A2-Level Enthalpy and Entropy
Enthalpy change and entropy change determines how likely a reaction is to happen.
Enthalpy change is the change in thermal energy that occurs during a reaction.
Entropy change refers to the change in disorder of a system that occurs during a reaction.
Entropy is a measure of disorder within in a chemical system.
If there a more ways of arranging particles in the system, it has a higher entropy than if there are fewer ways of arranging the particles.
Solids have low entropies and gases have high entropies. Going from solid to gas increases the entropy of a system as there are now more ways of arranging the particles.
In chemistry, energy is involved in reactions. We know from basic kinetic theory that in order for particles to react, they have to collide with the required activation energy. The products formed are either at a lower or higher energy than the reactants.
RECAP. If the products of a reaction are at a lower energy than the reactants, the reaction is exothermic and the overall enthalpy change for the reaction will be negative (-∆H). If the products of a reaction are at a higher energy than the reactants, the reaction is endothermic and the overall enthalpy change for the reaction be positive (+∆H).
In nature, exothermic reactions are generally preferred as they release energy. Nature is lazy and reactions that do not require an overall input of energy are more likely to occur than those that do.
Endothermic reactions can and do occur in chemistry though. Therefore, it is not just enthalpy changes that determine whether a reaction can or cannot occur. Entropy is also a factor that determines how likely a reaction is to happen.
Entropy is a way of measuring how many ways we can arrange particles in a system. Imagine a wardrobe with clothes in. The more clothes you have, the more ways you can hang them in the wardrobe – there are more possible ways of ‘arranging’ the clothes in the system (the system here being everything inside the wardrobe).
A wardrobe with more clothes in would have a higher entropy than a wardrobe with fewer clothes in.
This same theory applies to particles in a system, except now we need to think about not just the number of particles but also the state they are in.
In a solid, for example, particles cannot move around easily and have to be in a fixed, regular order. There are not many ways to arrange these particles, so the entropy of the system would be low.
In a system with gaseous particles, however, there are a vast number of ways to arrange these particles, so the entropy of the system would be high.
It is important to note that entropy, like enthalpy, is only really useful as a relative term. Comparing the entropy of one system to another can give us a good idea as to whether a reaction is likely to happen.
Nature is lazy but also completely chaotic – nature prefers systems with higher entropy.
In simple terms, the total energy of a system is lower if there are more ways to arrange the particles within it. We know that nature prefers to form lower energy systems, and because of this, systems with high entropy are more likely to form than systems with low entropy.
It is not that simple though!
Remember endothermic and exothermic reactions – the total enthalpy change of a reaction also has an impact on whether a reaction can or cannot happen.
We now have two factors that influence whether a reaction is likely to happen – enthalpy change and entropy change.
To describe whether a reaction is likely to occur we have to link these two factors together.