Problem 55 - Chemistry The Molecular Nature of Matter

Question

Hydrazine is a toxic substance that can form when household ammonia is mixed with a bleach such as Clorox. Its formula is \(\mathrm{N}_{2} \mathrm{H}_{4}\), and it is a weak base. Write a chemical equation showing its reaction with water.

Step-by-Step Solution

Answer

The balanced chemical equation for the reaction of hydrazine with water is: N2H4(aq) + H2O(l) ⇌ N2H5+(aq) + OH-(aq).

Step 1: Write the formula for the reaction of hydrazine with water

To write the formula, we recognize hydrazine (N2H4) as a weak base that will accept a proton from water (H2O), which acts as an acid. The reaction will result in the formation of the hydrazium ion (N2H5+) and a hydroxide ion (OH-): N2H4(aq) + H2O(l) ⇌ N2H5+(aq) + OH-(aq)

Step 2: Balance the chemical equation

Check if the chemical equation is already balanced by counting the atoms of each element on both sides of the reaction. For this reaction, it is already balanced as there are two nitrogen atoms and five hydrogen atoms on both sides of the equation, and one oxygen atom from water remains unchanged.

Key Concepts

Chemical Equation

A chemical equation is a symbolic representation of a chemical reaction, showing the reactants and products along with their respective quantities. Each molecule is expressed using its chemical formula, and the reaction is depicted by an arrow indicating the direction of the reaction. In the reaction of hydrazine (N₂H₄) with water (H₂O), the chemical equation is written as:
\[ \mathrm{N}_{2}\mathrm{H}_{4}(aq) + \mathrm{H}_{2}\mathrm{O}(l) \rightleftharpoons \mathrm{N}_{2}\mathrm{H}_{5}^{+}(aq) + \mathrm{OH}^{-}(aq) \]
Here we see hydrazine and water as the reactants, and the hydrazium ion and hydroxide ion as the products. Understanding how to construct and interpret chemical equations is crucial for studying chemical reactions, as it provides insight into the quantities of substances involved and their transformations.

Weak Base

Hydrazine is classified as a weak base because it only partially dissociates in water, accepting a proton to form the hydrazium ion. Rather than fully separating into ions, weak bases establish an equilibrium where both the unreacted base and the products coexist in solution. This equilibrium can be represented by a double-arrow in the chemical equation, suggesting that the reaction can proceed in either direction. Weak bases typically have higher pKa values, indicating less tendency to accept protons compared to strong bases, and they often result in solutions with a pH less than 14 but greater than 7, implying that they raise the hydroxide ion concentration to a lesser extent than strong bases.

Balancing Chemical Reactions

The importance of balancing chemical equations cannot be overstated. A balanced equation adheres to the law of conservation of mass, which states that mass is not created or destroyed in a chemical reaction. As a result, each element must have the same number of atoms on both sides of the equation. In the hydrazine reaction, we have:

• Two nitrogen atoms on both the reactants and products side.
• Five hydrogen atoms on both sides.
• One oxygen atom from water, which does not change.

Now that the chemical equation is balanced, it allows us to calculate the reactants' and products' quantities accurately. If the equation were not balanced, subsequent calculations, ranging from stoichiometric computations to energy changes, would be incorrect.

Acid-Base Reactions

An acid-base reaction involves the transfer of a proton (H+) from the acid to the base. When hydrazine acts as a base in water, it accepts a proton, causing the release of a hydroxide ion (OH-) and formation of a hydrazium ion (N₂H₅+). This type of reaction illustrates a fundamental concept in chemistry given by the Bronsted-Lowry acid-base theory, which defines acids and bases in terms of their ability to donate or accept protons, respectively. The reversible nature of the hydrazine and water reaction shows that the forward and backward reactions can occur, with the reaction settling into a state of dynamic equilibrium where the rates of the forward and reverse reactions are equal.