Problem 17 - Chemistry Matter and Change

Question

Describe the half-reactions that occur in a hydrogen fuel cell, and write the equation for the overall reaction.

Step-by-Step Solution

Answer

In a hydrogen fuel cell, the half-reactions occurring are the oxidation of hydrogen gas at the anode producing protons and electrons: \(2H_{2(g)} \rightarrow 4H^{+} _{(aq)} + 4e^- \) and the reduction of oxygen gas at the cathode to form water: \(O_{2(g)} + 4H^{+} _{(aq)} + 4e^- \rightarrow 2H_{2}O_{(l)} \) By combining these half-reactions, we obtain the overall reaction for the hydrogen fuel cell: \(2H_{2(g)} + O_{2(g)} \rightarrow 2H_{2}O_{(l)} \)

Step 1: 1. Half-reaction at the anode

In a hydrogen fuel cell, hydrogen gas (H2) is supplied at the anode. The half-reaction that occurs at the anode is the oxidation of hydrogen gas to produce protons (H+) and electrons (e-). Here is the half-reaction at the anode: \(2H_{2(g)} \rightarrow 4H^{+} _{(aq)} + 4e^- \)

Step 2: 2. Half-reaction at the cathode

Oxygen gas (O2) is supplied at the cathode in a hydrogen fuel cell. The half-reaction that occurs at the cathode is the reduction of oxygen gas by accepting electrons (e-) and protons (H+) to form water (H2O). Here is the half-reaction at the cathode: \(O_{2(g)} + 4H^{+} _{(aq)} + 4e^- \rightarrow 2H_{2}O_{(l)} \)

Step 3: 3. Equation for the overall reaction

To find the overall equation for the reaction in a hydrogen fuel cell, we need to combine the half-reactions at the anode and cathode. We can do this by adding the half-reactions together while cancelling out the ions and electrons that appear on both sides of the equation. Here is the equation for the overall reaction: \(2H_{2(g)} + O_{2(g)} \rightarrow 2H_{2}O_{(l)} \) In summary, the two half-reactions in a hydrogen fuel cell are the oxidation of hydrogen gas at the anode and the reduction of oxygen gas at the cathode. The overall reaction involves the conversion of hydrogen and oxygen gas into water, with the release of electrical energy during the process.

Key Concepts

Understanding Half-Reactions

In electrochemical cells like hydrogen fuel cells, the overall reaction is split into two simpler reactions called half-reactions. Each half-reaction takes place at a different electrode—one at the anode and the other at the cathode. This separation helps in understanding the flow of electrons and the transformation of molecules.

• **Anode Reaction:** At the anode of a hydrogen fuel cell, hydrogen gas is oxidized. This means hydrogen molecules lose electrons to become positively charged protons.
• **Cathode Reaction:** At the cathode, oxygen gas is reduced. This process involves oxygen molecules gaining electrons and combining with protons to form water.

By understanding half-reactions, one can see how reactions are broken down and how electrons move, which is crucial in studying electrochemical processes.

Exploring Oxidation

Oxidation is a chemical process where a molecule loses electrons. In a hydrogen fuel cell, this process occurs at the anode. Hydrogen gas undergoes oxidation as it loses electrons and forms protons.

• The reaction can be represented as: \[2H_{2(g)} \rightarrow 4H^{+} _{(aq)} + 4e^-\]
• Hydrogen gas splits into hydrogen ions and free electrons in this step.
• This release of electrons is what creates the electric current needed to power devices.

Understanding oxidation helps in identifying how energy is generated in fuel cells through the movement of electrons.

Understanding Reduction

Reduction is the opposite of oxidation. It involves the gain of electrons. In the hydrogen fuel cell, reduction occurs at the cathode. Oxygen gas accepts electrons and protons to form water.

• Reduction reaction is expressed as: \[O_{2(g)} + 4H^{+} _{(aq)} + 4e^- \rightarrow 2H_{2}O_{(l)}\]
• This reaction uses the electrons provided by the anode's oxidation process.
• The formation of water is a key part of completing the fuel cell’s operation.

Grasping reduction is essential as it highlights the balance of electron flow in chemical reactions within fuel cells.

Introduction to Electrochemistry in Fuel Cells

Electrochemistry studies the relationship between chemical changes and electrical energy. In hydrogen fuel cells, electrochemistry turns chemical energy from reactions into electrical energy.

• **Role of Fuel Cells:** They generate electricity through chemical reactions, not by burning fuel.
• **Electron Flow:** Electrons travel from the anode to the cathode, creating an electric current.
• **Overall Reaction:** Combining half-reactions gives the full picture of the fuel's conversion into energy, represented as: \[2H_{2(g)} + O_{2(g)} \rightarrow 2H_{2}O_{(l)}\]

Understanding electrochemistry is vital to appreciate how different components contribute to the efficient functioning of fuel cells, producing clean energy.