Grade 11 → Balance → Acid and Base Theory ↓
Bronsted-Lowry concept
The Bronsted-Lowry concept of acids and bases is an important model for understanding chemical reactions and equilibrium in chemistry. Developed by Danish chemist Johannes Nicolaus Bronsted and English chemist Thomas Martin Lowry in 1923, this theory focuses on the movement of protons (H + ions) between substances.
Basic definitions
According to the Bronsted-Lowry concept:
- Acid: A substance that donates protons (H + ions) to another substance.
- Base: A substance that accepts protons (H + ions) from another substance.
The ability to donate or accept a proton defines the acidic or basic nature of a compound in the Bronsted-Lowry theory. This concept broadens the Arrhenius theory and provides a more general approach that applies to a wide range of chemical reactions, including reactions in non-aqueous solutions.
Acid-base balance
An important aspect of the Bronsted-Lowry theory is the concept of equilibrium in acid-base reactions. An acid-base reaction can be represented as:
HA + B ⇌ A- + HB+
In this reaction:
HA
is the acid that donates a proton to baseB
, forming the conjugate baseA-
.B
accepts the proton and forms the conjugate acidHB+
.- This reaction is reversible, whereby a state of equilibrium is achieved.
Conjugate acid-base pair
Every acid has a conjugate base, and every base has a conjugate acid. This is due to the reversible nature of proton transfer reactions. The conjugate base is the species that remains after the acid donates its proton, and the conjugate acid is the species that forms when the base accepts a proton.
For example, in the reaction:
NH4+ + OH- ⇌ NH3 + H2O
Here, NH4+
is the acid and OH-
is the base. After the reaction:
NH3
(ammonia) is the conjugate base ofNH4+
.H2O
(water) is the conjugate acid of the baseOH-
.
Visualization of acid-base reactions
Let's represent the acid-base reaction graphically:
In this illustration, the solid blue line shows the forward reaction where HA
donates a proton to B
to form A-
and HB+
. The dashed line shows the reverse reaction, illustrating the dynamic nature of the equilibrium.
Water: A special case
In the Bronsted-Lowry framework, water is an amphoteric substance. Amphoteric substances can act as either acids or bases, depending on the reaction conditions. Here are some examples:
HCl + H2O ⇌ Cl- + H3O+
In the first example, water acts as a base, accepting a proton from hydrochloric acid (HCl
) to form a hydronium ion (H3O+
).
NH3 + H2O ⇌ NH4+ + OH-
In the second example, water acts as an acid, donating a proton to ammonia (NH3
) to form a hydroxide ion (OH-
).
This ability to behave as both an acid and a base makes water a versatile medium for acid–base reactions in aqueous solution.
Strength of acids and bases
The strength of acids and bases is determined by their tendency to donate or accept protons:
- Strong acid: Dissociates completely in the reaction, readily donating a proton. For example, hydrochloric acid (HCl).
- Weak acid: Partially dissociates, not all molecules donate protons. For example, acetic acid (CH3COOH).
- Strong base: Completely accepts protons in the reaction. For example, sodium hydroxide (NaOH).
- Weak base: Partially accepts protons, such as ammonia (
NH3
).
The strength of the acid and base affects the position of equilibrium in an acid-base reaction.
Examples of Bronsted-Lowry reactions
Example 1: Ammonia and water
NH3 + H2O ⇌ NH4+ + OH-
In this reaction:
NH3
is a base andH2O
initially acts as an acid.- The conjugate acid is
NH4+
and the conjugate base isOH-
.
Example 2: Acetic acid and water
CH3COOH + H2O ⇌ CH3COO- + H3O+
In this reaction:
CH3COOH
(acetic acid) is an acid andH2O
is initially a base.- The conjugate base is
CH3COO-
and the conjugate acid isH3O+
.
Role of solvents
Bronsted-Lowry reactions can occur in a variety of solvents, not just water. Solvents can affect the equilibrium by stabilizing or destabilizing acids and bases and their conjugates. The ability of the solvent to donate or accept protons affects the reaction kinetics, providing an additional level of complexity and flexibility in predicting reaction outcomes.
Advantages of the Bronsted-Lowry concept
The Bronsted-Lowry concept has several advantages over other theories:
- More general: It applies to a wider range of reactions than the Arrhenius theory, which is limited to aqueous reactions.
- Focusing on proton transfer: By focusing on proton transfer, it provides a clear understanding of the mechanisms involved in acid-base reactions.
- Conjugate pair: The concept of conjugate acid-base pairs provides insight into the reversibility and equilibrium of reactions.
Conclusion
The Bronsted-Lowry theory is one of the fundamental concepts in chemistry for understanding acids, bases, and their reactions. By emphasizing proton transfer and equilibrium, it provides a more comprehensive understanding of chemical behavior in diverse environments beyond aqueous solutions, which aligns with broader chemical principles and practices.