Chemical Reactions in Aqueous Solutions and Stoichiometry

Reactions in Aqueous Solutions

Acid and Base Definition According to Bronsted-Lowry

Acid: A substance capable of donating protons.

Base: A substance capable of accepting protons.

For a substance to act as an acid, it must be in the presence of another substance that acts as a base and accepts the proton transferred by the acid. In aqueous solutions, water plays the role of both acid and base.

• When an acid donates protons, it becomes a species capable of accepting protons, called the conjugate base.
• When a base accepts protons, it is transformed into a species capable of donating protons, i.e., an acid, called the conjugate acid.

Equilibrium of Water Dissociation and the Concept of Kw

Water behaves as both an acid and a base according to the reaction:

2H₂O ⇌ H₃O⁺ + OH^-

It dissociates into hydronium ions (H₃O⁺) and hydroxide ions (OH^-).

The ion product constant of water is defined as:

K_w = [H₃O⁺] x [OH^-]

At 25°C, the ionic product of water is 10^-14.

The pH of a solution is the negative logarithm of the molar concentration of H₃O⁺:

pH = -log[H₃O⁺]

pOH = -log[OH^-]

pH + pOH = 14

The concentrations of H₃O⁺ and OH^- ions in aqueous solutions are very small and difficult to manage. Therefore, a quick and practical way of measuring the acidity and basicity of these solutions is used: the pH scale.

• Acidic solution: pH < 7
• Neutral solution: pH = 7
• Basic solution: pH > 7

Concept of Oxidation and Reduction

From an electronic point of view, a reaction is considered an oxidation-reduction reaction when electrons are transferred.

• Oxidation is a chemical reaction in which a substance loses electrons. When a substance loses electrons, it is said to be oxidized. The substance that loses electrons is called the reducing agent.
• Reduction is a chemical reaction in which a substance gains electrons. When a substance gains electrons, it is said to be reduced. The substance that gains electrons is called the oxidizing agent.

Oxidation and reduction processes cannot occur in isolation. If a substance loses electrons, another must gain them. Therefore, we refer to these as oxidation-reduction reactions or redox reactions. In these reactions, there is a transfer of electrons from the substance that is oxidized to the substance that is reduced.

• The oxidizing agent is the chemical species (ion, atom, or molecule) that causes the oxidation of another substance. It gains the electrons lost by this substance and is itself reduced.
• The reducing agent is the chemical species that causes the reduction of another substance. It donates electrons and is itself oxidized.

Part 2, Item 4

Concentration Units

• g/L: The amount of solute, expressed in grams, per liter of solution. This represents the grams of solute contained in 1 L of solution.
• Molarity (M): The amount of solute, in moles, per liter of solution. This represents the number of moles of solute contained in 1 L of solution.
• Molality (m): The amount of solute, in moles, per kilogram of solvent.
• Mole Fraction: The number of moles of one component divided by the sum of the moles of all components.

Stoichiometry

Limiting Reagent

When a reaction occurs, the reagents are probably not present in the exact stoichiometric ratio (which is always constant). There may be an excess of one or more reagents. In this case, there will be one reagent that is consumed entirely, and this is called the limiting reagent.