(1) First Law of Thermodynamics, Basic Introduction - Internal Energy, Heat and Work - Chemistry

Introduction to the First Law of Thermodynamics

  • The first law states that energy cannot be created or destroyed; it can only be transferred between systems.

Energy Transfer Mechanisms

  • Energy can be transferred into or out of a system in two ways:

    • Heat (Q): When heat flows into a system, it increases the internal energy of that system.

    • Work (W): When work is performed on a system, it also increases the internal energy.

      • Example: If 300 joules of work is done on a system, its internal energy increases by 300 joules.

Conceptual Example

  • Analogy with Money:

    • Selling a laptop for $500 means your bank account increases by $500 while the buyer's account decreases by $500.

    • The concept of energy follows this model: energy is not created; it is transferred.

Different System Types

  • Open System:

    • Matter and energy can enter and exit.

  • Closed System:

    • Only energy can flow in and out; matter cannot.

  • Isolated System:

    • Neither matter nor energy can enter or leave; the total energy and mass remain constant.

Change in Internal Energy Equation

  • The formula for change in internal energy is:

    • ΔU = Q + W (Chemistry Perspective)

    • ΔU = Q - W (Physics Perspective)

Significance of Q and W

  • Q: Heat energy transfer

    • Positive when heat is absorbed (endothermic)

    • Negative when heat is released (exothermic)

  • W: Work done

    • Positive when work is done on the system

    • Negative when work is done by the system

Perspectives on Work

  • Chemistry Perspective:

    • From this viewpoint:

      • When work is done by the system, W is negative; internal energy decreases.

  • Physics Perspective:

    • From this viewpoint:

      • When work is done by the system, W is positive; surroundings gain energy.

Important Notes on System Energies

  • Q is positive for endothermic reactions (heat absorbed) and negative for exothermic (heat released).

  • W is negative when work is done by the system and positive for work done on the system.

Conclusion

  • Understanding the sign conventions of Q and W is essential for applying the first law of thermodynamics in practical scenarios, particularly when solving problems related to internal energy changes.