Problem 9 - Chemistry An Atoms First Approach

Question

You may have noticed that water sometimes drips from the exhaust of a car as it is running. Is this evidence that there is at least a small amount of water originally present in the gasoline? Explain.

Step-by-Step Solution

Answer

The presence of water dripping from a car's exhaust is not evidence of water originally present in the gasoline. The water is produced as a byproduct of the combustion reaction between the hydrocarbons in gasoline and oxygen from the air. The combustion reaction is represented as: \(C_n H_{2n+2} + \frac{3n+1}{2} O_2 \rightarrow n CO_2 + (n+1) H_2O\) The water produced during combustion is initially in a gaseous state due to high temperatures in the engine. As the exhaust gases cool down, the gaseous water condenses into liquid water, which drips from the exhaust pipe when the car is running.

Step 1: Understand the composition of gasoline

Gasoline is a mixture of hydrocarbons, which are molecules containing hydrogen and carbon atoms. There may be trace amounts of other substances present in gasoline, such as additives, but water is not one of them.

Step 2: Analyze the combustion process in a car engine

When gasoline is burned in a car engine, it undergoes a combustion reaction with oxygen. This process releases energy and produces carbon dioxide (CO2) and water (H2O) as byproducts. The chemical reaction is as follows: CnH2n+2 + (3n+1)/2 O2 → n CO2 + (n+1) H2O where n represents the number of carbon atoms in the hydrocarbon molecule.

Step 3: Identify the source of water in the exhaust

As mentioned earlier, water is a byproduct of the combustion process. Due to the high temperatures inside the engine, the water produced during combustion is in a gaseous state. When the exhaust gases cool down in the exhaust pipe, the gaseous water condenses into liquid water. This is the water that drips from the exhaust pipe when the car is running.

Step 4: Conclusion

The presence of water dripping from a car's exhaust is not evidence of water originally present in the gasoline. The water is produced as a result of the combustion reaction between the hydrocarbons in gasoline and oxygen from the air. This reaction produces carbon dioxide and water as byproducts, and the latter condenses into liquid water as the exhaust gases cool down.