BIOB111_CHBIO - Tutorial activity for Session 6:
General topics for the week:
•
Acids and Bases
•
pH and buffer
•
Interactive animations are used to reinforce the understanding
Instructions- interactive animations: Use the following link to learn the names and formulas for acids and bases
http://chemistry.about.com/od/acidsbases/a/strengthacids.htm (link verified 16 May 2011)
General tutorial questions:
1.
Define the terms acids, bases, pH and buffers.
Term
a. acid
b. base
c. buffer
a.
matches with ____, ______, _______
b.
matches with ____, ______, _______
c.
matches with ____, ______.
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Definition
(i) Maintains a constant pH by neutralizing any
added acid or any added base
(ii) Releases hydroxide ions in solution
(iii) Releases hydrogen ions in solution
(iv) Is made up of a weak acid and the salt of
its conjugate base
(v) Would cause pH to drop in a non-buffered
solution
(vi) Has a pH above 7.0
(vii) Would feel slippery and would taste bitter
(viii) Proton donor
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2.
Write the names of the following acids/ bases: Choose from: chloric acid, hydrochloric acid, phosphoric acid, nitric acid, barium
hydroxide, sodium hydroxide
3.
a)
HClO3 =
b)
H3PO4
c)
Ba(OH)2 =
d)
HNO3 =
e)
NaOH =
f)
HCl =
=
State if the following examples of body fluids are acidic, basic, or neutral, given each of these pHs?
a)
Saliva, pH 6.8 ________ (<7.0)
b)
Urine, pH 5.9 __________(<7.0)
c)
Pancreatic juice, pH 8.0 ____________ (> 7.0)
d)
Bile, pH 8.4 __________ (>7.0)
e)
Arterial blood, pH 7.35 to 7.45 ___________, (> 7.0)
f)
Concentrated NaOH, pH 13 ____________, (> 7.0)
g)
Concentrated HCl, pH 1 ___________ (<7.0)
4. Complete the missing items in the following table:
Characteristic
In solution, Arrhenius
definition
In solution, Brønsted-Lowry
definition
Electrolytes
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Acids
Produce H+ ions
Bases
Accept H+
Yes
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Taste
Feel
Litmus paper
Phenolphthalein indicator
Neutralization reaction
5.
6.
Sour
corrosive
red
Red
Neutralize acids
According to the Brønsted-Lowry definition, is an acid a proton donor or a proton acceptor?
a)
Proton donor
b)
Proton acceptor
According to the Brønsted-Lowry definition, is an base a proton donor or a proton acceptor?
a) Proton donor
b) Proton acceptor
7.
What is the difference between strong acids and weak acids?
a)
Strong acids dissociate only partially into ions in solution
b)
Weak acids dissociate completely into ions in solution
c)
Strong acids dissociate completely into ions in solution
d)
Weak acids do not form ions in solution
8. Which of the following statements about weak acids is correct?
a) Weak acids always contain C atoms
b) The percentage dissociation for weak acids is usually in the range of 40-60 %
c) Weak acid molecules only lose a small percentage of their acidic hydrogen atoms
d) More than one correct response
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9.
In the following reaction, carbonic acid dissociates only slightly in water. Is it a strong acid or a weak acid?
H2CO3 (aq) + H2O (l)
10.
a)
Strong acid
b)
Weak acid (reversible equilibrium, only partially dissociates in ionic solution)
What is one of the conjugate acid-base pairs when hydrofluoric acid (HF) dissolves in water?
HF (aq) + H2O (l)
11.
H3O+ (aq) + HCO3 – (aq)
a)
H2O and F–
b)
HF and H3O+
c)
HF and F–
H3O+ (aq) + F- (aq)
Does the equilibrium below favour the reactants or products?
HF (aq) + H2O (l)
H3O+ (aq) + F- (aq)
a)
Reactants favoured (HF and H2O) because HF is a weak acid
b)
Products favoured (hydronium ion and F – ion) because HF is a strong acid
12. Select from the response list the compound or ion that best fits the description: conjugate base of H3PO4.
a) H3PO4
b) H2PO4–
c) HPO42–
d) PO43–
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13.
Which one of the following would make a buffer solution?
a)
HCl and NaCl
b)
H3PO4
c)
H2CO3 and HCO3- weak acid and salt of its conjugate base
d)
NaCl and Na2CO3
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Conceptual multiple choice questions:
1.
Acids and bases react with each other in chemical reactions referred to as acid-base reactions. Acids are compounds that
donate H+ to a base or into solution, whereas bases are compounds that accept H+ from an acid or solution. Which of the
following correctly describes how an acid-base reaction proceeds?
a)
An acid donates a H+ to the base, which means the acid is converted to a product that is an acid and the base gets
converted to a product that is a base
b)
A base donates a H+ to the acid, which means that the base is converted into a product with one more H+ (acid) and the
acid is converted to a product with one less H+ (base)
c)
An base donates a H+ to the acid, which means the base is converted to a product that is a base and the acid gets
converted to a product that is an acid
d)
An acid donates a H+ to the base, which means that the acid is converted into a product with one less H+ (base) and the
base is converted to a product with one more H+ (acid)
2.
H2O is often one of the reactants in acid-base reactions. In acid-base reactions H2O sometimes acts as a base becoming the
hydronium ion (H3O+) and sometimes like an acid becoming the hydroxide ion (OH-). For the chemical reaction below, does
H2O function as an acid or base and why?
H2CO3 + H2O
H3O+ + HCO3–
a)
H2O accepts H+ from H2CO3, and is therefore acting as an acid
b)
H2O accepts H+ from H2CO3, and is therefore acting as a base
c)
H2O donates H + to H2CO3, and is therefore acting as an acid
d)
H2O donates H + to H2CO3, and is therefore acting as a base
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3.
H2O is often one of the reactants in acid-base reactions. In acid-base reactions H2O sometimes acts as a base becoming the
hydronium ion (H3O+) and sometimes like an acid becoming the hydroxide ion (OH-). For the chemical reaction below, does
H2O function as an acid or base and why?
NH3 + H2O
4.
NH4+ + OH–
a)
H2O accepts H+ from NH3, and is therefore acting as an acid
b)
H2O accepts H+ from NH3, and is therefore acting as a base
c)
H2O donates H + to NH3, and is therefore acting as an acid
d)
H2O donates H + to NH3, and is therefore acting as a base
Acid-base reactions each have two conjugate acid base pairs. The difference between the two compounds in a conjugate acidbase pair is the presence/absence of a H+. During an acid base reaction, the reactant acid loses a H+ to become a conjugate
base, whereas the reactant base gains a H+ to become a conjugate acid. For the below reaction, what is the conjugate acidbase pair that contains H2CO3?
H2CO3 + H2O
H3O+ + HCO3–
a)
H2CO3 is an acid and it’s conjugate base product is H3O+
b)
H2CO3 is a base and it’s conjugate acid product is HCO3-
c)
H2CO3 is a base and it’s conjugate acid product is H3O+
d)
H2CO3 is an acid and it’s conjugate base product is HCO3-
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5.
A compound that has the capacity to give away (donate) H+ is called an acid. However, strong acids are better than weak acids
at giving away H+. Which of the following is true about strong and weak acids?
6.
a)
Strong acids give away almost 100% of their H+, whereas weak acids only give away about 5% of their H+
b)
Both strong and weak acids will give away almost 100% of their H+
c)
Strong acids give away only about 5% of their H+, whereas weak acids give away almost 100% of their H+
d)
Both strong and weak acids only give away a small amount of their H+ (about 5%)
Each acid-base reaction involves two conjugate acid base pairs. In an acid-base reaction the acid becomes a conjugate base
by giving away H+, whereas the base becomes a conjugate acid by accepting H+ (see below). Which of the following correctly
describes a strong acid and its conjugate base?
aaaa
7.
a)
A strong acid is very reactive & readily gives away it’s H+ to the base, itself becoming a reactive strong conjugate base
b)
A strong acid is very stable & readily gives away it’s H+ to the base, itself becoming a more stable weak conjugate base
c)
A strong acid is very stable & readily gives away it’s H+ to the base, itself becoming a reactive strong conjugate base
d)
A strong acid is very reactive & readily gives away it’s H+ to the base, itself becoming a more stable weak conjugate base
A buffer contains two components, a weak acid (e.g. H2CO3) and its conjugate base (HCO3-). A buffer can prevent large pH
changes when a small amount of acid or base is added to the blood. How does a buffer prevent large pH change when a small
amount of acid is added to the blood?
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a)
Both the weak acid and conjugate base of the buffer react with the added acid to prevent a large pH change within the
blood
8.
b)
The weak acid of the buffer will neutralise the added acid through an acid base reaction
c)
Having the buffer in the blood is enough to prevent pH change from the added acid, an acid base reaction is not required
d)
The conjugate base of the buffer will neutralise the added acid through an acid base reaction
A buffer contains two components; a weak acid (e.g. H2CO3) and its conjugate base (HCO3-). A buffer can prevent large pH
changes when a small amount of acid or base is added to the blood. How does a buffer prevent large pH change when a small
amount of base is added to the blood?
a)
The conjugate base of the buffer will neutralise the added base through an acid base reaction
b)
Both the weak acid and conjugate base of the buffer react with the added base to prevent a large pH change within the
blood
c)
The weak acid of the buffer will neutralise the added base through an acid base reaction
d)
Having the buffer in the blood is enough to prevent pH change from the added base,
an acid base reaction is not required
9.
A buffer in our blood (H2CO3/HCO3-) assists in maintaining a tight pH range between 7.35-7.45. The buffer prevents drastic pH
change if a small amount of acid or base is added into the blood. What would happen if the pH of the blood was at 8.5 instead
of within the normal range?
a)
Tissues still get a normal supply of oxygen but the high pH will likely causes coma and death
b)
Tissues would be starved of oxygen but the individual would be unaffected
c)
Tissues would be starved of oxygen due insufficient oxygen delivery likely causing coma and death
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d)
10.
Tissues still get a normal supply of oxygen and the individual would be unaffected
pH is used to measure how acidic or basic a solution is, with low pH’s being acidic and high pH’s being basic. The pH of a
solution is determined by assessing the concentrations (amounts) of the hydronium ion (H3O+) and the hydroxide ion (OH-).
Which of the following correctly describes the pH for solution A that has a high concentration of hydronium ions and solution B
that has a high concentration of hydroxide ions?
a)
Both solutions A and B will have a neutral pH, as the concentration of the H3O+ ions and OH- ions is equal
b)
The pH of solution A will be high due to the basic nature of H3O+ ions, whereas the pH of solution B will be low due to the
acidic nature of the OH- ions
c)
Both solutions A and B will have an acidic pH, as neither solution has enough OH- ions to increase the pH sufficiently to
make the solution basic
d)
The pH of solution A will be low due to the acidity of H3O+ ions, whereas the pH of solution B will be high due to the basic
nature of the OH- ions
11.
When acid or base is added to a solution, the pH will change. In the absence of a buffer the pH change will be more drastic, as
buffers function to prevent large pH change. For a solution that has a pH of 11, what is the best way to decrease the pH to 5?
a)
To decrease the pH to 5, add hydronium ions (H3O+) in the presence of a buffer
b)
To decrease the pH to 5, add hydroxide ions (OH-) in the absence of a buffer
c)
To decrease the pH to 5, add hydronium ions (H3O+) in the absence of a buffer
d)
To decrease the pH to 5, add hydroxide ions (OH-) in the presence of a buffer
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12.
When acid or base is added to a solution, the pH will change. In the absence of a buffer the pH change will be more drastic, as
buffers function to prevent large pH change. For a solution that has a pH of 2, what is the best way to increase the pH to 12?
a)
To increase the pH to 12, add hydronium ions (H3O+) in the presence of a buffer
b)
To increase the pH to 12, add hydroxide ions (OH-) in the absence of a buffer
c)
To increase the pH to 12, add hydronium ions (H3O+) in the absence of a buffer
d)
To increase the pH to 12, add hydroxide ions (OH-) in the presence of a buffer
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