GREASE THOSE WHEELS (CHEMISTRY)
#2 ACID-BASE EQUILIBRIA
- show understanding of, and use the Brønsted-Lowry theory of acids and bases
- explain qualitatively the differences in behaviour between strong and weak acids and bases and the pH values of their aqueous solutions in terms of the extent of dissociation.
Before we start this component of your syllabus, let me give a brief introduction of this topic...
I hope you all are familiar with the basic acid-base theory (called the Arrhenius theory) that states that an acid is a substance that increases the concentration of H+ ions in a solution while a base is a substance that increases the concentration of OH- ions in a solution. With this and some concepts from the equilibria chapter we are going to give these acids and bases some more significant definations.
In 1923, Johannes N. Brønsted (1879–1947) and Thomas M. Lowry (1874-1936), while working completely independently gave acids and bases a defination that was completely different from the Arrhenius defination which had been widely accepted for years. The Brønsted-Lowry theory of acids and bases states that...
"An acid is a substance that acts as a proton doner and a base is a substance that readily accepts that proton. A proton being the nucleus of a hydrogen atom in this context."Several key points can be extracted from this defination and with the aid of the following acid/base reaction:
C3H6O2H (aq) + H2O (l) >>><<< H3O+ (aq) + C3H6O2- (aq)
Acid Base Acid Base
- The nucleus (or cation) of a hydrogen atom is basically very reactive in nature due its very small size and great density of the positive charge and thus will share electrons with any specie which has a lone pair of electrons within it.
- Water is not neutral in nature, it acts as a base in the presence of stronger acids and as an acid in the presence of stronger bases. Same is the case with an examination popular ethanoic acid.
- Some acid base reactions can be reversible.
- The Hydronium ion (H3O+) is formed when water acts as a base and accepts a proton from an acid (A normal water molecule has two lone pairs on the oxygen molecule but it donates only one of them to the approaching proton because then all the atoms involved in this polyatomic ion will be stable)
- Brønsted-Lowry acids and bases (as in the above mentioned reaction) exist in conjugate pairs or conjugate acid-base pairs. H2O---H3O+ and C3H6O2H---C3H6O2- are conjugate pairs in the above mentioned reaction. Note that conjugate pairs usually have a difference of one hydrogen between them.
- An aqueous solution is...
- Acidic, if it has more hydronium ions than hydroxide ions.
- Basic; if it has more hydroxide ions than hydronium ions.
- Neutral; when both hydronium and hydroxide ions are in an equal concentration.
As we have all come to know at our junior classes that there are both powerful and weak acids and bases, but what we have not been told is that what determines the strength of an acid or base...so now ladies and gentlemen we unveil these facts for you!!=)
- The equilibrium for a stronger acid's ionization is near 100% or in other words all the acid ionizes in water. While the equilibrium position for a weaker acid's ionization comes pretty close to 1% ionization and thus this difference in dissociation determines the strength of an acid.
- Most bases are very strong and purely ionic in nature (like NaOH, KOH etc) and dissociate completely in water. While some bases (like Ammonia) ionize partially in water giving weak bases.
- Another phenomenon associated with the strength and weakness of acids and bases is the strength of its conjugate. For acids it is the tendency of their conjugate base to react with the hydronium ion while for bases it is the degree of affinity their conjugate acids posses for a reaction with water to give hydroxide ions. A summary is given below...
- Strong Acid (Hydrochloric Acid) ---> Weakest Base (Cl-)
- Weak Acid (Ethanoic Acid) ---> Weaker Base (C2H3O2-)
- Weakest Acid (Boric Acid) ---> Strong Base (H2BO3-)
- Strong Base (Sodium Hydroxide) ---> Weakest Acid (Na+)
- Weak Base (Ammonia) ---> Strong Acid (NH4+)
- Solutions of some salts are also acidic and basic...we can tell this from the relative strength of their acids and bases...
- Solution of Ammonium Nitrate is acidic because the ammonium ion reacts with water as an acid and donates a proton to the water molecule to give Ammonia and Hydronium ion. While the nitrate ion has no tendency to react with water thus the solution has a higher concentration of hydronium ions that makes it acidic.
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