High School Kinetics and Equilibrium
IN THIS LESSON
Become familiar with the concept of equilibrium so you can apply it later.
1/4/25: I consider this lesson complete right now, but I am willing to edit it as needed.
Reversibility in reactions is the ability for a reaction to go from both the reactants to the products and the products to the reactants. Most reactions won’t only go in the reactants to products direction. They may not go 100% from reactants to products because they also occur from products to reactants. Reversibility is often denoted with the ⇌ symbol instead of an arrow in a reaction.
The natural tendency is for the enthalpy (ΔH) of a reaction to decrease (ΔH < 0).
Entropy is the scatteredness or chaos of a system. The natural tendency of entropy (ΔS) is to increase (ΔS > 0). Systems tend to change from an ordered state to maximum disorder.
Rules for figuring out if ΔS is positive or negative (without calculations):
When the substances are the same, they will be the most ordered in the solid state, and least ordered in the gas state. The liquid state and aqueous state are in between these two.
ΔS increases when a substance is broken into multiple pieces, such as during dissociation of molecules. For example, NaCl is in a more ordered state than Na- + Cl+.
ΔS is greater when there are more moles on that side of the reaction.
The signs of ΔH and ΔS can help us find if a reaction is spontaneous and if it’s reversible. The closer a reaction is to the natural tendencies of these values, the more likely it is to be spontaneous. If ΔH and ΔS are the same signs, they are also likely to be reversible.
For a reaction with a negative ΔH and a positive ΔS, it is most likely spontaneous and most likely not reversible.
For a reaction with both negative ΔH and ΔS or both positive ΔH and ΔS, it is hard to tell its spontaneity and most likely is reversible.
For a reaction with a positive ΔH and a negative ΔS, it is most likely not spontaneous or reversible.
Systems that are reversible tend to exist in equilibrium.
Most reactions are in dynamic equilibrium, or where all parts are in contact in a closed system and the reaction is ongoing.
Other reactions may be in a static equilibrium, where the reaction stops once equilibrium is reached, or steady equilibrium, where a reaction is ongoing and remains at equilibrium, but it exists in an open system, with new particles entering and exiting.
There are three main types of equilibrium: physical, solubility, and chemical. Physical equilibriums surround physical change, such as phase changes. Solubility equilibrium is when a substance exists as a solid within a solution of the same substance. Chemical equilibrium is the equilibrium of a chemical reaction, which is the equilibrium that has been discussed mostly in this lesson.
The three major factors that affect equilibrium are:
Concentration
Temperature
Pressure, if some molecules in the system are in the gas phase
Catalysts don’t affect equilibrium, because they increase the rate of reaction in both directions.