Bronsted-Lowry Theory, also called the Proton Theory of Acid and Base, is a theory that explains the concept of acid and base. It was given by Johannes Nicolaus Bronsted (Danish Chemist) and Thomas Martin Lowry (English Chemist) in 1923.
In this article, we will learn about, Bronsted Lowry’s Theory Definition, Examples, and others in detail.
Table of Content
What is Bronsted-Lowry Theory?
Bronsted-Lowry theory is an acid-base concept proposed by Danish chemists Johannes Nicolaus Bronsted and Thomas Martin Lowry in 1923. It defines acids and bases based on proton transfer during chemical reactions.
According to this theory:
- Acids are substances capable of donating protons (H⁺ ions) to other substances.
- Bases are substances capable of accepting protons (H⁺ ions) from other substances.
Bronsted Lowry Theory
Bronsted Lowry Theory Definition
In the Bronsted-Lowry theory, acids and bases are defined based on proton transfer. Acids donate protons while bases accept them. This concept expands the definition of acids and bases beyond aqueous solutions, allowing for a more versatile understanding of various chemical reactions. For instance, in the reaction between hydrochloric acid (HCl) and ammonia (NH3), HCl donates a proton to NH3, forming NH4⁺ and Cl⁻ ions, thereby illustrating the acid-base interaction.
Bronsted-Lowry Acids and Base
Various examples of Bronsted-Lowry Acids are,
| Bronsted-Lowry Acid | Conjugate Base |
|---|---|
| HCl | Cl⁻ |
| H2SO4 | HSO4⁻ |
| CH3COOH | CH3COO⁻ |
| H2O | OH⁻ |
Aqueous Solutions
Aqueous solutions play a pivotal role in various chemical processes, and the Bronsted-Lowry theory provides a robust framework for understanding acid-base interactions within these solutions. This theory extends beyond the limitations of the Arrhenius definition, allowing for a more comprehensive view of solute-solvent interactions, particularly in water-based systems.
Proton Transfer in Aqueous Solutions:
The Bronsted-Lowry theory’s applicability in aqueous solutions is rooted in its concept of proton transfer. In water-based environments, acids donate protons while bases accept them. This facilitates the understanding of how substances interact in solution and aids in predicting reaction outcomes based on the transfer of protons between molecules.
Water as Amphiprotic Substance:
Water is a quintessential example of an amphiprotic substance within the context of the Bronsted-Lowry theory. It showcases both acidic and basic properties in various chemical reactions.
As an Acid (Proton Donor): In certain reactions, water can donate a proton (H⁺ ion) to other substances. For example, in the reaction with ammonia (NH3), water acts as an acid by donating a proton to form the hydronium ion (H3O⁺) and the hydroxide ion (OH⁻).
As a Base (Proton Acceptor): Conversely, water can act as a base by accepting a proton from an acid. For instance, in the reaction with hydrogen chloride (HCl), water accepts a proton to form the hydronium ion (H3O⁺) and the chloride ion (Cl⁻).
Bronsted Lowry Acid Definition
Substances which donates a proton or H+ ion to another compound are called Bronsted Lowry Acids.
Acid ⇋ Proton + Conjugate Base
Bronsted Lowry Acids Examples
Some examples of Bronsted Lowry Acids are
- Hydrochloric Acid (HCl): It donates a proton (H⁺ ion) when dissolved in water.
- Sulfuric Acid (H2SO4): Contains two acidic hydrogen atoms and can release two protons sequentially.
- Acetic Acid (CH3COOH): An organic acid that donates a proton from its -COOH group.
- Water (H2O): Acts as both an acid (donating H⁺) and a base (accepting H⁺) in various reactions.
Bronsted Lowry Base Definition
Substances which accepts a proton or H+ ion from another compound are called Bronsted Lowry Bases.
Base + Protons ⇋ Conjugate Acid
Bronsted Lowry Bases Examples
Some examples of Bronsted Lowry Base are,
- OH– (Hydroxide Ion)
- S2- (Sulfide Ion)
- CO32- (Carbonate Ion)
Advantages of Bronsted Lowry Theory of Acid and Base
Various advantages of Bronsted Lowry Acids are,
- Detailed Understanding of Reactions: Helps us understand how acids and bases interact by exchanging protons (H⁺), making it easier to predict what happens in these reactions.
- Works in Different Solutions: Not just for water! It can be used to understand reactions in various liquids, not only in water-based solutions.
- Explains Organic and Biological Systems: It’s helpful in understanding how acids and bases behave in organic chemistry (like molecules in living things) and in biological processes.
Disadvantages of Bronsted Lowry Theory
Some disadvantages of Bronsted Lowry Acids are,
- Can’t Explain Some Reactions: It struggles with explaining reactions where protons aren’t involved or transferred, leaving gaps in understanding certain chemical interactions.
- Limits in Certain Cases: In some situations where reactions don’t involve proton transfer, this theory might not fit or fully explain what’s happening, leaving these scenarios outside of its explanation.
Applications of Bronsted Lowry Theory of Acid and Base
Various applications of Bronsted Lowry Theory
- Organic Chemistry: Helps chemists understand and predict how acids and bases react in making medicines, plastics, and other organic compounds.
- Biochemical Processes: Helps scientists understand how acids and bases play a role in important processes within living organisms, such as enzyme reactions or metabolic pathways.
- Industrial Catalysis: Guides industries in designing catalysts for chemical reactions by understanding how acids and bases can speed up or control reactions in manufacturing processes.
- Environmental Science: Helps in understanding natural processes like soil acidity, ocean pH levels, and pollution reactions in the environment.
- Food and Beverage Industry: Plays a role in food preservation, fermentation (like in brewing and baking), and understanding flavors related to acidity in foods and drinks.
Difference between Arrhenius Theory, Bronsted-Lowry Theory, and Lewis Acid-Base Theory
Various differences between Arrhenius Theory, Bronsted-Lowry Theory, and Lewis Acid-Base Theory is explained in the table added below,
| Acid-Base Theory | Acid Definition | Base Definition | Applicability |
|---|---|---|---|
| Arrhenius Theory | Substances releasing H⁺ ions in aqueous solutions. | Substances releasing OH⁻ ions in aqueous solutions. | Primarily limited to aqueous solutions. |
| Bronsted-Lowry Theory | Substances donating protons (H⁺ ions) in chemical reactions. | Substances accepting protons (H⁺ ions) in chemical reactions. | Applicable beyond aqueous solutions. |
| Lewis Acid-Base Theory | Species that accept electron pairs. | Species that donate electron pairs. | Focuses on electron pair transfer, broader application in various solvents and complex reactions involving non-aqueous systems. |
Summary on Bronsted Lowry’s Theory
Bronsted-Lowry theory defines the concept of acids as proton donors and bases as proton acceptors. In summary,
- Acids: Substances capable of donating protons (H⁺ ions) to other substances.
- Bases: Substances capable of accepting protons (H⁺ ions) from other substances.
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Bronsted Lowry Theory Examples
Example 1: Calculate the pH of a solution formed by mixing 50 mL of 0.1 M hydrochloric acid (HCl) with 150 mL of 0.05 M sodium hydroxide (NaOH). Given that the dissociation constants (Ka) of HCl and NaOH are 1.0 × 106 and 1.0 × 10-14 respectively.
Solution:
Step 1: Determine the number of moles of HCl and NaOH
Moles of HCl = 50 mL × 0.1 mol/L = 0.005 mol
Moles of NaOH = 150 mL × 0.05 mol/L = 0.0075 mol
Step 2: Identify the limiting reagent (reactant that gets used up first)
HCl is the limiting reagent (0.005 mol vs. 0.0075 mol)
Step 3: Calculate the excess moles of NaOH after the reaction with HCl
Excess moles of NaOH = Moles of NaOH – Moles reacted with HCl
= 0.0075 mol – 0.005 mol = 0.0025 mol
Step 4: Calculate the concentration of OH- ions from the excess NaOH.
Concentration of OH- ions = Excess moles of NaOH / Total volume of the solution
= 0.0025 mol / (50 mL + 150 mL)
= 0.0025 mol / 200 mL
= 0.0025 mol / 0.2 L
= 0.0125 mol/L
Step 5: Calculate pOH using the concentration of OH- ions
pOH = -log(OH- concentration) = -log(0.0125) ≈ 1.9
Step 6: Calculate pH using the relation pH + pOH = 14 (for water at 25°C)
pH = 14 – pOH ≈ 14 – 1.9 ≈ 12.1
Therefore, the pH of the solution ≈ 12.1.
Example 2: What volume of 0.2 M acetic acid (CH₃COOH) must be added to 100 mL of water to create a solution with a pH of 4.0? (Ka for acetic acid is 1.8 × 10-5).
Solution:
Step 1: Calculate the concentration of H+ ions corresponding to pH 4.0
[H+] = 10(-pH) = 10(-4.0) = 1.0 × 10(-4) mol/L
Step 2: Use the given Ka value to calculate the concentration of CH₃COOH
Ka = [H+][CH₃COO-] / [CH₃COOH]
Given [H+] = [CH₃COO-] (for a weak acid)
[CH₃COOH] = [H+] × [CH₃COOH] / Ka
= (1.0 × 10(-4)) × [CH₃COOH] / (1.8 × 10(-5))
[CH₃COOH] = 5.56 × 10-1 mol/L
Step 3: Use the formula for concentration (C = n/V) to find the volume of CH₃COOH needed.
Volume = n / C
Volume = 5.56 × 10-1 mol / 0.2 mol/L ≈ 2.78 L
Therefore, approximately 2.78 liters of 0.2 M acetic acid need to be added.
Example 3: Determine the pH of a solution obtained by mixing 25 mL of 0.1 M hydrochloric acid (HCl) with 75 mL of 0.2 M sodium acetate (CH₃COONa). (Ka for Acetic Acid is 1.8 × 10-5).
Solution:
Step 1: Identify the components that will react.
HCl will fully dissociate and contribute H+ ions.
Sodium acetate (CH3COONa) will partially dissociate into acetate ions (CH3COO–) and Na+ ions.
Step 2: Calculate the moles of H+ ions from HCl and CH3COO– ions from sodium acetate.
Moles of H+ ions from HCl = 25 mL × 0.1 mol/L = 0.0025 mol
Moles of CH3COO– ions from CH3COONa = 75 mL × 0.2 mol/L = 0.015 mol
Step 3: Calculate the total moles of CH3COO– ions.
Total moles of CH3COO– ions = Moles from sodium acetate – Moles reacted with HCl
= 0.015 mol – 0.0025 mol = 0.0125 mol
Step 4: Calculate the concentration of CH3COO– ions.
Total Volume of Solution = 25 mL + 75 mL = 100 mL = 0.1 L
Concentration of CH3COO– ions = Moles / Volume
= 0.0125 mol / 0.1 L = 0.125 mol/L
Step 5: Calculate pOH using the concentration of CH3COO– ions and Ka
pOH = -log(CH3COO– concentration) = -log(0.125) ≈ 0.9
Step 6: Calculate pH using the relation pH + pOH = 14 (for water at 25°C)
pH = 14 – pOH ≈ 14 – 0.9 ≈ 13.1
Therefore, the pH of the solution ≈ 13.1
Example 4. A 0.05 M solution of formic acid (HCOOH) has a pH of 2.6. Calculate the concentration of hydroxide ions ([OH-]) in this solution. (Ka for formic acid is 1.8 × 10-4).
Solution:
Given,
- pH = 2.6
pH = -log[H+]
[H+] = 10-pH
[H+] = 10(-2.6) ≈ 2.51 × 10-3mol/L
For a weak acid (HCOOH), use the relation Ka = [H+][HCOO–] / [HCOOH]
Given [H+] = [HCOO–] (for a weak acid)
Ka = [H+]2 / [HCOOH]
[HCOOH] = [H+]2 / Ka
[HCOOH] = (2.51 × 10(-3))2 / (1.8 × 10^(-4)) ≈ 3.54 × 10(-2) mol/L
Concentration of Hydroxide Ions is given by [OH–] = Kw / [H+].
Kw = 1.0 × 10-14 (at 25°C)
[OH–] = Kw / [H+] = 1.0 × 10^(-14) / 2.51 × 10(-3) ≈ 3.98 × 10(-12) mol/L
Therefore, the concentration of hydroxide ions ≈ 3.98 × 10^(-12) mol/L.
Example 5: Calculate the pH of a solution prepared by mixing 25 mL of 0.1 M hydrochloric acid (HCl) and 75 mL of 0.05 M sodium hydroxide (NaOH). (Ka for HCl is 1.0 × 106 and Kw for water is 1.0 × 10-14).
Solution:
Step 1: Determine the moles of H+ ions from HCl and OH- ions from NaOH.
Moles of H+ ions from HCl = 25 mL × 0.1 mol/L = 0.0025 mol
Moles of OH– ions from NaOH = 75 mL × 0.05 mol/L = 0.00375 mol
Step 2: Identify the limiting reagent (HCl or NaOH) based on the available moles.
HCl is the limiting reagent (0.0025 mol vs. 0.00375 mol)
Step 3: Calculate the excess moles of OH– ions after the reaction with HCl
Excess moles of NaOH = Moles of NaOH – Moles reacted with HCl
= 0.00375 mol – 0.0025 mol = 0.00125 mol
Step 4: Calculate the concentration of OH- ions from the excess NaOH
Concentration of OH– ions = Excess moles of NaOH / Total volume of the solution
= 0.00125 mol / (25 mL + 75 mL)
= 0.00125 mol / 0.1 L
= 0.0125 mol/L
Step 5: Calculate pOH using the concentration of OH– ions.
pOH = -log(OH- concentration) = -log(0.0125) ≈ 1.9
Step 6: Calculate pH using the relation pH + pOH = 14 (for water at 25°C)
pH = 14 – pOH ≈ 14 – 1.9 ≈ 12.1
Therefore, the pH of the solution ≈ 12.1
Bronsted Lowry Theory – FAQs
What is Bronsted-Lowry Theory?
Bronsted-Lowry Theory defines acids as proton donors, bases as proton acceptors in reactions.
How Does Proton Transfer Occur in Acid-Base Reactions?
Protons shift from acids to bases forming conjugate acid-base pairs.
What are Conjugate Acid-Base Pairs in Chemistry?
Acid donates proton to form its conjugate base; base accepts proton forming conjugate acid.
Why is Water Considered an Amphiprotic Substance?
Water acts as both an acid (donates H⁺) and a base (accepts H⁺).
What is an example of a Bronsted-Lowry Acid?
Bronsted–Lowry acid is generally written as HA, where H+ is the donatable proton, and A– is the anion of acid. Some examples of acids are HCl, H2SO4, HNO3, and CH3COOH.
What are Advantages of Bronsted-Lowry Theory?
Some advantages of Bronsted- Lowry Theory are,
- It explains basic property of substances without hydroxide ions.
- It explains role of water in acid-base reactions as more than just a solvent, etc.