10.4 Dynamic Equilibrium and le Chatlier's principle Flashcards
When ignited, what does hydrogen react with oxygen to form
Water
What is a reversible reaction
Reactions that take place in both ‘forward’ and ‘reverse’ directions.
What type of reaction is the Haber process
A reversible reaction
What is a product of the Haber process
Ammonia
Equation of the Haber process
N2(g) + 3H2(g) 2NH3(g)
In an equilibrium system:
The rate of the forward reaction is equal to the rate of the reverse reaction
The concentrations of reactants and products do not change
Equilibrium systems are ______. At equilibrium, both the forward and reverse reactions are taking place. As fast as the reactants are becoming products, the products are reacting to become ______. Therefore, in an equilibrium system, the concentrations of the reactants and products remain ______.
Dynamic
Reactants
Unchanged
What must be true for a reaction to remain in equilibrium
The system must be closed
What is a closed system
A system which is isolated from its surroundings
So the temp., pressure, and conc. of reactants and products are unaffected by outside influences
What does the position of equilibrium indicate
The extent of the reaction
What does le Chatlier’s principle state
That when a system in equilibrium is subjected to an external change, the system readjusts itself to minimise the effect of that change
When an equilibrium system adjusts as a result of a change:
IF THERE ARE MORE PRODUCTS FORMED
The position of equilibrium has shifted to the right
When an equilibrium system adjusts as a result of a change:
IF THERE ARE MORE REACTANTS FORMED
The position of equilibrium has shifted to the left
What will changing the concentration of a reactant or product affect a reversible reaction
The rate of the forward or reverse reactions will change
Investigating changes to the position of equilibrium with concentration:
Chromate ions and Dichromate ions
The two solutions have different colours so it is easy to see any shift in the position of equilibrium
They are sensitive to changes in acid concentration
Chromate ions - yellow
Dichromate ions - orange
Experiment to show that the position of equilibrium can be changed by altering the concentrations of the reactants or products
ADDING AN ACID
- Add solution of yellow potassium chromate K2CrO4 to a beaker
- Add dilute sulfuric acid, H2SO4, dropwise until there is no further change. Solution turns an orange colour
- Add aqueous sodium hydroxide, NaOH(aq) until there is no further change. Solution changes back to a yellow colour
Explanation of the experiment to show that the position of equilibrium can be changed by altering the concentrations of the reactants or products (ADDING AN ACID)
The shift decreases the concentration of the added reactant H+
The position of equilibrium shifts to the left, making more of the H+ reactant.
A new position of equilibrium is established (Solution turns yellow as CrO42- forms)
Change in temperature and equilibrium
What does the direction in which the equilibrium shifts depend on
The sign of ΔH
Forward and reverse directions have the same value for the enthalpy change - but the signs are opposite
An increase in temperature shifts the equilibrium position in the endothermic direction (ΔH is positive)
A decrease in temperature shifts the equilibrium position in the exothermic direction (ΔH is negative)
What colour is the solution of cobalt chloride and water
Pink
Experiment to investigate the changes to the position of equilibrium with temperature
- Dissolve cobalt chloride in water in a boiling tube. Add a small quantity of hydrochloric acid. Place boiling tube in iced water, solution is pink
- Set up boiling water bath and transfer boiling tube into boiling water. Solution turns a blue colour
- Transfer boiling tube back to iced water. Solution changes back to pink colour
In boiling water, you are increasing the heat energy of the system. This causes position of equilibrium to shift to minimise change:
As forward reaction is endothermic (ΔH is positive), position of equilibrium shifts to right in endothermic direction, to take heat energy in and minimise increase in temperature
Solution turns blue colour
Increase temperature shifts in…
…the endothermic direction
Decrease in temperature shifts in…
…the exothermic direction
In an exothermic forward reaction, ΔH is…
…negative
In an endothermic forward reaction, ΔH is…
…positive
How does equilibrium change if the temperature in an exothermic reaction increases
The position of equilibrium shifts to the left
More reactants are made
How does equilibrium change if the temperature in an exothermic reaction decreases
Position of equilibrium shifts to the right,
More products are made
How does equilibrium change if the temperature in an endothermic reaction increases
Position of equilibrium shifts to the right.
More products are made
How does equilibrium change if the temperature in an endothermic reaction decreases
Position of equilibrium shifts to the left
More reactants are made.
What is the pressure of a gas proportional to
Its concentration
Increasing the pressure of the system will shift the position of equilibrium to the side with…
…the fewer molecules
What colour is nitrogen dioxide NO2
Brown
What colour is dinitrogen tetroxide N2O4
Colourless
Does a catalyst change the position of equilibrium
NONONO
What does a catalyst do
Speeds up the rates of the forward and reverse reactions
Increases the rate at which an equilibrium is established however DOES NOT change the position of equilibrium
What can le Chatelier’s principle be used for
Predicting the best conditions of temperature and pressure to force the equilibrium to the right in order to produce the maximum yield of ammonia
What conditions does a typical ammonia plate operate under
Compromise conditions using a high enough temp. to give a reasonable rate without shifting the equilibrium position too fat away from ammonia and back to the reactiants.
Ensures ammonia achieved quickly, cheaply and safely
Typical temperatures and pressures used in an ammonia factory
350-500°C
100-200atm
Catalyst is also used (iron)
How do they improve the efficiency of ammonia factories
Unreacted nitrogen and hydrogen are recycled repeatedly so nearly all the nitrogen and hydrogen used is eventually converted into ammonia
What percentage of the nitrogen and hydrogen in an ammonia factory is converted to ammonia
15%
