Question

Write a chemical equation for the reaction between each pair of reactants, using single or double arrows as appropriate: (a) \(\mathrm{HNO}_{3}\) and \(\mathrm{OH}^{-}\) (b) \(\mathrm{HF}\) and \(\mathrm{OH}^{-}\) (c) \(\mathrm{NH}_{3}\) and \(\mathrm{H}_{2} \mathrm{O}\) (d) \(\mathrm{HCO}_{3}^{-}\) and \(\mathrm{H}_{2} \mathrm{O}\)

Short answer

The chemical equations for each reaction are: (a) \( \mathrm{HNO}_{3} + \mathrm{OH}^{-} \to \mathrm{H}_{2}\mathrm{O} + \mathrm{NO}_{3}^{-} \) (b) \( \mathrm{HF} + \mathrm{OH}^{-} \rightleftharpoons \mathrm{H}_{2}\mathrm{O} + \mathrm{F}^{-} \) (c) \( \mathrm{NH}_{3} + \mathrm{H}_{2}\mathrm{O} \rightleftharpoons \mathrm{NH}_{4}^{+} + \mathrm{OH}^{-} \) (d) \( \mathrm{HCO}_{3}^{-} + \mathrm{H}_{2}\mathrm{O} \rightleftharpoons \mathrm{H}_{3}\mathrm{O}^{+} + \mathrm{CO}_{3}^{2-} \)

Step-by-step solution

Pair (a): Reaction between \(\mathrm{HNO}_{3}\) and \(\mathrm{OH}^{-}\)

The nitric acid (\(\mathrm{HNO}_{3}\)) will react with the hydroxide ion (\(\mathrm{OH}^{-}\)) to form water (\(\mathrm{H}_{2}\mathrm{O}\)) and the nitrate ion (\(\mathrm{NO}_{3}^{-}\)). Since the nitric acid is a strong acid, it will ionize completely, and we can use a single arrow to represent the reaction as follows: \[ \mathrm{HNO}_{3} + \mathrm{OH}^{-} \to \mathrm{H}_{2}\mathrm{O} + \mathrm{NO}_{3}^{-} \]

Pair (b): Reaction between \(\mathrm{HF}\) and \(\mathrm{OH}^{-}\)

The hydrofluoric acid (\(\mathrm{HF}\)) reacts with the hydroxide ion (\(\mathrm{OH}^{-}\)) to form water (\(\mathrm{H}_{2}\mathrm{O}\)) and the fluoride ion (\(\mathrm{F}^{-}\)). As hydrofluoric acid is a weak acid, it doesn't ionize completely, so we should use a double arrow to represent that equilibrium exists between the reactants and products: \[ \mathrm{HF} + \mathrm{OH}^{-} \rightleftharpoons \mathrm{H}_{2}\mathrm{O} + \mathrm{F}^{-} \]

Pair (c): Reaction between \(\mathrm{NH}_{3}\) and \(\mathrm{H}_{2} \mathrm{O}\)

Ammonia (\(\mathrm{NH}_{3}\)) reacts with water (\(\mathrm{H}_{2}\mathrm{O}\)) to form the ammonium ion (\(\mathrm{NH}_{4}^{+}\)) and the hydroxide ion (\(\mathrm{OH}^{-}\)). The reaction is reversible, so we should use a double arrow to represent the equilibrium: \[ \mathrm{NH}_{3} + \mathrm{H}_{2}\mathrm{O} \rightleftharpoons \mathrm{NH}_{4}^{+} + \mathrm{OH}^{-} \]

Pair (d): Reaction between \(\mathrm{HCO}_{3}^{-}\) and \(\mathrm{H}_{2} \mathrm{O}\)

The bicarbonate ion (\(\mathrm{HCO}_{3}^{-}\)) reacts with water (\(\mathrm{H}_{2}\mathrm{O}\)) to form the hydronium ion (\(\mathrm{H}_{3}\mathrm{O}^{+}\)) and the carbonate ion (\(\mathrm{CO}_{3}^{2-}\)). This reaction is reversible, so we should use a double arrow to represent the equilibrium: \[ \mathrm{HCO}_{3}^{-} + \mathrm{H}_{2}\mathrm{O} \rightleftharpoons \mathrm{H}_{3}\mathrm{O}^{+} + \mathrm{CO}_{3}^{2-} \]

Key concepts

Acid-Base Reaction

Understanding acid-base reactions is fundamental in chemistry as they are among the most common types of chemical processes. These reactions involve an acid and a base that typically produce water and a salt. For instance, when you mix hydrochloric acid (HCl) with sodium hydroxide (NaOH), the hydrogen ion (H^+) from the acid combines with the hydroxide ion (OH^-) from the base to form water (H_2O), leaving sodium chloride (NaCl), common table salt, as the resultant ionic compound.

The example of nitric acid (HNO_3) reacting with hydroxide ion (OH^-) also depicts a quintessential acid-base reaction where water and nitrate ion (NO_3^-) are produced. This particular reaction completes without leftover reactants, indicative of a strong acid reacting with a strong base, hence the use of a single arrow in the chemical equation.

Chemical Equilibrium

The concept of chemical equilibrium is a state in a chemical reaction where the rate of the forward reaction equals the rate of the backward reaction. It means that the concentrations of the reactants and products remain unchanged over time, although particles continue to react with each other. This state can be observed in reactions such as the interaction between hydrofluoric acid (HF) and hydroxide ion (OH^-), where the reaction can progress both to the formation of water and fluoride ion (F^-), and the reverse reaction where the formed products can recombine to form the original reactants.

Uncaptured in a simple chemical equation is the dynamic nature of equilibrium; molecules are in a constant push and pull, a dance of breaking and forming bonds, endlessly cycling without change to the macroscopic properties. Double arrows (rightleftharpoons) in the equation are emblematic of this balance and signify that the reaction is reversible.

Reaction Arrows Significance

The symbols and notations in chemistry are not mere convention but encapsulate critical information about the processes they represent. Reaction arrows play a key role in conveying the directionality and completion of chemical reactions. A single arrow (rightarrow) implies that the reaction proceeds almost to completion, as seen in strong acid-strong base reactions, such as with nitric acid and hydroxide ion where there's a full transfer of protons. In contrast, double arrows (rightleftharpoons) indicate that the reaction is reversible and does not go to completion; it reaches a point of equilibrium as exemplified in the reaction of ammonia with water. It's vital for students to grasp the implications of these arrows as they provide deep insights into the behavior of the reacting species and the reaction conditions.

Ionization of Acids

The ionization of acids is a process where an acid releases hydrogen ions (H^+) into solution. The degree to which this happens can vary widely among acids. For example, strong acids like HCl ionize completely in water, releasing all their hydrogen ions to form hydronium ions (H_3O^+). On the other hand, weak acids like hydrofluoric acid (HF) only partially ionize, resulting in an equilibrium between the non-ionized and ionized forms of the acid in solution.

As presented in exercise pair (b), hydrofluoric acid in its reaction with hydroxide ion does not give away its hydrogen ion wholly which is depicted by the double arrow in its chemical equation. Such partial ionization is key in understanding the nuanced behavior of weak acids, particularly in biological systems and industrial applications where the strength of an acid can vastly influence its reactivity and the equilibrium state of the reaction.