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Solution equilibrium 2
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3. Acidic and basic solutions
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Acidic and basic solutions
Acids and bases in watery solutions have specific characteristics:
Acid solution
Basic solution
conducts electricity
tastes sour and has a piercing odor
tastes bitter
is corrosive
feels slippery, can burn
turns litmus paper red
turns litmus paper blue 3
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4. The ionic product of water
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The ionic product of water
Pure water undergoes autoprotolysis in a reversible reaction that produces hydronium and hydroxide ions.
The constant equilibrium of the reaction is the ionic product of water.
At 25 °C it is expressed with this formula:
Kw = [H3O+][OH–] = 1  10–14 4
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5. Acids and bases interact with water /1
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Acids and bases interact with water /1
Acids and bases interact with the autoprotolysis equilibrium of water, but they do not alter the value of Kw.
Acids are substances that increase the concentration of H3O+ and a decrease the concentration of OH–.
Ionization of an acid in water is usually: HA
H3O+ + A–
+H2O(l) 5
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6. Acids and bases interact with water /2
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Acids and bases interact with water /2
Bases are substances that cause an increase of the concentration of OH– and a decrease of the concentration of H3O+.
Ammonia and amines react according to this scheme: B
BH+ + OH–
+H2O(l) 6
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7. The strength of acids and bases /1
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The strength of acids and bases /1
The strength of an acid refers to its ability to increase the concentration of H3O+ in solutions.
The stronger the acid, the higher the value of the acid dissociation constant Ka
[HA]
Ka=
[H3O+][A–] 7
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8. The strength of acids and bases /2
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The strength of acids and bases /2
The strength of a base refers to its ability to increase the concentration of OH– in solutions.
The stronger the base, the higher the value of the base dissociation constant Kb
[B]
Kb=
[OH–][BH+] 8
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9. Brønsted–Lowry theory
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Brønsted–Lowry theory
According to the Brønsted–Lowry theory, an acid is a proton donor and a base is a proton acceptor.
In acid-base reactions there is an exchange of protons. They are reversible reactions.
AH + B
A– + HB+
When an acid loses a proton, it becomes its conjugate base.
When a base receives a proton, it becomes its conjugate acid. 9
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10. Amphiprotic and polyprotic substances
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Amphiprotic and polyprotic substances
Amphiprotic substances, like water, can react both as acids as well as bases, depending on the substances with which they interact.
A polyprotic acid is a substance that can lose two or more protons.
A polyprotic base is a substance that can receive two or more protons. 10
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pH scale
The acidic or basic properties of a solution depends not only on the strength of the acid or base, but also on its concentration.
The pH of a solution is the negative of the logarithm in base 10 of H3O+ in solution:
pH = –log[H3O+]
At 25 °C,
neutral solutions have pH = 7;
acid solutions have pH < 7;
basic solutions have pH > 7. 11
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How to measure the pH
The pH of a solution can be measured experimentally using two methods:
an instrument called a pH-meter;
the use of acid-base indicators.
It is possible to calculate the pH of a solution when knowing the concentration of the solution and the strengths of the acid and base. 12
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13. Neutralization reactions
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Neutralization reactions
When an acid and base are mixed in a solution, a neutralization reaction takes place, producing a salt and water.
acid + base → salt + water
Neutralization is achieved only if the number of H3O+ produced by the acid equals the number of OH– released by the base. 13
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14. Acid-base properties of salt solution
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Acid-base properties of salt solution
The anion of a salt can be considered the conjugate base of an acid; its cation is the conjugate acid of a base.
Salt solutions are neutral only if their anions and cations behave as acids and bases of similar strength.
If their anions and cations have different strengths, the solution is acidic (the cation is stronger than the anion) or basic (the anion is stronger than the cation). 14
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Buffer solutions
Buffer solutions are mixtures of solutes that counteract pH changes.
They are formed by a weak acid and its conjugate base or by a weak base and its conjugate acid.
pHbuffer = pKa for an acid buffer
pHbuffer = pKb for a basic buffer 15
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