Question

The "Chemistry in Focus" segment Water-Powered Fireplace discusses a fireplace that uses the electrolysis of water to produce hydrogen gas. Write the balanced chemical equation for the electrolysis of water. Which element in water is oxidized? Which is reduced? Find the oxidation states to answer these questions. Also, how is heat generated by the fireplace?

Short answer

The balanced chemical equation for the electrolysis of water is \(2H_2O (l) \rightarrow 2H_2 (g) + O_2 (g)\). In this reaction, hydrogen is oxidized (oxidation state: +1 to 0) and oxygen is reduced (oxidation state: -2 to 0). The heat generated by the water-powered fireplace mainly comes from the exothermic combustion of hydrogen gas, which reacts with oxygen in the air to form water (H2O) and release energy as heat: \(2H_2 (g) + O_2 (g) \rightarrow 2H_2O (l)\).

Step-by-step solution

Writing the Balanced Chemical Equation for the Electrolysis of Water

Electrolysis of water involves the splitting of water molecules (H2O) into hydrogen gas (H2) and oxygen gas (O2). To form the balanced chemical equation, we need to consider the conservation of mass of each element involved. In water, there are 2 hydrogen atoms and 1 oxygen atom in each molecule. By splitting the water molecules, we can form hydrogen gas (H2) and oxygen gas (O2). The balanced chemical equation for the electrolysis of water is: \(2H_2O (l) \rightarrow 2H_2 (g) + O_2 (g)\)

Finding the Oxidation States of Elements in Water and Products

To identify the element that is oxidized and the one that is reduced, we need to determine the oxidation states of each element in the water molecule and the products. In the water molecule H2O: Oxidation state of hydrogen (H) = +1 Oxidation state of oxygen (O) = -2 In the hydrogen gas molecule H2: Oxidation state of hydrogen (H) = 0 In the oxygen gas molecule O2: Oxidation state of oxygen (O) = 0

Identifying Oxidation and Reduction

Now that we have determined the oxidation states of each of the elements, we can identify which element is oxidized and which is reduced. Oxidation occurs when the oxidation state of an element increases. In this reaction: Hydrogen's oxidation state goes from +1 in H2O to 0 in H2. Reduction occurs when the oxidation state of an element decreases. In this reaction: Oxygen's oxidation state goes from -2 in H2O to 0 in O2. Thus, hydrogen is the element that is oxidized and oxygen is the element that is reduced during the electrolysis of water.

Explaining Heat Generation in the Fireplace

The heat generated by the water-powered fireplace is primarily due to the combustion of hydrogen gas. When hydrogen gas reacts with oxygen in the air, it forms water (H2O) and releases energy as heat: \( 2H_2 (g) + O_2 (g) \rightarrow 2H_2O (l)\) This reaction exothermic (releases energy), which gives off heat and warms the surrounding area. The energy produced in this process fuels the water-powered fireplace.

Key concepts

Balanced Chemical Equation

A balanced chemical equation ensures that the number of atoms for each element is the same on both sides of the equation. This complies with the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction.
For the electrolysis of water, we need to break down water molecules into hydrogen gas and oxygen gas. A water molecule, represented as \(H_2O\), consists of two hydrogen atoms and one oxygen atom.

• On the left side, we have water: \(H_2O,\) so we start with its molecules.
• During electrolysis, water splits into hydrogen \( (H_2)\) and oxygen \( (O_2)\) gases.
• In the balanced equation, we want to ensure equal atoms on both sides.

The balanced chemical equation for this process is:\[2H_2O (l) \rightarrow 2H_2 (g) + O_2 (g)\] This equation tells us that two molecules of liquid water decompose into two molecules of hydrogen gas and one molecule of oxygen gas.

Oxidation States

Oxidation states, also known as oxidation numbers, help in understanding electron transfer in a chemical reaction. These numbers can be thought of as the theoretical electrical charges that atoms would have if electrons were transferred completely, not shared.
In a water molecule (\(H_2O\)), the oxidation state of hydrogen is \(+1\), and oxygen is \(-2\). This reflects the usual charges: hydrogen tends to lose an electron (\(+1\)) and oxygen tends to gain electrons (\(-2\)).

• In hydrogen gas \((H_2)\), hydrogen has an oxidation state of \(0\) because the electrons are equally shared between the two atoms.
• In oxygen gas \((O_2)\), oxygen's oxidation state is also \(0\) due to equal electron sharing between oxygen atoms.

These states are crucial for determining how electrons shift during reactions, especially in redox reactions. Understanding oxidation states helps identify changes occurring during electrolysis or other chemical processes.

Oxidation and Reduction

Oxidation and reduction are parts of a chemical process where electron transfer occurs. These are often referred to as "redox" reactions because they happen simultaneously. For any element to be oxidized, another must be reduced.
In the electrolysis of water:

• **Oxidation** occurs when an element loses electrons, resulting in an increase in its oxidation state. In this process, hydrogen in water \((H_2O)\) is oxidized because its oxidation state changes from \(+1\) to \(0\) as it forms hydrogen gas \((H_2)\).
• **Reduction** occurs when an element gains electrons, resulting in a decrease in its oxidation state. Here, oxygen in water is reduced because its oxidation state changes from \(-2\) in \(H_2O\) to \(0\) in \(O_2\).

Understanding oxidation and reduction is critical for electrochemical processes, which involve electron transfers, affecting energy production and transformation.

Combustion of Hydrogen

The combustion of hydrogen is a chemical reaction that releases heat energy. It is described by the reaction of hydrogen gas with oxygen to produce water, releasing energy in the form of heat, making it an exothermic process.
When hydrogen gas \((H_2)\) combusts with oxygen \((O_2)\), it forms water \((H_2O)\):\[2H_2 (g) + O_2 (g) \rightarrow 2H_2O (l)\]In this reaction:

• Hydrogen and oxygen molecules react energetically, breaking and forming bonds.
• This results in a stable product (water) and the release of heat.
• "Exothermic" denotes the reaction gives off energy, often noticeable as heat.

This principle powers the "water-powered fireplace," where the heat from combusting generated hydrogen warms the immediate environment.