Topic 10: Chemical Equilibrium Flashcards
Describe and explain how the reaction between hydrogen and iodine can be described as a reversible reaction
If a mixture of hydrogen and iodine vapour in a 1:1 molar ratio is heated in a closed container, 90% reacts to form hydrogen iodide but 10% of the reactants remain unreacted
The hydrogen iodide formed will decompose and form a mixture of hydrogen and iodine that is equimolar to the starting mixture of hydrogen and iodine
How is equilibrium in the reversible reaction of hydrogen and iodine established ?
As hydrogen iodide is formed the conc of hydrogen and iodine decrease = rate of forward reaction decreases
As hydrogen iodide is formed it slowly starts to decompose, when conc of hydrogen iodide increases = rate of backward reaction increases
When there is no further change in the concentrations of reactants and products the system is said to be an equilibrium (rate of forward and backward reactions are equal) 
What are the two conditions that must be met for dynamic equilibrium to be established
- The reaction must be reversible
- The reaction mixture must be in a closed container
Which important features define a system that is in dynamic equilibrium
- The forward and backward reactions are continuously occurring
- The rate of the forward reaction is equal to the rate of the backward reaction
- The concentrations of reactants and products remain constant
What are the 4 factors that may affect the position of equilibrium of a reaction mixture?
- Concentration of a component
- Pressure of the system
- Temperature of the system
- Addition of a catalyst
Describe the effect of a change in concentration to the reactants
Increase = position of equilibrium shifts to the right
Decrease= position of equilibrium shifts to the left
Describe the effect of a change in concentration to the products
Increased= change in position of equilibrium to the left
Decreased= change in position of equilibrium to the right
Describe the effect of a change in pressure of reactants
Increase= change in position of equilibrium to the right
Decreased = change in position of equilibrium to the left
(Opposite for change in pressure of products) 
Describe the effect of a change in temperature (equilibria)
The rates of both the forward and backwards reaction will increase
However, the increase in rate of the endothermic reaction will be greater than the increase in rate of the exothermic reaction
Therefore, the change in position of equilibrium will depend on whether the forward reaction is exothermic or endothermic
Describe the effect of the addition of a catalyst (equilibria)
The rate of both the forward and backward reactions will increase so the position of equilibrium is not altered
The advantage of adding a catalyst at the beginning of a reaction is that it will reduce the time required to establish equilibrium
What is a homogeneous system
One in which all components are in the same phase ie. Gas phase, liquid phase or aqueous phase
What is a heterogeneous system
There are at least two different phases of components present
Give the equation for the equilibrium constant
. [C]^c [D]^d
Kc = ——————- For a reaction where aA + bB ⇌ cC + dD
[A]^a [B]^b
In order to maximise profits the major problems confronting chemists are to convert the reactants into products:
- as quickly as possible
- as completely as possible
Why are conditions of 450° C and 250atm used in the Haber process ?
The reaction between nitrogen and hydrogen is extremely slow at room temperature because of the very strong nitrogen triple bond producing a high activation energy
An iron catalyst is used and does not function very effectively at lower temperatures but a high temp would lead to increased costs so compromise temp of 450 is used
The higher the pressure, the higher the energy cost of compressing the gases but a low pressure would decrease yield. Therefore a compromise pressure of 250atm is used
