Experience tells us that the majority of chemical processes are exothermic. According to the law of conservation of energy, this means that the chemical potential energy must decrease in such processes. Hence, the products have less chemical potential energy than the reactants.
Syllabus reference R1.1.3Reactivity 1.1.3 - The relative stability of reactants and products determines whether reactions are endothermic or exothermic.
- Understand the difference between heat and temperature.
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- Structure 1.1 - What is the relationship between temperature and kinetic energy of particles?
Chemical potential energy
This is potential energy in that it is a function of the nature of the substance and its position with respect to all other substances. It is impossible to measure in absolute terms. We could draw an analogy with gravitational potential energy. When we lift a book of mass "m" above a table to a height of 1m above the table we say that it has a gravitational potential energy of m x 9.81 x 1.
But, if we then remove the table without touching the book (it's still in the same elevated position) then its gravitational potential energy increases.
The frame of reference is important. Are we measuring GPE with respect to the table, or the floor? The values are of course different. Chemical potential energy is similar in that it cannot be measured absolutely, but it can be measured with respect to a reference point. More of this later.
Stability
The concept of stability arises in many areas of chemistry, but what does it mean? In everyday language, stability refers to an object's resistance to change, its reliability. If we are talking about a person we mean that he/she is unlikely to react in an unexpected fashion.
The leaning tower of Pisa appears to be unstable - it looks like it could fall at any time.
The concept of stability is extended in science to refer to a substance's resistance to change when subject to external influences. When a substance is chemical stable, it resists chemical change. The more stable a substance is the less reactive it is.
The inert or noble gases, helium, neon, argon, krypton, xenon are all very stable. They have little or no tendency to form compounds - they are resistant to change.
Exothermic and endothermic processes
In exothermic processes, the chemical potential energy must decrease as heat energy is lost to the surroundings. Hence, the products have less chemical potential energy than the reactants.
In endothermic processes, the chemical potential energy must increase as heat energy is lost from the surroundings. Hence, the products have more chemical potential energy than the reactants.
This is shown graphically below:
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In an exothermic process the products have less potential energy than the reactants. We say that the products are more stable than the reactants. The reverse is true for an endothermic reaction - the products are less stable than the reactants.
Although all of the above discussion has focussed on chemical change, exactly the same arguments apply to physical processes. When a kettle boils, the water turns from liquid to gas. As it is an endothermic process, the steam formed has more chemical potential energy than the water.