lots of OH- ions as bases.  By adding chemicals with H+ ions acidic solutions can be made.  By adding chemicals with OH- ions basic solutions can be made.

Activity Tasks:

1. Review the Content Slides Acids and Bases on the D2L site.  

2. Complete the Teaching Idea “Concept Questions for Chemistry using PhET”  posted by Trish Loeblein on the pH Scale simulation at PHET (http://phet.colorado.edu/en/simulation/ph-scale). On your blog post the answers with your scientific explanations from the “Clicker Questions pH Scale” posted by Trish.

1. B. False. This is because when I moved the amount of acid to base ration on the PH scale simulation test, the color stayed the same.  So this would mean that the question is false, the color of a solution does not identified if it is an acid, base, or neutral solution.

2. D, because more than one of the photos shown above is a basic solution. 7 is natural on the pH scale, so anything higher than 7 is a basic.

3. C because the color red is acid, and there are a few bases (OH-) mixed in with the H₃O+. A is about 13 on the pH scale, B is showing a photo of water, and c could be a photo of vomit. 2 on the pH scale.

4. B, it cannot be A because that is showing equal amounts of acid to base. C is showing way more acid then bases.

5. D. A is showing the molecules of coffee, B is showing pop, and c is showing 13 in the pH scale. The correct answer is D because both coffee and pop are very acidic.

6. A. increases the pH. The more water lessens the acidity, so pH goes up.

7. B. more water lessens the basicity, so pH goes down. Decreases the pH.

8. A ABC, The lower numbers are acid, and the higher numbers are bases.

9. C BAC because water 7, acid 0 and base 14.  Water is 7 in pH scale, pop is 2.5 and b is showing 13 on a pH scale.

10. A, something (base) was added and made the equilibrium shift left. The pH scale from left to right goes 14-0.

 

3. Complete the Teaching Idea “Intro to Strong and Weak Acids and Bases” posted by Chris Bires on the Acid-Base Solutions simulation (http://phet.colorado.edu/en/simulation/acid-base-solutions) and post on your blog your data and answers to the questions posed.

Introduction to Strong and Weak Acids and Bases PhET Lab (rvsd 5/2011)

        How does the strength of an acid or base affect conductivity?pH?

Introduction:

When you test your pool’s pH, what are you those little vials or paper strips telling you?  When you hear an acid called “strong” or “weak”, what do those terms refer to?  In aqueous solutions, compounds can exist as molecules (undissociated) or ions (dissociated).  When an acid or a base exists in solution nearly completely as dissociated ions, we refer to that acid or base as strong.  A weak acid or base will donate ions to the solution, but will remain primarily as undissociated molecules.

 

Notation:

Acids are abbreviated HA, with the H representing the proton (H+) the acid donates to the solution.  The A is referred to as the acidic anion (A-) that is left in solution as the proton is donated.

Strong Bases are abbreviated MOH, with the OH representing the hydroxide ion (OH-) the base donates to the solution.  The M is cation (M+) that is left in solution as the hydroxide is donated. .

 

Autoionization:

Even without any acid or base added a very small number of water molecules will form protons (H+) and hydroxide ions (OH-).  The protons will then form hydronium ions, the acid ion.

 

Procedure: PhET Simulations à Play With Sims à Chemistryà Acid-Base Solutions à  

The concentration of the acids and bases used in the  at 0.010 (10^-2) Molar.

 

·         Begin with a strong acid and lower the pH probe into the beaker.  What is the pH of this solution? 

·         Test this strong acid with both pH paper and the conductivity probe.  What color does the pH indicator become?  Is this strong acid an electrolyte?  Does current travel through this solution?

·         Repeat the above tests with the weak acid, the strong base, and the weak base, and water.  Collect your observations in the table below:

 

	Strong Acid	Weak Acid	Strong Base	Weak Base	Water
pH meter read (value)	2.00	4.50	12.00	9.50	7.00
pH paper (color)	0.00 darkest red	6.00 light orange	11.00 medium blue	9.00 greenish-grey	7.00 dull yellow
Conductivity (bright/dim/none)	bright	dim	bright	dim	dull
Exists as Mostly (ions/molecules)	Mostly H30+	D, C

Procedure:

This simulation allows you to change the concentration of a strong and weak acid and base.

Complete the table below for some strong acids and bases and weak acids and bases by adjusting the concentration.

Strong Acids

Strength	Initial Acid Concentration (mol/L)	[HA] (mol/L)	[A-] (mol/L)	[H+] (mol/L)	pH
	.010 M	negligible	1.00 x 10 -2	1.00 x 10 -2	2.00
	.050 M	negligible	5.00 x 10-2	5.00 x 10-2	1.30
	.100 M	negligible	9.9 x 10-2	9.9 x 10-2	1.00
	1.00 M	negligible	9.90 x 100	9.90 x 100	0

Weak Acids

Strength (approximately)	Initial Acid Concentration (mol/L)	[HA] (mol/L)	[A-] (mol/L)	[H+] (mol/L)	pH
	.015 M	1.50 x 10-2	3.81 x 10-5	3.81 x 10-5	4.42
	.150 M	1.50 x 10-1	1.21 x 10-4	1.21 x 10-4	3.92
	.015 M	1.93 x 10-4	1.48 x 10-2	1.48 x 10-2	1.83
	.150 M	1.58 x 10-2	1.34 x 10-1	1.34 x 10-1	.87

Strong Bases

Strength	Initial Acid Concentration (mol/L)	[MOH] (mol/L)	[M+] (mol/L)	[OH-] (mol/L)	pH
	.010 M	neg	1.00 x 10-2	1.00 x 10-2	12
	.050 M	neg	5.00 x 10-2	5.00 x 10-2	12.70
	.100 M	neg	1.00 x 10-1	1.00 x 10-1	13
	1.00 M	neg	1.00 x 100	1.00 x 100	14

Weak Bases

Strength (approximately)	Initial Acid Concentration (mol/L)	[B] (mol/L)	[BH+] (mol/L)	[OH-] (mol/L)	pH
	.015 M	1.50 x 10-2	2.20 x 10-5	2.20 x 10-5	9.34
	.150 M	1.50 x 10-1	6.9 x 10-5	6.9 x 10-5	9.84
	.015 M	1.50 x 10-2	2.20 x 10-5	2.20 x 10-5	9.34
	.150 M	1.50 x 10-1	6.95 x 10-5	6.95 x 10-5	9.84

 

 

1.      A strong acid is very concentrated / exists primarily as ions.

 

2.      A weak base is a nonelectrolyte / weak electrolyte / strong electrolyte.

 

3.      A strong base is a nonelectrolyte / weak electrolyte / strong electrolyte.

 

4.      At the same concentration (Molarity) a strong acid will have a higher / lower / the same pH as a weak acid.

 

5.      As concentration of a weak acid increases, the pH increases / decreases / remains constant.

 

6.      As concentration of a weak base increases, the pH increases / decreases / remains constant.

 

7.      As the concentration of a weak acid increases, the number of ions increases /decreases / remains constant.

 

8.      As the concentration of a weak acid increases, conductivity increases / decreases / remains constant.

 

9.      As the strength of a weak acid increases, the proportion of ions to molecules increases / decreases.

 

10.  As the strength of a weak acid increases, the conductivity increases / decreases / remains constant.

 

11.  What are the pH values of a weak acid with a concentration of 0.10 and a strong acid with a concentration of 0.01, ten times lower? Weak acid, 0.10 M : 4 Strong Acid, 0.01 M : 2

 

12. Explain the significance of the results of your calculation above.

On the pH scale if you go ten times lower, the pH will only drop one digit lower. For someone that has never read the pH scale before, it can be confusion because each number is 10x times bigger (or smaller) than the number preceding. Depending on which way you are going on the pH scale always make sure that you are writing your answer with 10x. pH scale is a logarithmic. This is where base 2 crosses the x axis horizontal axis, and one passes through the points with (x,x) coordinates.