Equilibrium Basics
Define dynamic equilibrium.
Dynamic equilibrium is a state in a closed system where the rate of the forward reaction equals the rate of the reverse reaction, causing the concentrations of reactants and products to remain constant.
State Le Chatelier's Principle.
If a system at equilibrium is disturbed by a change in concentration, temperature, pressure or volume, the system responds by shifting equilibrium to oppose the change and establish a new equilibrium.
State the three requirements for a successful collision.
Particles must:
Write the equilibrium expression for:
H2(g)+I2(g)⇌2HI(g)
K= [HI2]^2 / [H2][I2]
Define enthalpy.
Enthalpy is the heat content of a system.
The change in enthalpy (ΔH) represents heat absorbed or released during a chemical reaction.
State TWO conditions required for a system to be in dynamic equilibrium.
For: N2(g)+3H2(g)⇌2NH3(g), predict the direction of shift when extra H₂ is added.
The equilibrium shifts to the right.
Adding H₂ increases the concentration of a reactant, so the system attempts to remove some of the added hydrogen by producing more ammonia.
Explain why increasing concentration increases reaction rate.
Increasing concentration places more particles in a given volume.
This increases collision frequency, leading to more successful collisions per second and therefore a faster reaction rate.
Write the equilibrium expression for:
N2(g)+3H2(g)⇌2NH3(g)
K= [NH3]^2 / [N2][H2]^3
State the sign of ΔH for:
(a) combustion
(b) photosynthesis
(a)
ΔH<0
Exothermic.
(b)
ΔH>0
Endothermic.
Explain why the following statement is incorrect: "At equilibrium the concentration of reactants equals the concentration of products."
At equilibrium, the concentrations remain constant, but they are not necessarily equal.
The ratio of reactants to products depends on the equilibrium position.
For example:
N2(g)+3H2(g)⇌2NH3(g)
At equilibrium there may be much more ammonia than nitrogen and hydrogen, but the system is still at equilibrium because the forward and reverse rates are equal.
The equilibrium is: 2NO2(g)⇌N2O4(g)2NO_2(g), Predict the effect of increasing pressure.
The equilibrium shifts to the right.
The left side contains 2 moles of gas.
The right side contains 1 mole of gas.
Increasing pressure favours the side with fewer gas particles, therefore more N₂O₄ forms.
Explain how a catalyst affects activation energy.
A catalyst provides an alternative reaction pathway with a lower activation energy.
This increases the number of successful collisions and speeds up the reaction.
Determine whether equilibrium favours products or reactants if:
K=2.5×10^5
Products are strongly favoured.
A very large K value means much more product exists at equilibrium than reactants.
Equilibrium lies to the right.
State what positive entropy means.
Positive entropy means the system becomes more disordered.
Particles become more randomly arranged and have greater freedom of movement.
A student mixes nitrogen and hydrogen in a sealed container. After several hours the concentrations become constant. Explain why the reaction has not stopped.
The reaction has not stopped because it has reached dynamic equilibrium.
Nitrogen and hydrogen molecules continue colliding and forming ammonia, while ammonia simultaneously decomposes into nitrogen and hydrogen.
The forward and reverse reactions continue occurring, but at equal rates.
As a result:
For the exothermic reaction: A + B ⇌ C + D + Heat, predict and explain the effect of increasing temperature.
The equilibrium shifts to the left.
Heat is a product.
Increasing temperature adds more "product" to the system.
To oppose this change, the equilibrium favours the reverse reaction and consumes some of the heat.
An equilibrium system is disturbed by adding reactants. Using collision theory, explain why the forward reaction initially becomes faster.
Adding reactants increases the number of reactant particles.
More reactant particles means more collisions between reactants.
The frequency of successful forward collisions increases immediately, causing the forward reaction rate to exceed the reverse reaction rate.
A reaction has:
K=50 and Q=2
Predict the direction in which the reaction proceeds.
Q<K
There are fewer products than required for equilibrium.
The forward reaction is favoured.
The system shifts right.
Using
ΔG=ΔH−TΔS
predict whether the reaction is spontaneous if
ΔH=−120kJ ΔS=+250J mol^−1 K^−1
The reaction is spontaneous.
A negative ΔH favours spontaneity.
A positive ΔS also favours spontaneity.
Both factors make ΔG negative.
The equilibrium: N2(g)+3H2(g)⇌2NH3(g) is established. Explain how collision theory accounts for the system reaching dynamic equilibrium.
Initially, only nitrogen and hydrogen molecules are present.
Because their concentrations are high, there are many successful collisions between reactant particles, causing the forward reaction rate to be greater than the reverse reaction rate.
As ammonia forms:
This causes:
Eventually, the forward and reverse reaction rates become equal.
At this point dynamic equilibrium is established, and the concentrations remain constant even though reactions continue to occur.
A closed equilibrium system experiences:
Discuss the effect of each change.
Temperature Increase
Product Removal
Pressure Decrease
To predict the final equilibrium position, you must know:
Explain why catalysts do not change the equilibrium position despite increasing reaction rates.
Catalysts lower the activation energy for both the forward and reverse reactions equally.
Both reaction rates increase by approximately the same amount.
Since neither direction is favoured, the equilibrium position remains unchanged.
The catalyst only allows equilibrium to be reached more quickly.
A reaction has:
K= 1.2×10^−4 and Q=4.5×10^−2
Determine the direction of reaction.
Q>K
There are too many products relative to equilibrium.
The reverse reaction is favoured.
The equilibrium shifts left.
Photosynthesis has:
ΔH=+2805kJ mol^−1 ΔS=−212J mol^−1 K^−1
Using Gibbs free energy, explain why it is non-spontaneous.
Photosynthesis has:
Therefore both terms oppose spontaneity.
Since:
ΔG=ΔH−TΔS\Delta G=\Delta H-T\Delta SΔG=ΔH−TΔS
the resulting ΔG is positive under standard conditions.
A positive ΔG indicates a non-spontaneous reaction.
Photosynthesis requires external energy from sunlight to occur.