First law of thermodynamics

Definition

The first law of thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another. This principle highlights the conservation of energy within a closed system, connecting changes in internal energy to heat transfer and work done during chemical reactions.

5 Must Know Facts For Your Next Test

1. The first law is mathematically expressed as $$\Delta U = Q - W$$, where $$\Delta U$$ is the change in internal energy, $$Q$$ is the heat added to the system, and $$W$$ is the work done by the system.
2. In an isolated system, the total energy remains constant, meaning any increase in internal energy must be balanced by a decrease in the energy of the surroundings.
3. During exothermic reactions, heat is released to the surroundings, which decreases the internal energy of the system, while endothermic reactions absorb heat, increasing internal energy.
4. The first law helps explain why enthalpy changes can indicate whether a reaction is endothermic or exothermic based on heat flow.
5. Heat capacity and specific heat are related concepts that describe how much heat a substance can absorb per unit temperature change, which plays a crucial role in applying the first law.

Review Questions

• How does the first law of thermodynamics apply to chemical reactions in terms of energy transformation?
  • The first law of thermodynamics emphasizes that during chemical reactions, the total energy of a closed system remains constant. This means that any energy released or absorbed during the reaction must be accounted for as either work done or heat transferred. For instance, in an exothermic reaction, energy is released as heat, causing a decrease in the system's internal energy while transferring that energy to the surroundings.
• Compare and contrast how the first law of thermodynamics applies to both endothermic and exothermic processes.
  • In exothermic processes, the first law indicates that heat is released into the surroundings, leading to a decrease in internal energy of the system. Conversely, in endothermic processes, heat is absorbed from the surroundings, resulting in an increase in internal energy. Both processes adhere to the conservation of energy principle outlined by the first law; thus, any change in internal energy is directly linked to heat flow and work done.
• Evaluate how understanding the first law of thermodynamics enhances our comprehension of real-world systems such as engines or refrigerators.
  • Understanding the first law of thermodynamics is crucial when analyzing real-world systems like engines and refrigerators because it helps explain how these devices transfer and convert energy. In an engine, chemical energy from fuel is transformed into mechanical work and heat according to this law. Similarly, refrigerators use work to remove heat from a cooler area and release it into a warmer one. This relationship between work and heat transfer showcases how efficiently these systems operate while adhering to the principle of energy conservation described by the first law.

Related terms

internal energy:

The total energy contained within a system, including kinetic and potential energies of its molecules.

enthalpy: A thermodynamic property that represents the total heat content of a system, often used to calculate heat changes during reactions at constant pressure.

system: A specific portion of matter chosen for analysis, separated from its surroundings by a boundary.