First Law of Thermodynamics: Videos & Practice Problems

The first law of thermodynamics states that energy cannot be created or destroyed, only transferred between a system, such as a chemical reaction, and its surroundings. Heat (q) flows from hot to cold, with positive q indicating heat gain and negative q indicating heat loss. Work (w) is the force exerted by reacting molecules; negative w occurs when the system does work on the surroundings, while positive w occurs when the surroundings do work on the system. Understanding these concepts is crucial for analyzing energy changes in chemical reactions.

First Law of Thermodynamics

The first law of thermodynamics is a fundamental principle stating that energy cannot be created or destroyed; rather, it is transferred between a system and its surroundings. In the context of chemistry, the system typically refers to the chemical reaction or substance being studied, while the surroundings encompass everything outside of that system, including the container and the environment.

For example, consider a container filled with gas molecules. In this scenario, the gas molecules represent the system under observation, while the container itself and everything external to it, such as the air and even observers, constitute the surroundings. This distinction is crucial for understanding how energy interacts within a chemical context.

When energy changes form—such as from kinetic energy of gas molecules to thermal energy—this transfer illustrates the first law of thermodynamics. It emphasizes that energy is conserved in a closed system, merely shifting from one type to another without any loss or gain in total energy. Thus, the key takeaway is that energy transformation is a continuous process that occurs between the system and its surroundings, reinforcing the idea of energy conservation in chemical reactions.

First Law of Thermodynamics Example 1

In this scenario, the chemist is investigating the thermal interaction between a metal ore and water. The metal ore, weighing 30 grams, is the primary focus of the study and is therefore classified as the system. The system is the part of the experiment that is being analyzed, which in this case is the metal ore itself.

The water, which weighs 615.5 grams and is initially at a temperature of 42.18 degrees Celsius, serves as the surroundings. The surroundings encompass everything outside the system that can exchange energy with it. In this case, the water is responsible for transferring heat to the metal ore, allowing the chemist to determine the final temperature of the metal.

To analyze the energy transfer, we can apply the principle of conservation of energy, which states that the heat gained by the metal ore must equal the heat lost by the water. The energy gained by the metal ore is given as 19.11 kilojoules. This energy transfer can be expressed mathematically as:

Q_metal = -Q_water

Where:

• Q_metal is the heat gained by the metal ore (19.11 kJ).
• Q_water is the heat lost by the water, which can be calculated using the formula:

Q = mcΔT

In this formula:

• m is the mass of the water (615.5 g),
• c is the specific heat capacity of water (approximately 4.18 J/g°C),
• ΔT is the change in temperature of the water.

By determining the change in temperature of the water, the final temperature of the metal ore can be calculated, allowing the chemist to complete the analysis of the thermal interaction between the two substances.

First Law of Thermodynamics

To grasp the concept of energy transfer between systems and their surroundings, it is essential to differentiate between heat and work. Heat, denoted by the variable q, refers to the transfer of thermal energy from an object at a higher temperature to one at a lower temperature. This process occurs naturally, as thermal energy flows from hot to cold objects.

On the other hand, work, represented by the variable w, involves the movement of molecules against an opposing force, such as gravity. When a system performs work, it requires energy to overcome this force, indicating that energy is being transferred in the process.

Understanding these two forms of energy transfer is crucial for analyzing how energy moves between systems and their surroundings. The next step involves exploring how the signs of q and w change under various conditions, which will further illuminate the dynamics of energy transfer.

First Law of Thermodynamics

Heat is defined as the flow of thermal energy from a hotter object to a colder one. In any heat transfer scenario, the hotter object loses heat while the colder object gains it. For instance, if we have two spheres, one at a higher temperature and the other at a lower temperature, heat will naturally move from the hotter sphere to the colder sphere. In this context, if a system loses heat, the heat transfer is represented by a negative sign (q < 0), indicating that it is evolving, releasing, or giving off heat. Conversely, if a system gains heat, it is represented by a positive sign (q > 0), indicating that it is absorbing or taking in heat. Thus, the sign of q is determined by whether the system is gaining or losing heat.

Work, on the other hand, is defined as the force exerted by reacting molecules on a frictionless piston. For example, if gas molecules in a container push against a piston to expand, they are doing work on the piston, which results in a negative work value (w < 0) because the system is exerting force on the surroundings. Conversely, if an external force compresses the gas by pushing down on the piston, the surroundings are doing work on the system, resulting in a positive work value (w > 0) since the system is not exerting force against an opposing force.

In summary, the signs of heat (q) and work (w) are crucial for understanding energy transfer in thermodynamic systems. A system that gains heat has a positive q, while one that loses heat has a negative q. Similarly, if the system does work on the surroundings, it is negative work, while if the surroundings do work on the system, it is positive work. This understanding is essential for analyzing energy changes in various physical and chemical processes.

What are the signs of q and w when a system loses heat while doing work on the surroundings?

• A. q = – , w = –
• B. q = + , w = +
• C. q = – , w = +
• D. q = + , w = –

Answer: (A)

Here’s what students ask on this topic:

What is the first law of thermodynamics?

The first law of thermodynamics states that energy cannot be created or destroyed, only transferred between a system and its surroundings. In other words, the total energy of an isolated system remains constant. This principle is also known as the law of energy conservation. In a chemical context, the system refers to the chemical reaction or substance being studied, while the surroundings include everything else. Energy can be transferred in the form of heat (q) or work (w), but the total energy change of the system and surroundings combined is always zero.

How does heat transfer between systems and surroundings?

Heat transfer between systems and surroundings occurs from a higher temperature object to a lower temperature object. This flow of thermal energy is represented by the variable q. If a system loses heat, it has a negative q, indicating heat is being released to the surroundings. Conversely, if a system gains heat, it has a positive q, indicating heat is being absorbed from the surroundings. The direction of heat flow is always from hot to cold, following the natural tendency to reach thermal equilibrium.

What is the difference between heat (q) and work (w) in thermodynamics?

In thermodynamics, heat (q) and work (w) are two forms of energy transfer between a system and its surroundings. Heat represents the flow of thermal energy from a higher temperature object to a lower temperature object. Work, on the other hand, is the energy transfer that occurs when a force is applied over a distance. Work is done by the system when it exerts a force on its surroundings, resulting in a negative w. Conversely, work is done on the system by the surroundings, resulting in a positive w. Both heat and work contribute to the overall energy change in a system.

How do the signs of q and w change under different circumstances?

The signs of q and w change based on the direction of energy transfer. For heat (q), if the system gains heat, q is positive, indicating heat absorption. If the system loses heat, q is negative, indicating heat release. For work (w), if the system does work on the surroundings, w is negative, indicating energy expenditure. If the surroundings do work on the system, w is positive, indicating energy gain. These sign conventions help in analyzing energy changes in chemical reactions and other thermodynamic processes.

What is the significance of the first law of thermodynamics in chemical reactions?

The first law of thermodynamics is significant in chemical reactions because it helps in understanding and quantifying energy changes. During a chemical reaction, energy can be transferred as heat or work between the system (the reacting substances) and the surroundings. By applying the first law, we can determine the energy balance and predict the behavior of the reaction. This principle is crucial for calculating enthalpy changes, understanding reaction spontaneity, and designing energy-efficient processes in chemistry and engineering.