One important category of chemical reactions is the reactions of acids and bases, called acid-base reactions or acid-base neutralization reactions.
Questions:
1. What is an acid? What is a base? Give an example of each to illustrate.
2. Why do we call an acid-base reaction a neutralization reaction?
3. How do we identify if a solution is acidic or basic?
4. Give some examples of acids and bases that you see or use in your daily life.
5. Give the general and common properties of acids and bases.
6. What’s a buffer solution?
7. Explain acid rain pollution and how it forms.
Solutions:
1. Acids and Bases are important categories of chemical substances. Many definitions or concepts of acids and bases have been developed, and the most common are the following:
Arrhenius’ acid-base theory
Acid: any substance, when dissolved in water, liberates hydrogen ions, H+.
Example:HCl(g) + H2O(l) → H+(aq) + Cl-(aq)
Base: any substance, when dissolved in water, liberates hydroxide ions, OH-.
Example: NaOH(s) + H2O(l) → Na+(aq) + OH-(aq)
In the Arrhenius theory of acids and bases, the presence of water is required.
It’s the presence of H+(aq) ions that is responsible of the acidic property, and it’s the presence of OH-(aq) ions that is responsible of the basic property. In this concept, for example, gaseous HCl (hydrogen chloride) as such is not an acid; it is an acid when it is dissolved in water. Solid NaOH is not a base, it is a base it is dissolved in water.
Strength of Acids and Bases
Acids and bases are categorized as strong or weak:
Strong acids and strong bases are acids and bases which, when dissolved in water, dissociate completely into their ions.
Examples:
HCl(aq) → H+(aq) + Cl-(aq) strong acid, complete dissociation
NaOH(aq) → Na+(aq) + OH-(aq) strong base
Weak acids and weak bases are acids and bases which, when dissolved in water, dissociate partially into their ions.
Example: H2CO3(aq) = H+(aq) + HCO3-(aq) weak acid, partial dissociation
NH4OH(aq) = NH4+(aq) + OH-(aq) weak base
2. According to the Arrhenius definition of acids and bases, the reaction between an acid and a base consists in the reaction between H+ and OH-ions:
H+(aq) + OH-(aq) → H2O(l)
Here we notice that the reaction is about the two properties (acid, base) neutralizing each other to produce water, H2O. The whole reaction acid-base
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
shows the formation of a salt and water. That is why it is called an acid-base neutralization reaction: acidic and basic properties cancel or neutralize each other.
3. How do we identify the acidic or basic/alkaline properties of a solution?
(i) pH scale: is a logarithmic scale for expressing the acidity or alkalinity of an aqueous solution; it is represented by the mathematical expression:
pH = -log[H+]
(ü) In pure water, [H+] = [OH-] = 10-7M
Therefore, pH of pure water is: pH = -log10-7 = 7.
Since in pure water, [H+] = [OH-], this means that pure water is neither acid nor base; it is neutral and pH = 7 is neutral pH, i.e. neither acid nor base.
(üi) In an acidic solution, [H+] > [OH-], or [H+] > 10-7M, and pH < 7; therefore pH < 7 is acidic pH.
(iv) In a basic or alkaline solution, [H+] < [OH-], or [H+] < 10-7M, and pH > 7; therefore pH > 7 is basic pH.
And this gives the pH scale:
|
pH |
Acid/Base |
|
pH
> 7 |
Base/Alkaline |
|
pH
= 0 |
Neutral |
|
pH
< 7 |
Acid |
(vi) Acid-base indicators: are chemical substances which have the property to show two different colors depending on the acidic or basic property of the solution to be tested.
Example:
Phenolphthalein - colorless in acidic solution, violet in basic solution.
4. Examples of acids and bases in our daily life (see Figure below).
In addition to the information given in the Figure:
Acidic soils, pH < 7, are not fertile, while basic soils, pH > 7, are fertile
Many aquatic organisms cannot survive in acidic water; fish cannot survive in acidic water with a pH ≤ 4.2
Milk is a little bit acidic, pH = 6.5-6.8
Fruit juices, pH = 2-3.
Our blood, if healthy, has a pH regulated between 7.35-7.46, almost neutral
pH of Some Substances and Commodities
(courses.lumenlearning.com)
5. Acids and bases, particularly Arrhenius acids and bases, are corrosive, i.e. can damage or eat away at materials and human body tissue on contact.
(a) Acids
· Aqueous solutions of acids conduct electrical current
· Acids have a sour taste (lemon, vinegar)
· Acids change the color of certain acid-base indicators
· Acid react with active metals to produce hydrogen gas and metal salt
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
· Acids react with bases (acid-base neutralization), and the reaction is fast and complete.
HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq)
HCl(g) + NH3(g) → NH4Cl(s)
· Acids react with most of metal oxides, because most of metal oxides are basic or amphoteric.
Fe2O3(s) + 6HCl(aq) → 2FeCl3 (aq) + 3H2O(l)
(a) Bases
· Aqueous solutions of bases conduct electrical current
· Bases have often a bitter taste (quinine taste) and a slippery touch (soapy water)
· Bases change also the color of certain acid-base indicators
· Bases react with metals but in a different way from that of acids
2NaOH(aq) + Zn(s) → Na2ZnO2(aq) + H2(g)
· Bases react with acids (acid-base neutralization)
· Bases react with oxides of non-metals, because oxides of non-metals are acidic.
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
· Bases react with oil and grease and that is why they are found in household cleaning agents (baking soda or sodium bicarbonate, household ammonia, lye or sodium hydroxide/caustic soda).
Observation: The Arrhenius acid-base theory was the first theory that introduced the concept of acid and base. Later on, came the Bronsted-Lowry and the Lewis theory which brought some generalization of the concept (not limited to the aqueous solutions); all of them are complementary one another and are used depending on the kind of the reactants involved in the reaction.
6. Buffer solution:
A buffer solution is a solution that resists change in pH when small amounts of an acid or alkali/base are added over a certain range or when the solution is diluted.
The other two important acid-base theories
# Bronsted-Lowry acid-base theory: an acid is a proton donor whereas a base is a proton acceptor.
Example:HCl(g) + NH3(g) → NH4Cl(s)
Here, HCl(g) is an acid because it is a proton donor, whereas NH3(g) is a base because it is a proton acceptor. Notice that both are not considered as acid and base in the Arrhenius theory, in absence of water. Their reaction doesn’t necessarily form water molecule.
#Lewis acid-base theory: a Lewis acid is a compound or a chemical species that can accept a lone pair of electrons, i.e. it is electron deficient, and a Lewis base is one that possesses a lone pair of electrons that it can donate and share with an electron deficient species/Lewis acid.
Examples:
H+ + :NH3 → [H3N:H]+ = [NH4]+
Lewis acid Lewis base
AlCl3 + :NH3 → H3N:AlCl3 = AlNH3Cl3
Lewis acid Lewis base
H+ + H2Ö: → [H2Ö:H]+ = H3Ö+
Lewis acid + Lewis base
7. Acid rain:
Acid rain is a form of air pollution.
The origin of that form of pollution is the different acid gases rejected in atmosphere by natural phenomena such as volcanism, and human activities where coal, charcoal, fossil fuel are used as source of energy in industries, transport, domestic needs, etc...
During the combustion reaction, acid gases are produced: mainly nitrogen oxides, NOx, and sulfur dioxide, SO2. Those gases are sent in the atmosphere where they mix with atmospheric water vapor and oxygen to form aqueous acid solutions of sulfuric acid and nitric acid: H2SO4(aq). HNO3(aq)
2SO2(g) + 2H2O(g) + O2(g) → 2H2SO4(aq) acid rain
3NO2(g) + H2O(g) → 2HNO3(aq) + NO(g) acid rain
The rainfall rendered acidic by that kind of atmospheric pollution is called acid rain.
Precipitation of acid rain makes lakes acidic, and when a lake is acidic, no aquatic life is possible and fish cannot survive in that lake. Acid rain makes soil acidic, and acidic soils lose their fertility. Acid rain destroys forest. Acid rain destroys civil works made in marble and limestone, buildings and bridges built with metals.