Question

"Jump-starting" a dead automobile battery can be dangerous if precautions are not taken, because of the production of an explosive mixture of _________ and _________ gases in the battery.

Short answer

The explosive mixture produced during the "jump-starting" of a dead automobile battery is formed by Hydrogen gas (H2) and Oxygen gas (O2). These gases are produced from the electrolysis of water in the lead-acid battery, and their reaction releases a large amount of energy, which can potentially cause an explosion if ignited.

Step-by-step solution

Understand the lead-acid battery chemistry

A lead-acid battery consists of lead (Pb) and lead dioxide (PbO2) plates submerged in a sulfuric acid (H2SO4) solution. The main reactions during discharge and charge of a lead-acid battery are as follows: Discharge: Pb (s) + PbO2 (s) + 2H2SO4 (aq) → 2PbSO4 (s) + 2H2O (l) Charge: 2PbSO4 (s) + 2H2O (l) → Pb (s) + PbO2 (s) + 2H2SO4 (aq) During the charging process, the battery may undergo water electrolysis, producing hydrogen gas (H2) and oxygen gas (O2). This process can be summarized in the following reactions: Anode (negative electrode): 2H2O (l) → 4H+ (aq) + 4e- + O2 (g) Cathode (positive electrode): 4H+ (aq) + 4e- → 2H2 (g)

Identify the explosive gases produced

From the reactions given in Step 1, we can now identify the gases produced during the charging process as: Hydrogen gas (H2) and Oxygen gas (O2). Both gases are produced from the electrolysis of water in the battery.

Explain the explosive nature of the mixture

When a battery is "jump-started", a rapid charging process takes place, which may result in a higher production of hydrogen and oxygen gases. When the concentration of these two gases is within the right proportions, they can mix to form an explosive mixture. The explosiveness comes from the reaction between hydrogen and oxygen gases, which can recombine to form water: 2H2 (g) + O2 (g) → 2H2O (l) This reaction releases a large amount of energy, which could potentially cause an explosion if ignited by a spark or a heat source during the "jump-starting" process. To sum up, we have identified the two gases that form an explosive mixture during the "jump-starting" of a dead automobile battery as Hydrogen gas (H2) and Oxygen gas (O2).

Key concepts

Electrolysis

Electrolysis is a fascinating process that involves using electricity to drive a non-spontaneous chemical reaction. In the context of a lead-acid battery, electrolysis occurs during the charging phase when electricity is passed through the battery's solution. This process splits water (\( H_2O \)) into hydrogen gas (\( H_2 \)) and oxygen gas (\( O_2 \)).

• The electrolyte in a typical lead-acid battery is sulfuric acid.
• As the battery charges, the electrical energy decomposes water into its elemental gases.
• Electrodes are placed in the electrolyte to facilitate this reaction: one serves as the anode and the other as the cathode.

Understanding electrolysis is key to grasping why explosive gases can form in a battery, as it lays the groundwork for separating water into volatile components.

Hydrogen Gas

Hydrogen gas is one of the lighter elements and happens to be a principal element produced during the electrolysis in a battery's charging process. In a lead-acid battery, this gas forms at the cathode, the positive part of the battery.

• It is represented by the symbol \( H_2 \) and comprises two hydrogen atoms bonded together.
• Hydrogen is a colorless, odorless gas that is highly flammable.
• In the presence of oxygen, it can lead to explosive reactions, which is why special precautions are necessary during charging.

This flammability and ability to produce explosions make hydrogen a key player in the dangers associated with "jump-starting" a battery improperly.

Oxygen Gas

Oxygen gas is another critical element involved in the electrolysis of water in lead-acid batteries. The gas is produced at the anode, the negative terminal of the battery.

• It's denoted by the chemical symbol \( O_2 \), consisting of two oxygen atoms bound together.
• Oxygen supports combustion, meaning it can cause fuels to burn more vigorously and rapidly.
• Like hydrogen, oxygen is colorless and odorless, making its presence hard to detect.

When combined with hydrogen in appropriate conditions, oxygen helps form an explosive atmosphere, so caution is advised around active batteries.

Chemical Reactions

Chemical reactions in a lead-acid battery involve the conversion of chemical energy into electrical energy and vice versa. These reactions are essential for the battery's operation during charging and discharging.

• During discharge: Lead dioxide and lead react with sulfuric acid to form lead sulfate and water.
• During charge: Lead sulfate and water revert to lead dioxide, lead, and sulfuric acid.
• The charging process can induce additional reactions like the decomposition of water into hydrogen and oxygen gases.

The chemical reactions during charging, specifically the formation of hydrogen and oxygen, can lead to explosive conditions if not carefully managed. Understanding these reactions provides a comprehensive view of the operational and safety dimensions of working with lead-acid batteries.