Question

Identify each substance as an Arrhenius acid, an Arrhenius base, or neither. a) \(\mathrm{NaOH}\) b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) c) \(\mathrm{H}_{3} \mathrm{PO}_{4}\)

Short answer

NaOH is a base, C2H5OH is neither, and H3PO4 is an acid.

Step-by-step solution

Understanding Arrhenius Acid

An Arrhenius acid is a substance that, when dissolved in water, increases the concentration of hydrogen ions, \( \mathrm{H^+} \). This can be represented by the general equation: \[ \mathrm{HA} \rightarrow \mathrm{H^+} + \mathrm{A^-} \] where \( \mathrm{HA} \) is the acid.

Understanding Arrhenius Base

An Arrhenius base is a substance that, when dissolved in water, increases the concentration of hydroxide ions, \( \mathrm{OH^-} \). This is represented by the general equation: \[ \mathrm{BOH} \rightarrow \mathrm{B^+} + \mathrm{OH^-} \] where \( \mathrm{BOH} \) is the base.

Identifying \\( \mathrm{NaOH} \\\)

\( \mathrm{NaOH} \) is sodium hydroxide. When dissolved in water, it dissociates into \( \mathrm{Na^+} \) and \( \mathrm{OH^-} \), thereby increasing the concentration of hydroxide ions in solution. Thus, \( \mathrm{NaOH} \) is an Arrhenius base.

Identifying \\( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH} \\\)

\( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH} \) is ethanol, a type of alcohol. It does not dissociate in water to produce \( \mathrm{H^+} \) or \( \mathrm{OH^-} \) ions. Therefore, it is neither an Arrhenius acid nor an Arrhenius base.

Identifying \\( \mathrm{H}_{3} \mathrm{PO}_{4} \\\)

\( \mathrm{H}_{3} \mathrm{PO}_{4} \) is phosphoric acid. When dissolved in water, it dissociates to release \( \mathrm{H^+} \) ions, increasing their concentration in the solution. Thus, \( \mathrm{H}_{3} \mathrm{PO}_{4} \) is an Arrhenius acid.

Key concepts

Arrhenius Acid

The concept of an Arrhenius acid is centered around the idea of hydrogen ions. According to the Arrhenius theory, an Arrhenius acid is a substance that, when added to water, increases the concentration of hydrogen ions \( \mathrm{H^+} \) in the solution.
In other words, an Arrhenius acid essentially releases \( \mathrm{H^+} \) ions into the water, making the solution more acidic. These \( \mathrm{H^+} \) ions are also referred to as protons, as they are simply a hydrogen atom that has lost an electron, leaving behind a proton only.
Some key points to remember about Arrhenius acids:

• They contribute to the acidity by providing \( \mathrm{H^+} \) ions.
• Example substances include strong acids like hydrochloric acid (\( \mathrm{HCl} \)) and sulfuric acid (\( \mathrm{H_2SO_4} \)).
• In the given context, phosphoric acid (\( \mathrm{H_3PO_4} \)) is an Arrhenius acid, as it dissociates in water to release \( \mathrm{H^+} \) ions.

Arrhenius Base

An Arrhenius base is a counterpart to an Arrhenius acid, with respect to its effect on water. A base increases the concentration of hydroxide ions \( \mathrm{OH^-} \) when dissolved in water.
These hydroxide ions are negatively charged molecules that interact with hydrogen ions \( \mathrm{H^+} \), which can decrease acidity and increase the basicity of a solution.
Here are some important points about Arrhenius bases:

• They increase the \( \mathrm{OH^-} \) concentration when dissolved in water.
• Sodium hydroxide (\( \mathrm{NaOH} \)) is a classic example—dissociating into \( \mathrm{Na^+} \) and \( \mathrm{OH^-} \).
• Other examples include potassium hydroxide (\( \mathrm{KOH} \)) and calcium hydroxide (\( \mathrm{Ca(OH)_2} \)).

Dissociation in Water

Dissociation in water is a key process that occurs when ionic substances split into ions as they dissolve. This behavior is essential for defining whether a substance acts as an Arrhenius acid or base.
When an acid dissolves, it splits into \( \mathrm{H^+} \) ions and its respective anions. Similarly, a base dissociates into \( \mathrm{OH^-} \) ions and its respective cations.
Key aspects of dissociation:

• The process depends on the nature of the substance.
• Ethanol (\( \mathrm{C_2H_5OH} \)), for example, does not dissociate significantly in water to release \( \mathrm{H^+} \) or \( \mathrm{OH^-} \) ions, hence is neither an acid nor a base.
• Fully understanding dissociation helps in predicting the solubility and reactivity of substances in water.