Question

Explain how the concentrations of hydrogen ions and hydroxide ions determine whether a solution is acidic, basic, or neutral.

Short answer

The nature of a solution (acidic, basic, or neutral) can be determined by comparing the concentrations of hydrogen ions (H+) and hydroxide ions (OH-). If [H^+] > [OH^-], the solution is acidic (pH < 7). If [H^+] < [OH^-], the solution is basic (pH > 7). If [H^+] = [OH^-], the solution is neutral (pH = 7). This is based on the ion product of water, \(K_w = [H^+][OH^-]\), which has a constant value of 1.0 x 10^-14 at 25°C in pure water.

Step-by-step solution

Understanding pH and Ion Concentration

First, let's understand what pH is. pH is the negative logarithmic scale that represents the concentration of hydrogen ions (H+) present in a solution. The pH scale ranges from 0 to 14, with the value of pH 7 being neutral (pure water). A pH value less than 7 indicates an acidic solution, while a pH value greater than 7 indicates a basic or alkaline solution.

Relationship between Hydrogen Ions and Hydroxide Ions

In any aqueous solution, the product of the concentrations of hydrogen ions (H+) and hydroxide ions (OH-) is constant and is known as the ion product of water (Kw). Mathematically, it can be written as: \[ K_w = [H^+][OH^-] \] For pure water, the value of Kw at 25°C is 1.0 x 10^-14.

Determining the Nature of the Solution (Acidic, Basic, or Neutral)

Using the relationship established in Step 2, we can determine the nature of a given solution. 1. If [H^+] > [OH^-]: Since the concentration of hydrogen ions is greater than the concentration of hydroxide ions, the solution is acidic. This corresponds to a pH value of less than 7. 2. If [H^+] < [OH^-]: With a higher concentration of hydroxide ions than hydrogen ions, the solution is basic or alkaline. In this case, the pH is greater than 7. 3. If [H^+] = [OH^-]: An equal concentration of hydrogen and hydroxide ions signifies a neutral solution. This can be observed in pure water, and it corresponds to a pH value of exactly 7. By determining the concentrations of hydrogen ions (H+) and hydroxide ions (OH-) in a solution, we can classify the solution as acidic, basic, or neutral according to the criteria mentioned above.

Key concepts

Acids and Bases

Acids and bases are two types of substances that have contrasting characteristics and are essential to many chemical reactions. An acid is a substance that increases the concentration of hydrogen ions (H^+) when dissolved in water, giving the solution a pH lower than 7.

Common examples of acids include hydrochloric acid (HCl) and citric acid found in citrus fruits. On the other hand, a base is a substance that increases the concentration of hydroxide ions (OH^-), resulting in a solution with a pH higher than 7. Substances like sodium hydroxide (NaOH) and baking soda (NaHCO_3) are typical bases.

The strength of an acid or base is usually determined by its ability to donate or accept hydrogen ions. Strong acids and bases dissociate completely in water, while weak acids and bases do not.

Hydrogen Ions

Hydrogen ions (H^+) play a pivotal role in the acidic properties of solutions. In essence, they are simply protons, as a hydrogen atom consists of one proton and one electron, and the ion has lost its single electron.

The concentration of hydrogen ions in a solution is measured by the pH scale, which is inversely related to the number of hydrogen ions; a high concentration of hydrogen ions results in a low pH, and a low concentration results in a high pH. This relationship is crucial in chemistry and is used to determine a solution's acidity. For example, gastric juice in our stomach has a very low pH because it is rich in hydrochloric acid, which dissociates to produce a high concentration of hydrogen ions.

Hydroxide Ions

Hydroxide ions (OH^-) are the counterpart to hydrogen ions in the discussion of acidity and basicity. They are negatively charged ions composed of one oxygen atom bonded to one hydrogen atom.

Bases, when dissolved in water, increase the concentration of hydroxide ions, which combine with hydrogen ions to form water (H_2O), thereby reducing the concentration of hydrogen ions and increasing the pH. In solutions where there is a higher concentration of hydroxide ions than hydrogen ions, the environment is considered basic or alkaline. Everyday products such as bleach and soap typically contain substances that produce hydroxide ions when dissolved in water.

Ion Product of Water

The ion product of water (K_w) is a fundamental constant that defines the relationship between hydrogen ions (H^+) and hydroxide ions (OH^-) in water. It is expressed as the product of their concentrations and remains constant at a given temperature.

Mathematically, the ion product of water is given by K_w = [H^+][OH^-], where [H^+] and [OH^-] represent the molar concentrations of hydrogen ions and hydroxide ions, respectively. At the standard temperature of 25°C, the value of K_w is always 1.0 x 10^-14. This relationship is particularly useful because it allows scientists and students to calculate one ion's concentration when the other is known, which helps in determining the solution's pH and thus its acidic or basic nature.