CHM 101 Chemistry of the Kitchen Acids and Bases Questions

Revised 6/24/19CHM 101 – Chemistry of the Kitchen: Acids and Bases
OBJECTIVES
•
•
•
Classify common household chemicals as either acids or bases using pH paper
and pH meter
Develop an understanding of the pH scale.
Examine the differences between strong and weak acids.
INTRODUCTION
Take a moment and look around your house. Many everyday chemicals used in your home
are acids or bases. Some of the most common acids and bases you may recognize and use
include vinegar (acetic acid), lemon juice (citric acid), and ammonia. Acids and bases are
essential substances in home, industry, and the environment. For example, the vast quantity of
sulfuric acid manufactured in the United States each year is needed to produce fertilizers,
polymers, steel, and many other materials. The influence of acids on the environment can be
seen through acid rain, which has caused numerous historic buildings and monuments to erode.
If you have ever had a goldfish, you know how important it is to monitor and control the
acidity of the water in the aquarium. A characteristic that acids and bases share is their ability
to turn certain organic compounds, such as vegetable materials, distinctive colors.
BACKGROUND
Since the 17th century, acids and bases have been characterized by their sour and bitter tastes,
respectively. In modern chemistry, these concepts have taken on considerably more precise
meaning. In fact, there are three definitions of acids and bases, the classical
(Arrhenius), the Bronsted-Lowrv, and the Lewis, which greatly expand our knowledge
of these chemicals. Acids and bases differ greatly in their strength in water, that is, in
the amount of H3O+ or OH−· produced per mole of substance dissolved. They are
generally classified as either strong or weak, depending on the extent of their dissociation
into ions in water. Acids and bases are electrolytes in water, so the classification of acid
and base strength correlates with the classification of electrolyte strength: strong
electrolytes such as the strong acids dissociate completely and weak electrolytes, such as
the weak acids undergo partial dissociation (Figure 1).
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Revised 6/24/19
Strong acid: HA (g) or (l) + H2O (l) → H3O+ (aq) + A− (aq)
Weak acid: HA (aq) + H2O (l) ⇌ H3O+ (aq) + A− (aq)
Figure 1. The behaviors of strong acid and weak acid in water
In a solution of a strong acid, virtually no HA molecules are present (Equation 1).
HA (aq) + H2O (l) → H3O+ (aq) + A− (aq)
Equation 1
The [H3O+] = [A−] ≈[HA]initial and the [HA]equilibrium ≈ 0. Since this process essentially goes to
completion, a single arrow is used (→) in the case of strong acids.
The situation is different in the case of a weak acid. From Figure 1, the majority of HA
molecules in a solution of a weak acid are undissociated. Thus, [H3O+] ≪ [HA]initial and
[HA]equilibrium ≈ [HA]initial. Notice in Equation 2 that the reaction is expressed as an
equilibrium (⇌), indicating that the reaction does not necessarily produce 100% products.
In fact, in the case of most weak acids, less than 5% of the original HA molecules will
actually dissociate to produce H3O+ (aq) and A− (aq) ions.
HA (aq) + H2O (l) ⇌ H3O+ (aq) + A− (aq)
Equation 2
The percentage of acid that is actually dissociated can be quantified in terms of the acid
dissociation constant, Ka, whose expression for the dissociation of a general weak acid, HA, in
water is shown in Equation 3. Notice [H2O] does not show up in the form of Ka.
𝐾𝑎 =
[𝐻3 𝑂 + ][𝐴− ]
[𝐻𝐴]
Equation 3
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Revised 6/24/19
Like any equilibrium constant, the magnitude of Ka tells how far to the right the
reaction has proceeded when equilibrium is reached. Thus, the stronger the acid, the
higher the [H3O+] at equilibrium, the larger the Ka:
Stronger acid => higher % HA dissociation => higher [H3O+] = larger Ka
Likewise, the weaker the acid, the smaller the Ka:
Weaker acid => lower % HA dissociated => lower [H3O+] = Smaller Ka
The pH Scale
In aqueous solution, [H3O+] can cary over an enormous range: from about 10 M to 10-15 M.
To handle numbers with negative exponents more conveniently in calculations, we convert
them to positive numbers using a numerical system called a p-scale, the negative of the
common (base -10) logarithm of the number. Applying this numerical system to [H3O+]
gives pH, the negative logarithm of [H3O+] or [H+] (Equation 4).
𝑝𝐻 = −log⁡[𝐻3 𝑂+ ]
Equation 4
For example, a solution with a [H3O+] of 5.4 x 10-4 M has a pH of 3.27 (Equation 4a).
𝑝𝐻 = − log[𝐻3 𝑂+ ] = − log(5.4⁡𝑥⁡10−4 ) = 3.27
Equation 4a
Likewise, the pH of a 1.0 x 10-12 M H3O+ solution is 12.00 on the pH scale. Note that the
higher the pH, the lower the [H 3O+]. Therefore, an acidic solution has a lower pH
(higher [H3O+]) than a basic solution. At 25°C the [H3O+] in pure water is 1.0 x 10-7 M,
so the pH of pure water at 25°C is 7.00 and aqueous solutions typically fall within a range
of 0 to 14. This information is summarized in Table 1.
Table 1.
Type of solution
Basic
Neutral
Acidic
pH
> 7.00
= 7.00
< 7.00 In the laboratory, pH values are usually obtained with an acid-base indicator or with a more precise Instrument called a pH meter. OVERVIEW In this series of experiments, you will examine several of the various methods in which the pH of a solution can be measured. You will determine the pH of some common household solutions by using pH paper and pH sensor. Page 3 of 4 Revised 6/24/19 PROCEDURE You will determine the pH of the following solutions: liquid Nox (soap), PowerAde lemon lime, sprite, baking soda, glass cleaner, vinegar, tap water, and DI water. 1. Test the pH of each substance using pH paper. Using a glass rod or disposable pipet to transfer a small amount of liquid onto pH paper. On the data sheet, record the pH by comparing the color to the scale associated with the paper. 2. Test the pH of each substance using pH sensor and LabQuest: BE GENTLE WITH THE ELECTRODES!!! They are sensitive and expensive. DO NOT BUMP THE ELECTRODES AGAINST THE CONTAINER OR ANYTHING!!! Connect the pH sensor to one of the channels of the LabQuest. To remove the pH sensor from its container, unscrew the cap. KEEP THE SOLUTION in the container. Rinse the electrode with DI water, then wipe the electrode with Kimwipe gently. Dip the electrode into the household solution to be tested. Record the pH value after the reading stabilizes. Remove the electrode, rinse it with DI water and wipe it with Kimwipe before testing the next solution. 3. After measuring the pH of all solutions, make sure to put the pH electrodes back into its container containing storage solution. DO NOT LET THE ELECTRODES DRY OUT!!! Page 4 of 4 Revised 6/19/17 Name _______________________________Station # _______ Date _________________ CHM 101 Chemistry of the Kitchen: Acids and Bases Pre-Lab Questions 1. Write the equations for the dissociation of the following acids and bases in water. a. HNO3 b. HF c. Mg(OH)2 2. Give the balance equation of the following acid/base neutralization reactions a. HNO3 + KOH b. H3PO4 + Ca(OH)2 3. Calculate [H3O+] and [OH−] from the following pH a. pH = 3.25 b. pH = 8.45 Page 1 of 1 Revised 4/18/19 Name ____________________________Station # ________ Date __________________ CHM 101 CHEMISTRY OF THE KITCHEN: ACIDS AND BASES Report Sheet Table 1. pH of household products Name pH paper reading pH meter reading PowderAde lemon lime Glass cleaner Liquid Nox (soap) Baking soda Sprite Vinegar Tap water DI water 1) How is the pH read by pH meter compared to pH read by pH paper? 2) What are properties of acids and bases? 3) Calculate concentrations of H3O+ and OH- and write the answer in Table 2. Show your work using the pH meter reading. Page 1 of 2 Revised 4/18/19 Table 2. Concentrations of H3O+ and OH- for household products (using the pH meter reading) Name [H3O+] [OH-] PowderAde lemon lime Glass cleaner Liquid Nox (soap) Baking soda Sprite Vinegar Tap water DI water Page 2 of 2 CHEMISTRY OF THE KITCHEN: ACIDS AND BASES Make-up Data Instructions: Use the following data to complete the data /calculation sheet of the experiment. Write your weekly lab report for this lab, using this data and following the lab syllabus instructions Table 1. pH of household products Name pH paper reading pH meter reading Powder Ade lemon lime 4 3.70 Glass cleaner 9 9.24 Liquid Nox (soap) 6 6.35 Baking soda 8 8.57 Sprite 4 4.30 Vinegar 4 3.95 Tap water 6 6.64 DI water 5 5.70 Revised 6/20/17 Name _________________________ Station # ______ Date ______________ CHM 101 Chemistry of the Kitchen: Acids and Bases Post-Lab Questions 1. Which household solutions are acidic? Which one is the strongest acid among them? 2. Which household solutions are basic? Which one is the strongest base among them? 3. List 2 acids and 2 bases you encounter everyday (do not use the items from this experiment). Page 1 of 1