Problem 25 - Chemistry and Chemical Reactivity

Question

A method recently suggested for the preparation of hydrogen (and oxygen) from water proceeds as follows: (a) Sulfuric acid and hydrogen iodide are formed from sulfur dioxide, water, and iodine. (b) The sulfuric acid from the first step is decomposed by heat to water, sulfur dioxide, and oxygen. (c) The hydrogen iodide from the first step is decomposed with heat to hydrogen and iodine. Write a balanced equation for each of these steps, and show that their sum is the decomposition of water to form hydrogen and oxygen.

Step-by-Step Solution

Answer

The overall balanced equation for the decomposition of water is \(2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2\).

Step 1: Write the Reaction for Step (a)

For the first step, sulfuric acid (H₂SO₄) and hydrogen iodide (HI) are produced from sulfur dioxide (SO₂), water (H₂O), and iodine (I₂). The chemical equation is: \(\text{SO}_2 + \text{H}_2\text{O} + \text{I}_2 \rightarrow \text{H}_2\text{SO}_4 + 2\text{HI}\).

Step 2: Write the Reaction for Step (b)

In this step, sulfuric acid is decomposed by heat to produce water, sulfur dioxide, and oxygen. The chemical equation for this decomposition is: \(\text{H}_2\text{SO}_4 \rightarrow \text{H}_2\text{O} + \text{SO}_2 + \frac{1}{2}\text{O}_2\).

Step 3: Write the Reaction for Step (c)

Hydrogen iodide is decomposed by heat into hydrogen and iodine. This can be represented by: \(2\text{HI} \rightarrow \text{H}_2 + \text{I}_2\).

Step 4: Sum the Reactions

Add the balanced equations from Steps 1, 2, and 3: \[ \begin{align*} \text{SO}_2 + \text{H}_2\text{O} + \text{I}_2 &\rightarrow \text{H}_2\text{SO}_4 + 2\text{HI} \ \text{H}_2\text{SO}_4 &\rightarrow \text{H}_2\text{O} + \text{SO}_2 + \frac{1}{2}\text{O}_2 \ 2\text{HI} &\rightarrow \text{H}_2 + \text{I}_2 \end{align*} \] When added, the intermediate products and reactants cancel out, leaving: \[ \text{H}_2\text{O} \rightarrow \text{H}_2 + \frac{1}{2}\text{O}_2 \]. Multiply this entire reaction by 2 to balance: \(2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2\).

Key Concepts

Chemical Equation Balancing

Balancing chemical equations is an essential skill in chemistry that ensures the same number of atoms for each element on both sides of the equation. This is based on the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a chemical reaction. In the exercise, we deal with three different chemical reactions across multiple steps.

• For Step 1: Sulfur dioxide, water, and iodine react to form sulfuric acid and hydrogen iodide. The balanced equation is: \( \text{SO}_2 + \text{H}_2\text{O} + \text{I}_2 \rightarrow \text{H}_2\text{SO}_4 + 2\text{HI} \).
• For Step 2: Sulfuric acid decomposes into water, sulfur dioxide, and oxygen: \( \text{H}_2\text{SO}_4 \rightarrow \text{H}_2\text{O} + \text{SO}_2 + \frac{1}{2}\text{O}_2 \).
• For Step 3: Hydrogen iodide breaks down into hydrogen and iodine: \( 2\text{HI} \rightarrow \text{H}_2 + \text{I}_2 \).

By adding these equations together, the intermediate compounds cancel, showing the overall decomposition of water. Multiplying to ensure all coefficients are integers, the balanced decomposition of water becomes: \( 2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2 \). This confirms that we successfully conserved mass across the reactions.

Hydrogen Production

Hydrogen production is a significant focus in chemical processes due to its applications as a clean energy source. In the exercise, hydrogen is produced from the decomposition of hydrogen iodide.

• Step 3 specifically handles this transformation, where heating hydrogen iodide results in the formation of hydrogen gas and iodine\( : 2\text{HI} \rightarrow \text{H}_2 + \text{I}_2 \).
• The reaction involves breaking the bonds in hydrogen iodide and reforming hydrogen molecules. This is an example of a decomposition reaction, where a single compound breaks down into simpler substances.

Hydrogen generated through this method can be used in various applications, such as fuel cells for vehicles and industrial chemical processes. Understanding each step's role in producing hydrogen helps us appreciate decompositions' broader implications for sustainable energy.

Oxygen Production

Oxygen production is a vital aspect of both natural and industrial processes. In this particular exercise, oxygen is generated through the decomposition of sulfuric acid.

• During Step 2, sulfuric acid is decomposed under heat to release oxygen as well as water and sulfur dioxide: \( \text{H}_2\text{SO}_4 \rightarrow \text{H}_2\text{O} + \text{SO}_2 + \frac{1}{2}\text{O}_2 \).
• This demonstrates a classic decomposition reaction, where a complex molecule breaks into simpler products. This process is efficient for producing oxygen gas, potentially useful for medical and industrial purposes.

Knowing how oxygen is produced in chemical reactions helps us connect to practical applications, from medical oxygen supply to enhancing combustion in engines. The elegance of chemical reactions lies in their ability to transform one substance into another with valuable outcomes, showcasing chemistry's transformative power.