Question

Gasoline can be thought of as having the formula \(\mathrm{C}_{8} \mathrm{H}_{18}\). When gasoline is used in a car's engine, it reacts with oxygen \(\left(\mathrm{O}_{2}\right)\). If you look closely at the exhaust pipe of a running car, you will see water dripping out of it. Where did this water come form (what kind of change is involved)?

Short answer

The water observed dripping from a car's exhaust pipe comes from the combustion of gasoline (\(\mathrm{C}_{8}\mathrm{H}_{18}\)) with oxygen (\(\mathrm{O}_{2}\)) in the engine. The balanced chemical equation for this combustion reaction is \(\mathrm{C}_{8}\mathrm{H}_{18} + \frac{25}{2}\mathrm{O}_{2} \rightarrow 8\mathrm{CO}_2 + 9\mathrm{H}_{2}\mathrm{O}\), which shows that water (\(\mathrm{H}_{2}\mathrm{O}\)) is one of the products. This process is a chemical change, as new substances with different properties than the reactants are formed.

Step-by-step solution

In this step, we identify the chemical reaction happening in a car's engine when gasoline, \(\mathrm{C}_{8}\mathrm{H}_{18}\), reacts with oxygen, \(\mathrm{O}_{2}\). This type of reaction is called a combustion reaction, where a hydrocarbon (like gasoline) reacts with oxygen, producing carbon dioxide \(\left(\mathrm{CO}_{2}\right)\) and water \(\left(\mathrm{H}_{2}\mathrm{O}\right)\) as the products. #Step 2: Write the balanced chemical equation for the combustion of gasoline#

For a combustion reaction involving gasoline and oxygen, we need to balance the chemical equation to ensure that the number of atoms for each element on the reactant side matches the number of atoms for the same element on the product side. The balanced chemical equation is given by: \[ \mathrm{C}_{8}\mathrm{H}_{18} + \frac{25}{2}\mathrm{O}_{2} \rightarrow 8\mathrm{CO}_2 + 9\mathrm{H}_{2}\mathrm{O} \] This equation shows that one molecule of gasoline reacts with 12.5 molecules of oxygen to produce 8 molecules of carbon dioxide and 9 molecules of water. #Step 3: Identify the source of water in the exhaust pipe#

It can be observed from the balanced chemical equation in Step 2 that water is one of the products of the combustion reaction between gasoline and oxygen. This means that the water observed dripping from the exhaust pipe of a car comes from the combustion of gasoline in the engine. #Step 4: Describe the chemical change involved#

The chemical reaction that occurs during the combustion of gasoline in a car's engine involves breaking the chemical bonds in the gasoline molecules and oxygen molecules and forming new bonds to produce carbon dioxide and water molecules. This process is known as a chemical change, as new substances with different properties than the reactants are formed.

Key concepts

Balanced Chemical Equation

Understanding a balanced chemical equation is crucial in chemistry, especially for reactions like the combustion of gasoline. When writing a balanced equation, it is essential to ensure that the number of atoms for each element is the same on both the reactants' and products' sides. This obeys the Law of Conservation of Mass, which states that matter cannot be created or destroyed in an isolated system.

In the case of gasoline combustion, the balanced equation is \[\mathrm{C}_{8}\mathrm{H}_{18} + \frac{25}{2}\mathrm{O}_{2} \rightarrow 8\mathrm{CO}_2 + 9\mathrm{H}_{2}\mathrm{O}\]
This indicates that each carbon (\(\mathrm{C}\)) and hydrogen (\(\mathrm{H}\)) atom from the gasoline molecules pairs with oxygen (\(\mathrm{O}_{2}\)) to produce carbon dioxide and water. Varying the coefficients systematically while maintaining the balance is a methodical process called balancing the equation.

Chemical Change

Chemical change is a fundamental concept in chemistry that describes the process in which one or more substances convert into new substances with distinct physical and chemical properties. The transformation during a chemical change involves the breaking of old bonds and the creation of new ones.

For example, in a car engine, the chemical change occurs when gasoline, composed of carbon and hydrogen, combines with oxygen, leading to the production of carbon dioxide and water:\[\mathrm{C}_{8}\mathrm{H}_{18} + \frac{25}{2}\mathrm{O}_{2} \rightarrow 8\mathrm{CO}_2 + 9\mathrm{H}_{2}\mathrm{O}\]
This is a classic example of a combustion reaction, where fuel (a hydrocarbon) and oxygen result in new products with properties that are entirely different from those of the reactants.

Hydrocarbon Combustion

Hydrocarbon combustion is a specific type of chemical reaction where a hydrocarbon, such as gasoline \(\mathrm{C}_{8}\mathrm{H}_{18}\), reacts with oxygen \(\mathrm{O}_{2}\) to produce carbon dioxide \(\mathrm{CO}_{2}\) and water \(\mathrm{H}_{2}\mathrm{O}\). It is an exothermic process, meaning it releases energy in the form of heat and light, which is essential for powering engines.

The combustion of hydrocarbons is a critical reaction in the automotive industry, as it is how cars generate the power needed to move. This process is highly efficient but also raises concerns about carbon dioxide emissions, contributing to greenhouse gases and global warming. The equation below depicts the complete combustion reaction:
\[\mathrm{C}_{8}\mathrm{H}_{18} + \frac{25}{2}\mathrm{O}_{2} \rightarrow 8\mathrm{CO}_2 + 9\mathrm{H}_{2}\mathrm{O}\]
The water visible at the car's exhaust, a byproduct of this reaction, is indicative of the hydrocarbon's complete combustion.

Stoichiometry

Stoichiometry is an area of chemistry that involves the calculation of reactants and products in chemical reactions. It's foundational for understanding how much of each substance is needed and what quantity of each product will be formed.

Using the balanced equation for the combustion of gasoline, stoichiometry allows us to determine the exact amounts of oxygen needed to react with a certain amount of gasoline, and the quantities of carbon dioxide and water produced:\[\mathrm{C}_{8}\mathrm{H}_{18} + \frac{25}{2}\mathrm{O}_{2} \rightarrow 8\mathrm{CO}_2 + 9\mathrm{H}_{2}\mathrm{O}\]

This stoichiometric balance is crucial not only for theoretical calculations but also for practical applications such as designing engine fuel systems, controlling emissions, and understanding environmental impacts of combustion processes.