Equilibrium and reversible reactions Flashcards
What happens in a reversible reaction?
Some reactions go to completion, where the reactants are used up to form the product molecules and the reaction stops when all of the reactants are used up
In reversible reactions, the product molecules can themselves react with each other or decompose and form the reactant molecules again
What happens in reversible reactions? (both ways)
It is said that the reaction can occur in both directions:
The forward reaction forming the products
The reverse reaction forming the reactants
Are the reactions exothermic or endothermic? In reversible reactions
If the forward reaction is exothermic, then the reverse reaction will be endothermic
The same amount of heat is transferred in both directions
What happens when writing equations for reversible reactions?
When writing chemical equations for reversible reactions, a symbol with arrows pointing on both sides is used
Explain an example of a reversible reaction.
An example is, the reaction for the Haber process which produces ammonia from nitrogen and hydrogen
N2 + 3H2 ⇌ 2NH3
The forward reaction, producing ammonia, is exothermic
So, the reverse reaction is endothermic
What are hydrated salts?
Hydrated salts are salts that contain water of crystallisation which affects their molecular shape and colour
Water of crystallisation is the water that is stoichiometrically included in the structure of some salts during the crystallisation process
Explain the reaction of the hydrated salt:
hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water
CuSO4*5H2O ⇌ CuSO4 + 5H2O
What colour are the crystals?
Which is the hydrated salt and which one is anhydrous
The hydrated salt is copper(II) sulfate pentahydrate, CuSO4*5H2O
These are usually seen as blue crystals
The hydrated salt can be heated / dehydrated to form anhydrous copper(II) sulfate, CuSO4
This reaction is endothermic as energy is taken in to remove the water
The anhydrous salt is copper(II) sulfate
This is usually seen as white crystals / powder
Adding water to the anhydrous salt forms the hydrated copper(II) sulfate pentahydrate, CuSO4*5H2O
This reaction is highly exothermic
Explain this reaction:
Another example is cobalt(II) chloride:
hydrated cobalt(II) chloride ⇌ anhydrous cobalt(II) chloride + water
CoCl2*6H2O ⇌ CoCl2 + 6H2O
The hydrated salt is cobalt(II) chloride hexahydrate, CoCl2*6H2O
These are usually seen as pink crystals
The hydrated salt can be heated / dehydrated to form anhydrous cobalt(II) chloride, CoCl2
This reaction is endothermic as energy is taken in to remove the water
The anhydrous salt is cobalt(II) chloride, CoCl2
This is usually seen as blue crystals
Adding water to the anhydrous salt forms the hydrated cobalt(II) chloride hexahydrate, CoCl2*6H2O
This reaction is highly exothermic
The hydration of CoCl2 and CuSO4 are chemical tests used to detect the presence of water. What are the colour changes?
You should remember the equations and colour changes:
CoCl2 + 6H2O ⇌ CoCl2.6H2O Blue to pink
CuSO4 + 5H2O ⇌ CuSO4.5H2O White to blue
Where can a reversible reaction reach equilibrium, what does it mean?
Reversible reactions occur in both the forward and backward directions
A reversible reaction can reach equilibrium in a closed system
This is so none of the participating chemical species can leave the reaction vessel and nothing else can enter
What happens to the rate of the reaction at equilibrium?
The rate of the forward reaction is equal to the rate of the reverse reaction
What happens to the concentration of reactants at equilibrium? and why?
The concentration of reactants and products remains constant (given there is no other change to the system such as temperature and pressure)
What does it mean when we say equilibrium is dynamic?
This means that the molecules on the left and right of the equation are changing into each other by chemical reactions constantly and at the same rate
An example of a reaction reaching equilibrium is the reaction between H2 and N2 in the Haber process. Explain
At the start of the reaction, only nitrogen and hydrogen are present
This means that the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest
As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease
So, the rate of the forward reaction will decrease
However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase
Ammonia will decompose to reform hydrogen and nitrogen
In a closed system, the two reactions are interlinked and none of the gases can escape
So, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached:
What does Le chateliers principle state?
The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction. It states that when a change is made to the conditions of a system at equilibrium, the system automatically moves to oppose the change
What does the chateliers principle allow us to predict and do?
The principle is used to predict changes to the position of equilibrium when there are changes in:
Temperature
Pressure
Concentration
Knowing the energy changes, states and concentrations involved allows us to use the principle to manipulate the outcome of reversible reactions
What happens when the pressure is changed? According to CP
if pressure is increased, the position of equilibrium moves in the direction which has the smallest amount of gaseous molecules
When is the position of equilibrium said to shift to the right?
The position of equilibrium is said to shift to the right when the forward reaction is favoured
This means that there is an increase in the amount of products formed
When is the position of equilibrium said to shift to the left?
The position of equilibrium is said to shift to the left when the reverse reaction is favoured
So, there is an increase in the amount of reactants formed
How can we predict the effects of changes in temperature on systems of equilibrium?
We can predict the effect of changes in temperature on systems in equilibrium
To make this prediction it is necessary to know whether the reaction is exothermic or endothermic
What happens when the temperature is raised?
If the temperature is raised:
The yield from the endothermic reaction increases
The yield from the exothermic reaction decreases
Equilibrium moves in the endothermic direction to reverse the change
What happens when the temperature is lowered?
If the temperature is lowered:
The yield from the endothermic reaction decreases
The yield from the exothermic reaction increases
Equilibrium moves in the exothermic direction to reverse the change
What happens when a change in temperature is made to a system?
When a change in temperature is made to a system, the system will oppose the change
E.g. If the temperature is increased, the system will oppose the change by decreasing the temperature
It will do this by favouring the endothermic reaction
Iodine monochloride reacts reversibly with chlorine to form iodine trichloride.
The forward reaction is exothermic:
ICl + Cl2 ⇌ ICl3
dark brown yellow
What colour will the mixture turn when heated? Explain your answer.
The system will oppose the increase in temperature
Increasing the temperature of an equilibrium reaction favours the endothermic reaction
If the forward reaction is exothermic, then the backward reaction must be endothermic
Therefore, the equilibrium will move to the left and produce more of the reactants
This means that the colour of the mixture will become increasingly brown as the temperature increases
In which reactions does pressure have an effect?
Changes in pressure only affect reactions where the reactants or products are gases
How to predict the effect of changes in pressure on system in equilibrium
We can predict the effect of changes in pressure on systems in equilibrium
To make this prediction, the balanced symbol equation must be known
Effect of increase in pressure on system of equilibrium
Equilibrium shifts in the direction that produces the least number of molecules.This reduces the effects of the change because the pressure decreases as the number of molecules decreases.
Effect of decrease in pressure on system of equilibrium
Equilibrium shifts in the direction that produces the greatest number of molecules. This reduces the effects of the change because the pressure increases as the number of molecules increases.
Nitrogen dioxide molecules can dimerise and form dinitrogen tetroxide in the following equilibrium reaction:
2NO2 (g) ⇌ N2O4 (g)
brown gas colourless gas
What will the colour change be if the pressure is increased? Explain your answer.
The number of gas molecules produced by the forward reaction = 1
The number of gas molecules produced by the reverse reaction = 2
An increase in pressure will cause equilibrium to shift in the direction that produces the least number of molecules of gas
This is the forward reaction
So, the equilibrium shifts to the right
This means that:
The mixture will become increasingly colourless
The concentration of N2O4 will increase
Effect of changing concentration on equilibrium
The effect of changing concentration can be thought of as a balance, with the reactants on the left and the products on the right
If the concentration of a reactant increases, then the equilbrium shifts to the right to balance this balance
Effect of Increase in concentration of a reactant
Equilibrium shifts to the right
Effect of decrease in concentration of a reactant
Equilibrium shifts to the left
Effect of Increase in concentration of a product
Equilibrium shifts to the left
Effect of Decrease in concentration of a product
Equilibrium shifts to the right
Iodine monochloride reacts reversibly with chlorine to form iodine trichloride
ICl + Cl2 ⇌ ICl3
dark brown yellow
Explain what happens when:
- The concentration of ICl3 increases
- Some Cl2 is removed
- The concentration of ICl3 increases
There are more molecules of ICl3 on the products side
So, the position of the equilibrium moves to the left
This produces more ICl and Cl2
So, the reaction mixture gets darker / turns dark brown
- Some Cl2 is removed
There are less molecules of Cl2 on the reactants side
So, the position of the equilibrium moves to the left
This produces more Cl2 (and ICl)
So, the reaction mixture gets darker / turns dark brown
How does the presence of a catalyst affect equilibrium?
The presence of a catalyst:
Does not affect the position of equilibrium
Increases the rate at which equilibrium is reached
Why does a catalyst increase the rate at which equilibrium is reached?
This is because the catalyst increases the rate of both the forward and backward reactions by the same amount by providing an alternative pathway requiring lower activation energy
As a result, the equilibrium concentration of reactants and products is the same as it would be without the catalyst
What happens when the conditions of equilibrium are changed, how does the system respond?
When the conditions at equilibrium are changed, the system always responds by doing the opposite.
For example if the concentration is increased the system tries to reduce it by changing the direction of the reaction or if the temperature is increased the system will try to reduce the temperature by absorbing the extra heat.
volatile
(of a substance) easily evaporated at normal temperatures.
“volatile solvents such as petroleum ether, hexane, and benzene”
Source of nitrogen and hydrogen in haber process
Hydrogen is obtained from methane while nitrogen from air
