Energy is defined as the ability to do work, and work is defined as the application of a force over a distance. When energy is transferred from one system to another, some energy is always "lost" as heat. Chemical processes and reactions always involve a transfer of energy. Reactions or processes that require energy, or heat, are endothermic, and those that release heat are exothermic. An exothermic reaction will cause a temperature rise in the surroundings, and an endothermic reaction will cause a temperature drop. The temperature change of a reaction or process can be measured using a calorimeter. This temperature change and the heat capacity of the materials can be used to calculate the energy change of the process.
At A Glance
Energy can take many forms and is always conserved. Potential energy is the energy due to position. Kinetic energy is the energy due to motion. In many systems, potential and kinetic energy are converted to each other.
Energy transferred over a distance is work; the energy of molecular motion is thermal energy. The transfer of thermal energy is called heat.
Thermodynamic work done by or on a gas is equal to −PoutΔV.
- Energy changes during reactions are described by thermochemistry. An exothermic process releases heat and raises the temperature of the surroundings. An endothermic process absorbs heat and lowers the temperature of the surroundings.
- The temperature change in the surroundings can be used to calculate the energy change of a chemical process. These changes are described by calorimetry.
- Chemical thermodynamics is the study of the flow of energy during chemical reactions or phase changes between the system and the surroundings.
Internal energy is the sum of the kinetic and potential energies of the particles in a system.
- The change in the internal energy of a system (U) is equal to heat in or out of a system (q) plus the work done on or by the system (w): ΔU=q+w.