Acids, Bases, and pH - SS2 Chemistry Past Questions and Answers - page 1
According to the Brønsted-Lowry theory, an acid is defined as a substance that:
Accepts a proton (H+)
Donates a proton (H+)
Dissociates into positive and negative ions
Forms a covalent bond with a base
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 defined as a substance that:
Accepts a proton (H+)
Donates a proton (H+)
Dissociates into positive and negative ions
Forms a covalent bond with an acid
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 defined as a substance that:
Accepts a proton (H+)
Donates a proton (H+)
Dissociates into positive and negative ions
Accepts a pair of electrons
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.
According to the Brønsted-Lowry theory, which of the following is a conjugate acid-base pair?
HCl and NaOH
H2O and OH-
H3O+ and OH-
HCl and Cl-
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.
Compare and contrast the Arrhenius, Brønsted-Lowry, and Lewis acid-base theories. Discuss their definitions of acids and bases and provide examples to illustrate each theory.
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.
Discuss the strengths and limitations of the Arrhenius, Brønsted-Lowry, and Lewis acid-base theories. How do these theories enhance our understanding 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.
What does the pH scale measure?
Concentration of hydrogen ions (H+)
Concentration of hydroxide ions (OH-)
Concentration of both hydrogen and hydroxide ions
Concentration of water molecules (H2O)
A solution has a pH of 3. What is the concentration of hydrogen ions in the solution?
10-3 M
10-10 M
103 M
1010 M
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.
A solution has a hydroxide ion concentration of 10-9 M. What is the pH of the solution?
5
9
14
Cannot be determined with the given information
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.
If a solution has a pH of 9, it can be classified as:
Acidic
Neutral
Basic
Amphoteric
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.