Science 11th grade
LEARNING UNIT
What is everything around
us made of?
S/K
LEARNING OBJECT
Why is the pH scale non-linear?
Cognitive
Objective:
To analyze the ionic
equilibrium of water and its relation with the pH of
aqueous solutions.
SKILL 1: Contrast the three existing theories that
define acids and bases.
SKILL 2: Interpret the ionic equilibrium of pure water
and its relation with the Kw constant.
SKILL 3: Interpret curves of titration to explain the
process of neutralization of strong and week acids
with strong bases, and strong and weak bases with
strong acids.
Psychomotor Objective: To calculate the pH value
based on acidity and basicity constant.
SKILL 4: Use the Ka and Kb constant to estimate the
pH of aqueous solutions based on the calculation of
concentrations of hydronium and hydroxide ions.
SKILL 5: Compare the behavior observed in a titration
monitored with potentiometers and pH indicators.
Attitudinal Objective: To argue the importance of
pH in living systems.
SKILL 6: Research into the pH of human blood and its
buffer system.
Language
Socio cultural context of
the LO
Curricular axis
Standard competencies
Background Knowledge
English
Colombia
Living environment
Relate the structure of organic molecules to their
physical and chemical properties and their capacity
for chemical change.
Dilution, mixing atom, chemical reactions.
Vocabulary Box
ü Acceptor: Chemistry. An atom, ion, group of
atoms, or compound that combines with, or
accepts, another entity, thereby profoundly
affecting physical and chemical properties.
ü Blood pressure: the pressure of the blood
against the inner walls of the blood vessels,
varying in different parts of the body during
different phases of contraction of the heart and
under different conditions of health, exertion,
etc.
ü Gorge: a narrow space with rocky walls,
especially one through which a stream runs.
ü Heartburn: an uneasy burning sensation in the
stomach, typically extending toward the
esophagus, and sometimes associated with the
eructation of an acid fluid.
ü Junk food: food, as potato chips or candy, that
is high in calories but of little nutritional value.
ü Realize: to understand clearly.
Retrieved
on
June
http://www.dictionary.com
English Review Topic
01,
2016
Passive voice
NAME: _________________________________________________
GRADE: ________________________________________________
from
Introduction
Avatar 1: My stomach hurts. The doctor said it was heartburn.
Avatar 2: What is it?
Avatar 1: He said there is excess of hydrochloric acid in my stomach.
Avatar 2: Oops, and how did it reached your stomach?
Avatar 1: well... the doctor said that heartburn could be caused by
eating too much, or having irritating food. This also happens to people
suffering abnormal blood pressure, that is, someone suffering stress,
insomnia, anxiety problems, and emotionally unstable.
Avatar 2: I think that all the junk food we have and the stress of school
exams is causing your stomachache.
Avatar 2: Did the doctor give you a prescription?
Avatar 1: Yes, I can have Mylanta or milk of magnesia to neutralize my
stomach.
Avatar 2: I hope you get better, see you tomorrow at school. Bye!
Avatar 1: Thanks, bye!
Objectives
•
•
•
To analyze the ionic equilibrium of water and its relation with the
pH of aqueous solutions.
To argue the importance of pH in living systems.
To calculate the pH value based on acidity and basicity constant.
Activity 1:
Skill 1: Contrast the three existing theories that define acids and bases.
Skill 2: Interpret the ionic equilibrium of pure water and its relation with
the Kw constant
What are the theories that explain the definition of acids and
bases?
For many years, scientists have tried to explain what an acid and a base
is. Initially they were differentiated based on their sensory properties;
for example, they could differentiate an acid by its sour flavor (vinegar)
and a base by its soapy touch (soap). However, they did not know their
chemical properties. This is how they began generating scientific
theories to explain acids and bases in the XIX century.
Figure 1. Illustration of the hierarchy of acid-base theories. Arrhenius
definition of acids and bases is a subclass of the Brønsted-Lowry
definition, and Lewis includes the definitions of Arrhenius and BrønstedLowry.
Arrhenius
According to Svante Arrhenius (Swedish chemist), most substances form
ions when they are dissolved in water or other solvents, and he called
them electrolytes. From 1880 to 1890, he related acids to the presence
of H+ ions, and bases to the presence of OH- ions. So he defined acids
as substances that produce H+ ions when dissolved in water, and bases
as substances that produce OH- ions. However, his definitions were
limited because they were only for aqueous dissolutions (Brown et al.,
2004; Gallego Picó et al., 2013).
Brønsted-Lowry
In 1923, Danish chemist Johannes Brønsted, and English chemist
Thomas Lowry suggested a new and more general definition for acids
and bases. The base of this new theory stated that acid-base reactions
implied the transfer of H+ ions from one substance to another. They
defined an acid as a substance able to donate protons and a base as a
substance able to accept them. For example, when hydrochloric acid is
mixed in water, one proton from the acid molecule is transferred to the
water molecule. In this case water behaves as a base (Brown et al.,
2004; Gallego Picó et al., 2013).
Figure 2. Reaction of proton transfer from hydrochloric acid.
This definition is not only applicable to aqueous dissolutions, but also to
other type of dissolutions.
Lewis
In 1923, Gilbert Newton Lewis, American physical chemist, realized that
the meaning of bases and acids have to be a lot broader. For Lewis, an
acid is an acceptor of a pair of electrons and a base is a donor of a pair
of electrons. When a base donates a pair of electrons, an acid forms a
coordinated covalent bond. Some of the acid-base reactions of Lewis
are: 1) Acid-base reaction of Brønsted-Lowry, where a proton is
transferred to a base. 2) Formation of complex metal-ions. 3)
Nucleophile reactions (base) - electrophile (acid) (American Chemical
Society, 2005; Atkins & Jones, 2006).
Figure 3. Types of acid-base reactions of Lewis. Retrieved from:
(American Chemical Society, 2005).
Learning activity 1
Match the concepts on the left to the contents on the right. For this
purpose, drag the concept (left) to the content (right).
1 Svante Arrhenius
A An acid is an acceptor of a pair
of electrons and a base is a donor
of a pair of electrons.
2 Brønsted-Lowry
B Acids are substances that
produce H+ ions when they are
dissolved in water, and bases are
substances that produce OH-ions
3 Gilbert Newton Lewis
4 More generalized definition of
acid and base.
5 Definition of acid base only
applicable to aqueous substances.
C Lewis
D Arrhenius
E They defined an acid as a
substance able to donate protons
and a base as a substance able to
accept them.
Did you know that…?
Water can behave as an acid with some substances and as base with
some others. For example, water behaves as a base with hydrochloric
acid, and as an acid with ammonia. The substances are called
amphoteric or amphiprotic (Gallego Picó et al., 2013).
Can water self-dissociate?
Considering that water is amphoteric, the transfer of protons from a
water molecule to another can occur in pure water, for example, in a
glass of water. This type of reaction is called autoionization. Considering
that the O-H bond is strong, the fraction of protons transferred is very
small. H3O+ and OH- concentrations are very small, for this reason water
is a very poor conductor of electricity.
Figure 4. Reaction of autoionization of water.
Since the autoionization of water is a process of equilibrium, it is
possible to calculate the water equilibrium constant or constant of the
ionic point of water, and it is symbolized as Kw.
Then:
Kw= [(H3O+ (aq)) (OH-(aq)]
What is the pH of water at 25oC? 7, that is, (H3O+ (aq)) in the reaction
of equilibrium is 10-7,00 M. As OH-(aq) is formed for every H3O+ (aq),
then (OH-(aq)) is also 10-7,00 M.
Kw= [(10-7.00 M x 10-7.00 M)]= 10-14.00 M = 1,0 x 10-14 M
Then, (OH-(aq)) is greater than 10-7.00 M in a basic dissolution, and
lower than 10-7.00 M in an acid dissolution, and neutral when the value of
[OH-] is equal to [H3O+] (American Chemical Society, 2005; Atkins &
Jones, 2006; Brown et al., 2004).
Learning activity 2
Multiple-choice questions with a single answer:
1. A dissolution at 25oC contains [OH-]= 1,0 x 10-9 M, the value of
[H+].
A 1.0 x 10-9 M
B 1.0 x 10-7 M
C 1.0 x 10-5 M
D 1.0 x 10-3 M
2. In acid dissolutions at 25oC:
A [OH-] is greater than [H3O+]
B [H3O+] is greater than [OH-]
C [H3O+] is less than [OH-]
D The value of Kw is other than 1.0 x 10-14 M
3. What is the concentration of [H+] in a dissolution where [OH-] is
equal to 2 x 10-5?
A 10 x 10-9 M
B 5 x 10-9 M
C 10 x 10-10 M
D 5 x 10-10 M
Activity 2:
Skill 4: Use the Ka and Kb constant to estimate the pH of aqueous
solutions based on the calculation of concentrations of hydronium and
hydroxide ions.
Are there substances more acid than others?
Figure 5. pH scale.
We can determine qualitatively which solutions are acid or basic, for
example, vinegar and soap. However, we can determine quantitatively
how acid or basic a solution can be in a scale from 0 to 14. Chemists
express the molar concentration of a hydronium ion in terms of the pH
of a solution, which is equal to the negative logarithm of this ion:
pH= -log [H3O+]
For example, water at 25oC has [H3O+] = 1.0 X 10-7 M, then the pH:
pH= -log [1.0 X 10-7]
= - [-7.00]
= 7.00
We could also calculate the pH of a basic solution, where [OH-]= 2.0 X
10-, as follows:
[H3O+] = Kw / [OH-]
= 1.0 X 10-14/ 2.0 X 10-3
= 5.0 X 10-12 M
In this way, the higher the molar concentration of [H3O+], the lower the
pH. A value lower than 7 means that the solution is acid and higher than
7 means that it is basic. In case the pH is 7, as in water, this means that
the solution is neutral (Atkins & Jones, 2006; Brown et al., 2004).
Learning activity 1
Calculated the pH of the following solutions:
•
•
A solution where [OH-]= 0.001M
A solution where [H3O+] = 2.0 X 10-6
How strong or weak can an acid or base be?
We can identify there are some acids that are better proton donors than
others and certain bases that are better proton receivers than others.
We can also identify that the easier it is for a substance to donate
protons, the harder it is for its conjugate base to accept a proton, that
is, the stronger the acid, the weaker the conjugate base, and vice versa
(Brown et al., 2004).
Most acids and bases existing in nature are weak, for example, water in
rivers, streams, gorges and torrents have a natural acidity due to the
presence of carbonic acid (H2CO3), ions HPO42- and HPO4- from dripping
of fertilizers and carboxyl acids formed from plant degradation(Atkins &
Jones, 2006).
Figure 6. Strong and weak acids and bases.
Learning activity 2
Find the name of each molecule in image 6.
How to quantify the strength of acidity or basicity in a
substance?
This can be done based on the acidity (Ka) or basicity (Kb) constant.
They are equally real constants for the transfer of protons between
solute and solvent. This calculation can be performed because acids and
conjugate bases are in equilibrium in the solution (Brown et al., 2004).
Figure 7. Equations to calculate the constant of acidity and basicity.
Learning activity 2
Phenylacetic acid (HC8H7O3) is a substance that accumulates in the
blood of patients suffering Phenylketonuria (PKU). What is the Ka of a
dissolution of 0.085 M HC8H7O3, with pH= 2.68 at 25oC?
Did you know that…?
Phenylketonuria is an autosomal recessive disease caused by a defect in
the gene encoding the phenylalanine hydroxylase protein. Blocking of
this metabolic path causes excesses of phenylalanine in blood. If this
disease is not treated, it can lead to intellectual disability or even death
(Müller-Esterl, 2008).
Very important…
Usually, the acid or basic dissociation constants are expressed as pKa
or pKb, that is, as –log Ka or –log Kb. If the acid or base is strong, the
value of pKa or pKb is low; and when the acid or base is weak, the
value of pKa or pKb is high.
Learning activity 3
1. Calculate the pH of a solution of 0.20 M HCN, where Ka = 4.9 x 10-10
at 25oC.
2. Calculate the pH of a 0.05 M sodium acetate solution, where the Ka of
acetic acid is 1.75 x 10-5 and Kw= 1 x 10-14.
Activity 3:
Skill 3: Interpret curves of titration to explain the process of
neutralization of strong and week acids with strong bases, and strong
and weak bases with strong acids.
Skill 5: Compare the behavior observed in a titration monitored with
potentiometers and pH indicators.
Skill 6: Research into the pH of human blood and its buffer system.
What is a buffer solution?
If we add a drop of hydrochloric acid (approximately 0.01mL) to 1L of
pure water, the pH would change from 7 to 5, that is, around 100 times
the concentration of [H+]. If drastic changes in pH as above occur in
biological systems such as cells, they could affect cell functions and
structures. Consequently, it is vital to keep a constant pH in living
systems, which is possible through buffer solutions. For example, a
person cannot survive more than a few minutes if the pH goes down to
7 (neutral). Blood can maintain an constant pH because of certain buffer
substances that are part of blood (Campbell & Reece, 2007).
Let's watch this video:
https://www.youtube.com/watch?v=8Fdt5WnYn1k
Learning activity 1
Query about the blood buffer system.
What is titration?
Chemists use titration to know the concentration of a solute given a
solution. This implies adding a solution called titrant (reagent of known
concentration) to a sample. For example, we have 20 mL of HCl at a
known concentration, and then add a dissolution of 0.1 M NaOH until the
neutralization reaction between HCl and NaOH is total. How do we know
when this occurs?
Let's analyze the following graph of titration of buffer solutions:
• pH of the midpoint or equivalence point is equal to pKa of its
corresponding acid.
• The slope at that the equivalence point is less in relation to the ends.
This indicates that the sample is neutral at this point and will not
change easily by adding a strong base or acid. These solutions are
called buffers.
• The equivalence point indicates that the amount of [OH-] added is
equal to the amount of acid present in the initial point.
Figure 7. Titration curves of the acetic acid, phosphate and
ammonia.
How to detect when the equivalence point has been reached in a
titration?
Figure 8. Methods to detect the equivalence point.
We can use colorants called indicators such as phenolphthalein, which is
colorless in an acid dissolution and turns pink in a basic dissolution. The
color change indicates that the acid has been neutralized.
These are other indicators:
pH
Figure 9. Acid-base indicators.
We can use a pH meter to measure pH in a solution precisely. It
measures the concentration of [H+] ions. The pH meter acts as a
galvanic cell, which is generally a combined electrolyte. It has a
reference electrode and a glass electrode. The lower section has a round
glass bulb. The internal and external tubes contain a reference solution,
but only the external tube is in contact with the sample in which pH will
be measured, through a porous barrier acting as a salt bridge (GarciaSoto, Sierra, Lorente, & Requejo, 2006).
Figure 9. Parts of a pH meter
Learning activity 2
In the chemistry laboratory, a student wants to know the concentration
of acid in a water sample from the school's pond. For this purpose, the
student makes 2 titration curves:
She uses a color indicator for the first curve, and the pH of the sample is
monitored through a pH Meter in the second curve.
1. Name the two types of indicators that can be used and explain how
they work.
2. Explain how you can determine the equivalence point of titration
using the two methods.
3. What is more accurate the use of indicators or the pH meter?
Are there different curves of acid-base titration?
https://www.youtube.com/watch?v=BBIGR0RAMtY
Analyze the following graph and answer true (T) or False (F), as
appropriate:
Figure 10. Titration curve
1 Point D corresponds to the equivalence point ____
2 Point B corresponds to the initial point ____
3 The curve corresponds to a titration of a weak acid and a strong base
___
4 The base used may be NaOH ____
5 <10 mL of base are necessary to reach the equivalence point ___
Abstract
Homework
•
•
To manipulate acids and bases, you must wear robe, gloves and
biosecurity glasses.
Since we are dealing with small amounts and low concentrations,
any remaining reagent can be disposed through the sink.
1. Organize groups of five students.
2. Each group is going to make the following preparations:
• Vinegar: take 10 drops of vinegar and dilute in 10 mL of distilled
water.
•
•
•
•
Yogurt: take 10 drops of yogurt and dilute in 10 mL of distilled
water.
10mL of rainwater: do not dilute the sample.
1 aspirin tab: pulverize and add 10 mL of distilled water.
Lemon juice: take 10 drops of lemon juice and dilute in 10 mL of
distilled water.
•
Note: You can measure volumes using a syringe and a dropper.
3. Place the samples in two test tubes or glass containers.
4. Add three drops of the indicator: solution of purple cabbage. For this
purpose, each group must prepare the indicator as follows:
• Take to fresh leaves of purple cabbage.
• Cut the leaves and add 50 mL of distilled water.
• Heat the cabbage and water to the boiling point.
• When the solution is purple, allow cooling down and filter the
liquid.
5. Add 5 drops of 0.1 M NaOH to one tube and 5 drops of HCl to the
other tube slowly.
6. Record the data obtained in the table.
Indicator
Color
when Color
when Color
when
the indicator the base is the acid is
is added to added
added
the sample
Purple
cabbage
7. Write a conclusion about the practice.
8. Answer: What is the importance or implication of the pH from the
analyzed sample?
9. Socialize the results in class; no more than 7 minutes per group.
.
Note: consider the pH scale of the sample using purple cabbage
juice as indicator.
Figure 12. pH scale of the sample using purple cabbage juice as
indicator.
Evaluation
This test evaluates individually the student's performance in relation to
the contents studied in this module.
Skill 4 Use the Ka and Kb constant to estimate the pH of aqueous
solutions based on the calculation of concentrations of hydronium and
hydroxide ions.
1 The pH of 0.5 M acetic acid (HC2H3O2) at 25oC is:
A 2.52
B 2.62
C 2.42
D 2.72
Multiple-choice questions with a multiple answer:
Skill 3 Interpret curves of titration to explain the process of
neutralization of strong and week acids with strong bases, and strong
and weak bases with strong acids.
Two samples from Bogotá River and Medellin River were analyzed in the
laboratory. The following titration curves were obtained:
2 What can you say about this graph?
a. The equivalence points are pH 7 and 8.72.
b. The titrant is a weak base.
c. The red curve corresponds to a weak acid.
d. The blue curve corresponds to a weak acid.
Observe the titration curve of a buffer solution of acetic acid.
3 What can you say about this graph?
a. If an indicator is used in titration, the color change would occur in
point A.
b. The equivalence point is B.
c. In point B [HA]=[A-].
d. In point C the solution has an acid pH.
Skill 1 Contrast the three existing theories that define acids and bases.
Choose true (T) or False (F), as appropriate:
4 According to Brønsted-Lowry, the acid-base reactions implied the
transfer of H+ ions from one substance to another _____
5 The Arrhenius definition of acid and base is currently the most general
definition compared to that of Lewis and Brønsted-Lowry, since it
defines acids as substances that produce H+ ions when they are
dissolved in water, and bases as substances that produce OH-. ions.
Skill 2 Interpret the ionic equilibrium of pure water and its relation with
the Kw constant
6 A solution of 0.05 M NaOH prepared in water at 25oC
contains (H+)=1.0 x 10-7 M and Kw = 1.0 x 10-14 _____
Bibliography
American Chemical Society. (2005). Química: un proyecto de la
American Chemical Society. Reverte.
Atkins, P. W., & Jones, L. (2006). Principios de química: los caminos del
descubrimiento. Ed. Médica Panamericana.
Brown, T. L., LeMay, H. E., Jr., Bursten, B. E., & Burdge, J. R. (2004).
Química. Pearson Educación.
Campbell, N. A., & Reece, J. B. (2007). Biología. Ed. Médica
Panamericana.
Gallego Picó, A., Garcinuno Martinez, M. R., Morcillo Ortega, M. J., &
Vázques Segura, M. Á. (2013). QUÍMICA BÁSICA. Editorial UNED.
Garcia-Soto, M. D. M. D. L. F., Sierra, A. N., Lorente, V. M. D., &
Requejo, F. P. (2006). Experimentación en química general. Editorial
Paraninfo.
Garritz, A., & Chamizo, J. A. (1998). Química. Pearson Educación.
Gennaro, A. R. D. (2003). Remington: Farmacia, Volume 1. Buenos
Aires, Argentina: Ed. Médica Panamericana.
Müller-Esterl, W. (2008). Bioquímica. Fundamentos para Medicina y
Ciencias de la Vida. Reverte.
Wikipedia, C. de. (2016). Nucleófilo. Retrieved May 24, 2016, from
https://es.wikipedia.org/w/index.php?title=Especial:Citar&page=Nu
cle%C3%B3filo&id=89827759
Glossary
•
•
•
•
•
Protons: Positively-charged subatomic particles found in the
nucleus of an atom (Brown et al., 2004).
Electrons: Negatively-charged subatomic particles found in the
nucleus of an atom (Brown et al., 2004).
Nucleophile: chemical species that donates a pair of free
electrons to another species (Wikipedia, 2016).
Fertilizer: Organic and inorganic substance applied to soil,
containing large amounts of nutrients that are absorbed by plants
(López, 2016).
Galvanic cell: cell that works based on a reaction of oxidereduction and electrons passing through an external circuit (Brown
et al., 2004).