Acids, Bases, and pH - SS2 Chemistry Past Questions and Answers - page 1

According to the Brønsted-Lowry theory, an acid is a substance that donates a proton (H+) to another substance, known as a base. The acid loses a proton, while the base gains a proton.

According to the Arrhenius theory, a base is a substance that, when dissolved in water, dissociates to produce hydroxide ions (OH-). This theory defines bases based on their ability to generate hydroxide ions in aqueous solutions.

According to the Lewis theory, an acid is a substance that can accept a pair of electrons during a chemical reaction. This theory broadens the definition of acids beyond proton transfer, emphasising the role of electron pair acceptance.

In the Brønsted-Lowry theory, a conjugate acid-base pair consists of an acid and a base that are related by the transfer of a proton. In this case, H3O+ (hydronium ion) acts as the conjugate acid, donating a proton, while OH- (hydroxide ion) acts as the conjugate base, accepting the proton.

Arrhenius Theory:

●     Definition: According to Arrhenius, an acid is a substance that dissociates in water to produce hydrogen ions (H+), while a base is a substance that dissociates in water to produce hydroxide ions (OH-).

●     Examples: Hydrochloric acid (HCl) dissociates in water to produce H+ ions, making it an Arrhenius acid. Sodium hydroxide (NaOH) dissociates in water to produce OH- ions, making it an Arrhenius base.

Brønsted-Lowry Theory:

●     Definition: Brønsted-Lowry defines an acid as a substance that donates a proton (H+) and a base as a substance that accepts a proton (H+).

●     Examples: Hydrochloric acid (HCl) donates a proton to water, forming the hydronium ion (H3O+), making it a Brønsted-Lowry acid. Ammonia (NH3) accepts a proton from water, forming the ammonium ion (NH4+), making it a Brønsted-Lowry base.

Lewis Theory:

●     Definition: Lewis defines an acid as a substance that accepts an electron pair, and a base as a substance that donates an electron pair.

●     Examples: Boron trifluoride (BF3) accepts an electron pair from a molecule like ammonia (NH3), making it a Lewis acid. Ammonia donates an electron pair, making it a Lewis base.

While all three theories provide different perspectives on acids and bases, they are interconnected. The Arrhenius theory focuses on ionisation in water, while the Brønsted-Lowry theory extends the concept to include proton transfer in any solvent. The Lewis theory broadens the definition further by considering electron pair transfer, which encompasses a wider range of acid-base reactions.

Arrhenius Theory:

●     Strengths: The Arrhenius theory provides a simple and straightforward concept of acids and bases in aqueous solutions. It is suitable for understanding common acid-base reactions and is based on observable ionisation behaviour.

●     Limitations: The Arrhenius theory is limited to aqueous solutions and does not account for acid-base reactions in non-aqueous solvents or those without ionisation. It also does not explain acid-base behaviour in the absence of water.

Brønsted-Lowry Theory:

●     Strengths: The Brønsted-Lowry theory expands the concept of acids and bases beyond water and explains acid-base reactions in various solvents. It focuses on proton transfer, which is fundamental to many chemical reactions. It also allows for the understanding of acid-base behaviour in non-aqueous systems.

●     Limitations: The Brønsted-Lowry theory does not address acid-base reactions involving species that do not involve proton transfer, such as Lewis acids and bases. It does not fully explain acid-base behaviour in the absence of solvents.

Lewis Theory:

●     Strengths: The Lewis theory provides a comprehensive understanding of acid-base reactions by including species that do not involve proton transfer, such as metal cations and complex ions. It accounts for acid-base behaviour in non-aqueous systems and is applicable to a wide range of chemical reactions.

●     Limitations: The Lewis theory may be complex and less intuitive than the Arrhenius and Brønsted-Lowry theories. It requires a deeper understanding of electron pair transfer and coordination chemistry.

These acid-base theories enhance our understanding of acid-base reactions by providing different perspectives and explanations for their behaviour. The Arrhenius theory focuses on ionisation in water, the Brønsted-Lowry theory extends the concept to include proton transfer in any solvent, and the Lewis theory encompasses electron pair transfer. Together, they offer a comprehensive framework for understanding acid-base interactions in various chemical systems.

On the pH scale, each decrease in pH value by 1 represents a tenfold increase in the concentration of hydrogen ions. Since the pH is 3, the concentration of hydrogen ions is 10^-3 M.

The pH of a solution can be calculated from the concentration of hydrogen ions (H+), but in this case, we are given the concentration of hydroxide ions (OH-). To calculate the pH, we need the concentration of hydrogen ions. Therefore, the pH cannot be determined with the given information.

On the pH scale, values below 7 indicate acidity, 7 represents neutrality, and values above 7 indicate alkalinity or basicity. Since the pH is 9, the solution is considered basic.