Question

When gasoline burns, it reacts with oxygen in the air and forms hot gases consisting of carbon dioxide and water vapor. How does the potential energy of the gasoline and oxygen compare with the potential energy of the carbon dioxide and water vapor?

Short answer

The potential energy of the gasoline and oxygen is higher than that of the carbon dioxide and water vapor; the difference in energy is released as thermal energy and work done by the system during combustion.

Step-by-step solution

Identify the Types of Energy Involved

When gasoline burns, chemical potential energy is converted into thermal energy (heat) and light. The chemical potential energy is stored in the molecular bonds of the gasoline and oxygen molecules.

Examine the Conservation of Energy Principle

According to the law of conservation of energy, energy cannot be created or destroyed in an isolated system. The potential chemical energy of the gasoline and oxygen before the reaction is equal to the sum of the energy of the products (carbon dioxide and water vapor), the thermal energy released, and any work done by the system.

Compare Potential Energy Before and After Combustion

The potential energy of the gasoline and oxygen is higher than the potential energy of the carbon dioxide and water vapor after the reaction. The difference in potential energy appears as the thermal energy and any work that is done by the expansion of the gases.

Key concepts

Chemical Potential Energy

Chemical potential energy refers to the stored energy within chemical substances, which can be released or absorbed during chemical reactions. This type of energy is bound within the bonds that hold atoms together in molecules.

In the context of gasoline combustion, the chemical potential energy is initially stored in the molecular bonds of gasoline and oxygen. When gasoline combusts, the bonds between the atoms are broken and new bonds are formed, creating carbon dioxide and water vapor. The process of breaking and forming bonds involves the transfer of energy. Energy is required to break the bonds of the reactants, and energy is released when new bonds are formed in the products.

It's crucial for students to understand that during combustion, while the chemical potential energy of the reactants is higher, it's primarily this energy that is transformed into other forms such as heat and light, sometimes causing confusion if one only considers the products' potential energy. The apparent 'loss' of potential energy is actually a transformation, consistent with the conservation of energy principle.

Conservation of Energy

The law of conservation of energy states that energy cannot be created or destroyed, only transformed from one form to another or transferred from one object to another, within an isolated system.

When examining the combustion of gasoline, this principle holds true. Before ignition, the chemical potential energy of gasoline and oxygen is all the energy that exists in the system. Upon combustion, this energy is neither lost nor gained but is conserved as it transitions into different forms—mainly heat energy that we can feel, light energy that we can see, and the kinetic energy of the expanding gas molecules.

To fully grasp the conservation of energy, students can imagine energy as currency. Just as money can be exchanged for various goods or services without disappearing, energy is 'spent' during chemical reactions to create different forms of energy, such as motion or heat, but the total 'amount' of energy remains constant in the system.

Enthalpy of Combustion

The enthalpy of combustion is a specific term in thermochemistry that refers to the heat released when one mole of a substance burns completely in oxygen. It's a crucial concept because it quantifies the amount of energy produced as heat during combustion.

For substances like gasoline, the enthalpy of combustion can be measured and is essential for calculating energy efficiency and the impact of various fuels on the environment. This figure tells us how much potential energy in the fuel is available for conversion to other forms of energy. Understanding the enthalpy of combustion helps in visualizing the energy landscape of the reactants and products – it's a snapshot of the heat change involved.

Students should note that even though the enthalpy of combustion is usually expressed as a negative number because it represents an exothermic reaction (one that releases heat), it is actually indicative of the quantity of energy that the fuel contained and could transfer into useful work or heat.

Thermochemistry

Thermochemistry is the branch of chemistry that deals with the relationship between chemical reactions and energy changes involving heat. It is concerned with the measurement and study of the heat evolved or absorbed in chemical processes.

In thermochemical terms, the burning of gasoline in an engine is a combustion reaction that is highly exothermic, releasing a large amount of heat. By studying the thermochemistry of this reaction, we can determine the heat (enthalpy) change of the system, predict the energy transferred as work and comprehend the energy efficiency of the process. For students, understanding the basics of thermochemistry is vital for practical applications like engine design, environmental assessments, and even in culinary science where heat alters the chemical makeup of food.

When considering a gasoline engine, thermochemistry helps explain how the high temperatures and pressures from the combustion process convert chemical energy into mechanical energy that powers the vehicle.