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Summary Chemistry Equilibrium Notes
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Notes on Equilibrium
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EquilibriumTheoretically, all reactions can happen in both directions and are reversible (⇌). However, in
reality some reactions can only proceed in one direction as the other direction requires so
much energy as to be impractical. Thus, these are referred to as irreversible reactions.
Reversible Reactions: Irreversible Reactions:
Weak acids
Hydrated and anhydrous forms Combustion
State changes
Dissolution Strong acids
Reversible reactions must take place in closed Photosynthesis
systems, otherwise matter will escape and
equilibrium cannot be reached. In a closed
system, energy cannot escape. Equilibrium is achieved when both directions of a reversible
reaction are occurring at the same rate. This is known as dynamic equilibrium. Dynamic
equilibrium have ΔG = 0
Gibbs Free Energy
ΔG is the measure of whether a reaction is spontaneous or not. If ΔG is negative, the
reaction is spontaneous – if ΔG is positive, the reaction is non-spontaneous. If it is 0, the
reaction is at equilibrium.
The formula for finding Gibbs Free Energy is ΔG = ΔH – TΔS
ΔH ΔS ΔG Reaction
Negative (-) Positive (+) (-) Always spontaneous
Energy is Energy is Negative
released dispersed
Positive (+) Positive (+) (+) or (–) Spontaneous if ΔS is large and T is high
Energy is Energy is Positive or
absorbed dispersed negative
Negative (-) Negative (-) (+) or (–) Spontaneous if T is low
Energy is Energy is Positive or
released concentrated negative
Positive (+) Negative (-) (+) Never spontaneous
Energy is Energy is Positive
absorbed concentrated
Alternately, if no reaction is occurring in either direction it is static equilibrium. A static
equilibrium occurs at the completion of an irreversible reaction, where all particles are at
rest and the chemical reaction cannot continue. In this state, all reactants have been used. If
a system is not closed, it can reach a steady state, but not equilibrium.
A saturated solution is a solution where no more solute can be added (maximum
concentration). It is in a state of dynamic equilibrium, as the solute is both dissolving and
crystallising at the same rate.
, The two methods of graphing equilibrium are concentration/time graphs and rate-of-
reaction/time graphs.
Concentration is shown with [square brackets] around the chemical in question.
In a homogenous equilibrium all species (reactants and products) involved in the reaction
are in the same state (i.e. H2O + CO ⇌ H2 + CO2), while in a heterogeneous equilibrium the
species are in different states.
When observing an equilibrium system, it will be characterised by constant macroscopic
properties – i.e. physical properties. The concentrations of reactants/products, the colour,
pressure, pH and electrical conductivity will remain constant from our perspective.
On an atomic level, there is a continuous change taking place in both directions, leading to
no net change.
An equilibrium can be approached from either direction.
Le Chatelier's Principle: When a change (stress) is imposed on a system at equilibrium, the
position of the equilibrium shifts to reduce the effect of the change. Therefore, when a
reactant on one side of an equation increases, the products (on the other side) increase, and
the other reactants (same side) decrease.
The changes that can affect the equilibrium include concentration, temperature, and (for
gases) pressure.
A + B ⇌ C + D + heat
If you add more A, the concentration of B will decrease and the concentration of C + D will
increase.
If you add more heat, the concentration of C + D will decrease and the concentration of A +
B will increase.
If you increase the pressure by decreasing volume, the reaction will go to the side with the
least moles of gas.
If you increase the pressure by adding an inert or spectator gas, it has no effect!
Adding a catalyst also has no effect.
H2O + Cr2O72- ⇌ 2H+ + 2CrO42-
When hydrogen ions are neutralised, the mixture becomes bluer. When hydrogen ions are
added, the mixture becomes redder.
These changes in equilibrium position can be described in three ways:
Shifts to the right/left
Favours the forward/reverse
Favours formation of products/reactants
In written responses, use all of these and also quote “The system in equilibrium will shift to
counteract the change” – Le Chatelier
Collision Theory
Reactions occur when particles collide with enough kinetic energy (the minimum being the
activation energy) and the correct orientation to break the bonds and form an activation
complex for new substances to be formed. If the activation energy is low enough in both
directions, the reaction can occur in equilibrium.
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