Chemical potential energy
Experimental observations reveal that many (if not all) chemical and physical changes proceed with a corresponding change in the temperature of the reacting system. If the temperature changes, then energy is either appearing as particle motion, or disappearing due to reduction in particle motion.
As the law of conservation of energy states that 'energy cannot be created (or destroyed)' then it is apparent that the energy must be coming from (or going to) some other form. This other form of energy must be inherent in the particles of substances and for this reason it is given the name chemical energy, or chemical potential energy.
When a reacting mixture increases its temperature, energy is being transformed from chemical potential energy to kinetic energy of the particles. For simplicity's sake we usually refer to the kinetic/motion energy of the particles as the heat energy.
If the final temperature is higher than the initial temperature then energy has been transformed from chemical potential energy to heat energy. The TOTAL energy of the system remains constant (providing the reaction system is isolated and nothing is allowed to enter or leave). The total energy of a systems is referred to as the Internal energy of the system, it is given the symbol 'U'.
|Total energy (internal energy) = chemical energy + heat energy|
Consequently, if the chemical potential energy goes down, the heat energy must go up. If the chemical potential energy goes up the heat energy must go down.
Reactions that transform chemical to heat energy are called EXOthermic, and reactions that transform heat energy into chemical potential energy are called ENDOthermic.
Chemical elements and compounds have chemical potential energy. This energy is due to its nature and position in the universe.
REM However, chemicals DO NOT 'contain' energy in the same way as a bottle contains water, and they cannot release energy by being broken - this is one of the most common misconceptions of students at this level. Chemicals have the potential to cause an increase (or decrease) in the heat energy of the particles in the local environment through reaction.
Chemicals, then, have chemical potential energy that can be turned into heat energy. The chemical potential energy that is involved in any process of transformation is called the enthalpy, H.
|The enthalpy change, ΔH, is defined as the change in heat energy at constant pressure.|
Hence, in a reaction in which chemical energy turns into heat energy, the extra heat energy is due to a decrease in the chemical energy. As the enthalpy has decreased, this change is given a negative value by convention, ΔH = -. The opposite change is from heat energy to chemical energy. In this case, the enthalpy has increased and the change is given a positive sign, ΔH = +.
- Chemical potential energy is energy possessed by a substance as a consequence of its nature.
- Heat energy is the kinetic motion of the particles in the system.
- Enthalpy is that part of the chemical potential energy that can be transformed into heat energy, or vice versa.
Reactions involving volume changes
Not required for the IB exams
If a chemical reaction produces a gas, which is allowed to expand against a pressure, it uses up energy in the form of work.The definition of work done by gases against a pressure, P,:
Work done = PΔV, where ΔV is the change in gas volume.
Hence the total enthalpy change is affected by the work done.
ΔH = ΔU + PΔV
When dealing with solids and liquids, volume changes are negligible and ΔH = ΔU. This is useful, as most chemical reactions are carried out in open vessels under atmospheric pressure.