First Law of Thermodynamics: Videos & Practice Problems

The First Law of Thermodynamics, a fundamental concept in chemistry, states that energy cannot be created or destroyed, only transferred between a system and its surroundings. The system typically refers to the substance or chemical reaction under study, while the surroundings encompass everything else. Energy transfer can occur in two forms: heat ($Q$) and work ($W$). Heat is the flow of thermal energy from a higher temperature object to a lower temperature object, with the sign of $Q$ being positive when the system gains heat and negative when it loses heat. Work involves movement against gravity or an opposing force, with $W$ being negative when the system does work on the surroundings and positive when work is done on the system by the surroundings. Understanding the signs of $Q$ and $W$ is crucial for analyzing energy changes during 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 thermal energy to kinetic energy—it is a transfer of energy between the system and its surroundings. This concept reinforces the idea that energy is conserved in a closed system, merely shifting from one form to another rather than being lost or gained. Thus, the first law of thermodynamics emphasizes the conservation of energy and the dynamic interactions that occur in chemical processes.

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, which is the substance of interest, is classified as the system. This is the component undergoing analysis, specifically regarding its temperature change as it absorbs energy. In contrast, the water, which is in contact with the metal ore and provides the thermal environment, is considered the surroundings.

To determine the final temperature of the metal ore, we can apply the principle of conservation of energy, which states that the energy gained by the metal ore must equal the energy lost by the water. The energy gained by the metal ore is given as 19.11 kilojoules. The specific heat capacity of water is approximately 4.18 J/g°C, and the mass of the water is 615.5 grams. The equation used to calculate the heat transfer is:

Q = mcΔT

Where:

• Q is the heat energy (in joules),
• m is the mass (in grams),
• c is the specific heat capacity (in J/g°C),
• ΔT is the change in temperature (in °C).

For the water, the heat lost can be expressed as:

Q_water = m_water * c_water * (T_initial - T_final)

Substituting the known values:

Q_water = 615.5 g * 4.18 J/g°C * (42.18°C - T_final)

Since the energy gained by the metal ore equals the energy lost by the water, we can set up the equation:

19,110 J = 615.5 g * 4.18 J/g°C * (42.18°C - T_final)

By solving this equation, the final temperature of the metal ore can be determined, illustrating the energy transfer between the system and its surroundings.

First Law of Thermodynamics

To grasp the transfer of energy between systems and their surroundings, it's essential to differentiate between heat and work. Heat, denoted by the variable q, refers to the flow of thermal energy from an object at a higher temperature to one at a lower temperature. This process illustrates the natural tendency for energy to move from warmer to cooler areas, emphasizing the concept of thermal equilibrium.

On the other hand, work, represented by w, involves the movement of reacting molecules against an opposing force, such as gravity. When work is performed, energy is transferred as a result of this movement, indicating that energy is being expended to overcome resistance.

Understanding these two forms of energy transfer is crucial for analyzing how energy interacts within a system and its surroundings. The next step involves exploring how the signs of q and w change under various conditions, which will further clarify their roles in 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 thermodynamic processes. 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 transfers in thermodynamic systems.

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. This principle is fundamental in chemistry and physics. The system refers to the substance or chemical reaction being studied, while the surroundings include everything else. Energy transfer can occur in two forms: heat (Q) and work (W). Heat is the flow of thermal energy from a higher temperature object to a lower temperature object, and work involves movement against gravity or an opposing force. Understanding this law is crucial for analyzing energy changes during chemical reactions.

How is energy transferred between a system and its surroundings?

Energy transfer between a system and its surroundings occurs in two primary forms: heat (Q) and work (W). Heat represents the flow of thermal energy from a higher temperature object to a lower temperature object. When a system gains heat, Q is positive, and when it loses heat, Q is negative. Work involves the movement of reacting molecules against gravity or an opposing force. If the system does work on the surroundings, W is negative, and if the surroundings do work on the system, W is positive. These transfers are essential for understanding energy changes in chemical reactions.

What is the difference between heat and work in thermodynamics?

In thermodynamics, heat (Q) and work (W) are two forms of energy transfer. Heat is the flow of thermal energy from a higher temperature object to a lower temperature object. It is represented by the variable Q, with positive Q indicating heat gain and negative Q indicating heat loss. Work, represented by W, involves the movement of reacting molecules against gravity or an opposing force. When the system does work on the surroundings, W is negative, and when the surroundings do work on the system, W is positive. Both heat and work are crucial for analyzing energy changes in chemical reactions.

How do the signs of Q and W change under different circumstances?

The signs of Q (heat) and W (work) change based on the direction of energy transfer. For heat (Q), if the system gains heat, Q is positive, and if the system loses heat, Q is negative. For work (W), if the system does work on the surroundings, W is negative, and if the surroundings do work on the system, W is positive. These sign conventions help in understanding the energy changes during chemical reactions and are essential for applying the First Law of Thermodynamics.

What is the significance of the First Law of Thermodynamics in chemistry?

The First Law of Thermodynamics is significant in chemistry because it provides a fundamental understanding of energy conservation during chemical reactions. It states that energy cannot be created or destroyed, only transferred between a system and its surroundings. This principle helps chemists analyze how energy changes in the form of heat (Q) and work (W) affect chemical reactions. By understanding these energy transfers, chemists can predict reaction behavior, optimize reaction conditions, and develop more efficient chemical processes.