Acid-Base: An Introduction
sjcbjb
Acids are compounds that give out hydrogen ions or a proton when mixed in an aqueous solution and have a pH below 7. These acids generally contain dissociable protons or hydrogen groups, also called acidic hydrogen that readily dissociates in solution or in presence of bases.
Bases are compounds that produce hydroxyl ions when mixed in an aqueous solution. They have a pH above 7. A base, therefore, bears a dissociable hydroxyl group or a basic group that dissociates in an aqueous medium. Bases are also compounds that can steal or accept hydrogen ions, thus, bases can also be defined as compounds which are hydrogen acceptors.
The strength of an acid or base is measured by the pH scale, which indicates its degree of dissociation. A strong acid or a strong base is one which dissociates completely in the aqueous medium furnishing hydrogen or hydroxyl ions in large amounts. The dissociation of acid or base is measured by a dissociation constant. A strong acid or strong base will have a high dissociation constant.
Strong Acid
Strong acids HA have high dissociation due to the presence of highly acidic hydrogens. These hydrogens are usually connected to highly electronegative groups (often halogens like chlorine, fluorine, and iodine). In the solution, the resulting anion after losing a proton is called the Conjugate base of the acid; they are extremely weak bases and therefore, can sustain longer in the solution.
The conjugate base of an acid, the anion, can usually carry a negative charge comfortably due to the high electronegativity of the group or charge stabilisation mechanism.
HA + H2O → H3O+ + A-
Strong acids have low pH. The pH is associated with the concentration of hydrogen ions in a solution. Mathematically, pH is represented as the negative algorithm of the concentration of hydrogen ions.
pH= -log [H+]
Therefore, large hydrogen ion concentration corresponds to low pH value. The concentration of hydrogen ions in a solution is related to what extent the acid dissociates and releases the protons.
The degree of dissociation is accounted for by a parameter Ka called the dissociation constant of acids. A large Ka value indicates higher dissociation and therefore, higher acidity. In chemistry, for convenience pKa value is considered, which is the logarithmic acid dissociation constant, mathematically, represented as the negative logarithm of Ka. So, the high value of acid dissociation constant Ka will correspond to a low pKa and thus, stronger acids will have a small pKa value.
pKa = -log Ka
Some strong acids can be remembered with help of a mnemonic device:
“Chemistry needs preparation sometimes, help!”
Chemistry – Chloric acid
Needs - Nitric acid
Preparation - Perchloric acid
Sometimes - Sulphuric acid
Help - Hydro acid
Table: Few Examples of Strong Acids
Strong Base
Strong bases BOH are compounds that dissociate completely in solution producing large concentrations of hydroxyl ion. Moreover, the bases can be powerful proton acceptors, so in an aqueous solution, they can steal a proton from a water molecule H2O producing an OH- ion.
BOH(aq) → B+ (aq) + OH- (aq)
The conjugate acid of a strong base, i.e., the cation is a weak acid. In an aqueous solution, these cations are solvated by the water molecules surrounding them and the charge is stabilised.
Similar to pH, pOH is associated with hydroxyl ion concentration. Being a negative logarithm of hydroxyl ion concentration, a strong base furnishing large amounts of hydroxyl ion would have low pOH.
pOH= -log [OH-]
The degree of dissociation in the case of bases is captured by the base dissociation constant Kb. The logarithmic base dissociation constant, pKb is similar to pKa and low pKb relates to stronger bases.
pKb = -log Kb
In an aqueous solution pH and pOH is related by the equation:
pH + pOH = 14
Either pH or pOH can be calculated if the other is known. In chemistry, both the strength of acid and base is indicated by the pH scale, where the pH for bases is calculated from the above equation.
Strong acids usually correspond to a pH range 1-2, while strong bases have a pH 13-14. Strong acids and bases both are extremely dangerous and harmful. They are corrosive and cause serious burns on contact.
Table: Few Examples of Strong Bases
Interesting Facts
Fluoroantimonic acid is the world’s strongest acid, it is a superacid
o-diethynylbenzene dianion is the strongest proton acceptor in the gas phase and can be considered the strongest base in the world, right now.
Key Features
Strong acids and Strong Bases dissociate completely in aqueous solution
Strong acids produce large concentrations of hydrogen ions in solutions
Strong acids have low pH (1-2)
Strong bases produce large concentrations of hydroxyl ions in solutions
Strong bases high pH (13-14)
FAQs on Strong Acid and Base Solutions
1. What defines a solution as a strong acid or a strong base?
A solution is classified as a strong acid or a strong base based on its degree of dissociation in water. A strong acid is a substance that ionises or dissociates almost completely in an aqueous solution to produce a high concentration of hydrogen ions (H+). Similarly, a strong base dissociates completely to yield a high concentration of hydroxide ions (OH-). This complete dissociation is the key characteristic that distinguishes them from weak acids and bases.
2. Why do strong acids and bases completely dissociate in aqueous solutions?
Strong acids and bases dissociate completely due to the nature of their chemical bonds and the stability of the resulting ions in water. For a strong acid (HA), the bond between H and A is highly polarised, making it easy to break. Once dissociated, its conjugate base (A-) is very stable and weak, meaning it has little tendency to re-associate with a proton. For strong bases, they readily release hydroxide ions (OH-) because the resulting cation is stable when solvated by water molecules.
3. What are the key differences between a strong acid and a concentrated acid?
The terms 'strong' and 'concentrated' describe different properties of an acid.
- Strength refers to the degree of dissociation. A strong acid, like HCl, dissociates completely in water, regardless of its concentration.
- Concentration refers to the amount of acid dissolved in a specific volume of solvent. You can have a dilute solution of a strong acid (low concentration) or a concentrated solution of a weak acid (high concentration). Therefore, a strong acid is not always concentrated, and a weak acid is not always dilute.
4. How do the values of Ka and pKa help in determining the strength of an acid?
The acid dissociation constant (K_a) and its logarithmic form (pK_a) are quantitative measures of acid strength.
- A large K_a value indicates that the acid dissociates to a great extent, producing many H+ ions, which signifies a stronger acid.
- The pK_a is calculated as -log(K_a). Due to the negative logarithm, a stronger acid with a large K_a will have a small or even negative pK_a value. Therefore, the lower the pK_a, the stronger the acid.
5. Could you provide some examples of common strong acids and strong bases?
Certainly. Here are some common examples of strong acids and bases encountered in chemistry:
- Strong Acids: Perchloric acid (HClO₄), Hydroiodic acid (HI), Hydrobromic acid (HBr), Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), and Nitric acid (HNO₃).
- Strong Bases: Lithium hydroxide (LiOH), Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Rubidium hydroxide (RbOH), and Caesium hydroxide (CsOH). These are typically the hydroxides of Group 1 and Group 2 metals.
6. What is the role of the conjugate base in determining the strength of an acid?
The stability of the conjugate base is crucial in determining the strength of an acid. When a strong acid (HA) donates a proton, it forms its conjugate base (A⁻). For the acid to be strong, this conjugate base must be very weak and stable. Its stability means it can comfortably hold the negative charge and has a very low affinity to accept a proton back. This is often due to factors like high electronegativity or resonance, which delocalise the charge. An unstable conjugate base would quickly re-associate with H⁺, resulting in a weak acid.
7. How is the pH scale used to represent the strength of acids and bases in a solution?
The pH scale is a logarithmic scale from 0 to 14 that measures the hydrogen ion [H⁺] concentration in a solution to indicate its acidity or basicity.
- A low pH (typically 0-2) signifies a high concentration of H⁺ ions, characteristic of a strong acid solution.
- A high pH (typically 12-14) signifies a low concentration of H⁺ ions and consequently a high concentration of OH⁻ ions, characteristic of a strong base solution.
- A pH of 7 is considered neutral. The scale provides a simple, standardised way to compare the relative strengths of different acidic and basic solutions.
