Problem 19 - 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 sum of reactions results in the equation for water decomposition: \(2\, \text{H}_2\text{O} \rightarrow \text{H}_2 + \text{O}_2\).

Step 1: Write the Equation for Reaction (a)

The first reaction involves sulfur dioxide (SO₂), water (H₂O), and iodine (I₂) reacting to form sulfuric acid (H₂SO₄) and hydrogen iodide (HI). The balanced chemical equation for this step is: \[\text{SO}_2 + 2\, \text{H}_2\text{O} + \text{I}_2 \rightarrow \text{H}_2\text{SO}_4 + 2\, \text{HI}\]

Step 2: Write the Equation for Reaction (b)

In the second reaction, the sulfuric acid (H₂SO₄) produced in Step 1 decomposes when heated to form water (H₂O), sulfur dioxide (SO₂), and oxygen (O₂). The balanced chemical equation is: \[\text{H}_2\text{SO}_4 \rightarrow \text{SO}_2 + \text{H}_2\text{O} + \frac{1}{2}\text{O}_2\]

Step 3: Write the Equation for Reaction (c)

The third reaction involves the decomposition of hydrogen iodide (HI) under heat to produce hydrogen (H₂) and iodine (I₂). The balanced chemical equation is given by: \[2\, \text{HI} \rightarrow \text{H}_2 + \text{I}_2\]

Step 4: Sum the Equations

To find the sum of these reactions and show it equates to the decomposition of water, add together the equations from Steps 1, 2, and 3.\[\text{SO}_2 + 2\, \text{H}_2\text{O} + \text{I}_2 \rightarrow \text{H}_2\text{SO}_4 + 2\, \text{HI}\]\[\text{H}_2\text{SO}_4 \rightarrow \text{SO}_2 + \text{H}_2\text{O} + \frac{1}{2}\text{O}_2\]\[2\, \text{HI} \rightarrow \text{H}_2 + \text{I}_2\]Cancel out the intermediates (SO₂, I₂, H₂SO₄, and HI) present on both sides of the equations.\[2\, \text{H}_2\text{O} \rightarrow \text{H}_2 + \text{O}_2\]This confirms the overall reaction as the decomposition of water into hydrogen and oxygen.

Key Concepts

Balanced Chemical Equation

A balanced chemical equation represents the reactants and products in a chemical reaction with equal numbers of each type of atom on both sides. This is essential due to the law of conservation of mass, which states that matter cannot be created nor destroyed in a closed system.
To balance an equation, examine the number of each type of atom in both reactants and products and adjust coefficients (the numbers before molecules) accordingly. For example, to balance the decomposition of water into hydrogen and oxygen, you would showcase:

• 2 molecules of water (H₂O) on the reactant side
• leading to 2 molecules of hydrogen gas (H₂)
• and 1 molecule of oxygen gas (O₂) on the product side

Thus, the balanced equation becomes:\[2 \, \text{H}_2 \text{O} \rightarrow 2 \, \text{H}_2 + \text{O}_2\]By ensuring all equations are balanced, you're showing exactly what happens during a chemical reaction.

Sulfuric Acid Decomposition

Sulfuric acid decomposition is the process in which sulfuric acid (H₂SO₄) is broken down into its simpler components under the application of heat. In the context of the exercise, sulfuric acid decomposes into water (H₂O), sulfur dioxide (SO₂), and oxygen (O₂).The balanced chemical equation for this reaction is:\[\text{H}_2\text{SO}_4 \rightarrow \text{SO}_2 + \text{H}_2\text{O} + \frac{1}{2}\text{O}_2\]This equation shows that for every molecule of H₂SO₄ decomposed, it produces one molecule of SO₂, one molecule of H₂O, and half a molecule of O₂. When balancing a reaction such as this, ensuring that each type of atom is equal on both sides is crucial to accurately completing the chemical reaction.Some key points about sulfuric acid decomposition:

• Heat acts as the primary driving force to disrupt bonds in H₂SO₄.
• The products are simpler molecules than the reactant, indicating a step towards decomposition.
• This reaction is useful in processes requiring the production of oxygen or sulfur dioxide.

Hydrogen Iodide Decomposition

Hydrogen iodide (HI) decomposition refers to the breakdown of HI into hydrogen (H₂) and iodine (I₂) upon heating. In many applications, this reaction is essential for producing pure hydrogen gas.
The balanced equation for hydrogen iodide decomposition is straightforward:

• The given balanced equation is: \[2 \, \text{HI} \rightarrow \text{H}_2 + \text{I}_2\]
• This indicates that two molecules of HI break down to form one molecule of H₂ and one molecule of I₂.

What stands out in this equation is its simplicity: the equation is symmetric, meaning it balances easily without fractional coefficients.Key things to remember about this process:

• Heat is the necessary energy input to make the reaction occur.
• It's an example of a straightforward decomposition reaction.
• Produces elemental hydrogen and iodine, both valuable substances in various industrial applications.

Understanding this reaction enriches knowledge of simple decomposition reactions and their practical uses.