Reactivity 1.1.1 – Chemical reactions involve heat transfer
📌 Heat versus temperature
- Heat can be defined as a form of energy transfer occurring as a result of temperature difference. The transfer of heat to a system causes an increase in kinetic energy in molecules in the system.
- Temperature is defined as the measure of the average kinetic energy of these molecules.
📌 System and the surroundings
- The system is the area of ‘interest’ of a reaction while the surroundings are theoretically, everything else in the universe.
- There are 3 types of systems :
- Open systems : where energy and matter can be exchanged with the surroundings
- Closed systems : where energy can be exchanged with the surroundings but matter cannot
- Isolated systems : where neither energy nor matter can be exchanged with the surroundings
- The first law of thermodynamics states that energy cannot be created or destroyed, therefore the total energy cannot change during a reaction. However, heat can be exchanged between a system and the surroundings. The total heat content of a reaction is known as its ‘enthalpy’
📌 Enthalpy changes
- When a system transfers heat to its surroundings, the total enthalpy of the system decreases
- When a system gains heat from its surroundings, the total enthalpy of the system increases
- Changes in enthalpy are denoted by the sign Δ𝐻 (where 𝐻 is enthalpy)