Question

A common demonstration in chemistry courses involves adding a tiny speck of manganese(IV) oxide to a concentrated hydrogen peroxide, \(\mathrm{H}_{2} \mathrm{O}_{2},\) solution. Hydrogen peroxide is unstable, and it decomposes quite spectacularly under these conditions to produce oxygen gas and steam (water vapor). Manganese(IV) oxide is a catalyst for the decomposition of hydrogen peroxide and is not consumed in the reaction. Write the balanced equation for the decomposition reaction of hydrogen peroxide.

Short answer

The balanced equation for the decomposition of hydrogen peroxide (H2O2) is: \(2 \mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow \mathrm{O}_{2} + 2 \mathrm{H}_{2}\mathrm{O}\).

Step-by-step solution

Write the unbalanced equation

We will start by writing down the unbalanced chemical equation for the decomposition of hydrogen peroxide: \[\mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow \mathrm{O}_{2} + \mathrm{H}_{2}\mathrm{O}\] Step 2: Balancing hydrogen atoms

Balance hydrogen atoms

The number of hydrogen atoms is not equal on both sides of the equation. There are 2 hydrogen atoms in H2O2 and 2 hydrogen atoms in H2O. To balance this, we will add a coefficient of 2 in front of H2O: \[\mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow \mathrm{O}_{2} + 2 \mathrm{H}_{2}\mathrm{O}\] Step 3: Balancing oxygen atoms

Balance oxygen atoms

The number of oxygen atoms is not equal on both sides of the equation after balancing the hydrogen atoms. There are 2 oxygen atoms in H2O2, 2 oxygen atoms in O2, and 2 oxygen atoms in 2H2O. To balance this, we will add a coefficient of 2 in front of H2O2: \[2 \mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow \mathrm{O}_{2} + 2 \mathrm{H}_{2}\mathrm{O}\] Step 4: Check if the equation is balanced

Check if the equation is balanced

Now, we need to check if the equation is balanced. We have the following atoms on both sides of the equation: Left side: 4 hydrogen atoms and 4 oxygen atoms Right side: 4 hydrogen atoms (2H2O) and 4 oxygen atoms (1 O2 + 2 H2O) Since the number of each type of atom is the same on both sides of the equation, the equation is balanced. So, the balanced equation for the decomposition of hydrogen peroxide (H2O2) is: \[2 \mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow \mathrm{O}_{2} + 2 \mathrm{H}_{2}\mathrm{O}\]

Key concepts

Chemical Equation Balancing

Understanding how to balance chemical equations is essential in the study of chemistry. In a balanced chemical equation, the number of atoms for each element is the same on both sides of the reaction arrow, satisfying the law of conservation of mass. To balance equations, coefficients are added before the chemical formulas to ensure the number of atoms of each element is equal on both sides.

The decomposition of hydrogen peroxide is a prime example to illustrate this process. Starting with an unbalanced equation, \(\mathrm{H}_{2}\mathrm{O}_{2} \longrightarrow \mathrm{O}_{2} + \mathrm{H}_{2}\mathrm{O}\), the hydrogen at first appears balanced with two atoms on each side. However, oxygen presents a challenge with two atoms on the left but on the right, one from oxygen gas and two from water, totaling three atoms.

To rectify this, we place a coefficient of 2 in front of hydrogen peroxide, resulting in four oxygen atoms. Then, we place a coefficient of 2 in front of water on the right-hand side, which brings the hydrogen atoms total to four, aligning with the left side's count. The balanced equation, therefore, is \(2 \mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow \mathrm{O}_{2} + 2 \mathrm{H}_{2}\mathrm{O}\), reflecting the same number of hydrogen and oxygen atoms on both sides.

Catalysis

Catalysis is a process that speeds up the rate of a chemical reaction without undergoing any permanent chemical change itself. Catalysts achieve this by providing an alternative reaction pathway with a lower activation energy. This means that a greater number of molecules have the necessary energy to undergo the reaction, thus increasing the rate.

In our specific scenario, manganese(IV) oxide, \(\mathrm{MnO}_{2}\), acts as the catalyst for the decomposition of hydrogen peroxide. Though it plays a crucial role in expediting the reaction, it remains unchanged upon completion. Catalysts like manganese(IV) oxide are particularly valuable because they can be used repeatedly, making them highly efficient for both laboratory demonstrations and industrial processes.

Manganese(IV) Oxide

Manganese(IV) oxide, often represented as \(\mathrm{MnO}_{2}\), is a black or brown solid used widely as a catalyst in various reactions, including the decomposition of hydrogen peroxide. Apart from its catalytic properties, manganese(IV) oxide is used in dry-cell batteries, such as the alkaline battery and the zinc-carbon battery.

In the context of our reaction, the addition of a tiny speck of \(\mathrm{MnO}_{2}\) to hydrogen peroxide leads to a swift decomposition, producing water and oxygen gas. Importantly, manganese(IV) oxide remains unchanged at the end of the reaction and does not appear in the balanced chemical equation, highlighting its role as a catalyst rather than a reactant.

Oxygen Gas Production

The production of oxygen gas is a crucial component of the decomposition reaction of hydrogen peroxide. When hydrogen peroxide breaks down, it generates oxygen gas, \(\mathrm{O}_{2}\), and water in the form of steam. The generation of oxygen gas is a visible and often dramatic result of this exothermic reaction, as it may produce bubbles and foam, especially when a catalyst is present.

In practical applications, oxygen gas produced from chemical reactions like this one can be used for medical purposes, water treatment, and rocket propulsion. Its liberation from the decomposition of hydrogen peroxide is integral to demonstrating the principles of chemical reactions and stoichiometry in educational settings.