6 Reversible reactions Flashcards
What is a reversible reaction?
Where products of the reaction can react with each other and convert back to the original reactants- the reaction can go in both directions
What is the symbol for a reversible reaction?
The thermal decomposition of ammonium chloride is a reversible reaction
When heated NH4Cl can thermally decompose into NH3 and HCl gases.
What would be observed if both these gases meet in a cooler enviroment?
white cloud or powder forms
NH3 and HCl reacts to form NH4Cl which is a white solid
Hydrated copper (II) sulfate can be heated (endothermic) which will drive off the water leaving anhydrous copper (II) sulfate.
What would happen if you added water to the white anhydrous copper (II) sulfate?
It would form blue hydrated copper (II) sulfate
This is the reverse reaction.
This reaction is exothermic- heat released to the surroundings
Hydrated copper (II) sulfate can be heated which will drive off the water leaving anhydrous copper (II) sulfate.
Is this an endothermic or exothermic reaction?
endothermic- heat taken in
Anhydrous copper (II) sulfate can react with water to form hydrated copper (II) sulfate.
What is the colour change in this reaction?
white to blue
Hydrated copper (II) sulfate can be heated which will drive off the water leaving anhydrous copper (II) sulfate.
What is the colour change in this reaction?
blue to white
How would you write the reversible reaction for the thermal decomposition of ammonium chloride?
(don’t forget the state symbols)
If a reversible reaction takes place in a closed system then a state of ____________ equilibrium will always be reached
Dynamic equilibrium
When a reaction of gases are carried out in a closed system, dynamic equilibrium occurs.
What is dynamic equilibrium?
This is when the concentration of the reactants and products remain the same because the rate of the forward reaction = the rate of the reverse reaction.
Don’t forget, the reaction are still taking place in both directions, but the overall effect is nil.
State three features of a reaction that is in dynamic equilibrium
- The forward and reverse reaction occur at the same time
- The rate of the forward reaction = the rate of the reverse reaction
- The concentration of the products and reactants is constant- not changing.
What three conditions can shift an equilibrium position during dynamic equilibrium?
temperature
pressure
changing the concentration of the reactants or products
In reversible reactions the ‘position’ of equilibrium can shift by changing the temperature or pressure of the closed vessel.
Shifting the equilibrium position to the right….
A) increases the percentage yield
B) decreases the percentage yield
A) increases the percentage yield
What enthalpy value indicates an endothermic reaction?
positive ethalphy value
What enthalpy value indicates an exothermic reaction?
negative ethalphy value
When the mixture of gases are added to a hot water bath, the mixture turns a dark brown colour.
Use the ‘teenager rule’ OR Le Chatelier’s principle to work out if the reaction is exothermic or endothermic.
Increasing the temperature favours the endothermic reaction.
The dark brown colour shows that the equilibrium was shifted to the right.
This means the reaction is endothermic (in the forward direction)
Which reversible reaction results in a greater yield?
The first reaction shows a greater yield
The concentration of products is higher in the first reaction than the second reaction.
The energy profile diagram below represents an (endothermic/exothermic) reaction
Exothermic
The energy profile diagram below represents an (endothermic/exothermic) reaction
Endothermic
Use the ‘teenager rule’ OR Le Chatelier’s Principle to determine what would happen to the percentage yield for the reaction below if the pressure is increased.
Increasing pressure favours the direction with fewer moles (this will decrease the pressure)
4 moles of gas on left –> 2 moles of gas on the right
The equilibrium will shift right
percentage yield will increase
What is the difference between an exothermic and endothermic reaction.
Exothermic reactions heat up the surrounding
Endothermic reactions cool down the surrounding
A catalyst provides an alternative route with a lower activation energy. This helps speed up a reaction because there are more particles with the required activation energy increasing the number of successful collisions per unit time.
Explain how a catalyst affects the percentage yield of a reaction at dynamic equilibrium.
Adding a catalyst does not shift (or affect) the equilibrium position therefore it does not alter the percentage yield.
It does, however, speed up the reaction and dynamic equilibrium is reached more rapidly.
During some industrial processes, product is removed from the reaction vessel. Any unreacted reactants in this is then recycled and put back into the reaction vessel.
Using the ‘teenage rule’ or Le Chatelier’s Principle, explain why adding reactants into the reaction vessel increases the percentage yield.
Adding reactants to the reaction vessel, increases the concentration of the reactants.
The equilibrium will shift right to decrease the concentration of reactants
This increases the percentage yield
In the graph below, state when dynamic equilibrium occurs.
How do you know?
Why?
At B
Concentration of reactants and products remain constant
Rate of forward reaction = rate of reverse reaction
Use the ‘teenager rule’ or Le Chatelier’s Principle to explain what happens to the percentage yield when temperature is decreased.
The enthalpy value is negative- exothermic reaction
Decreasing the temperature favours the exothermic reaction
This reaction is exothermic in the forward direction
equilibrium shift to the right
percentage yield increases.
Use the ‘teenager rule’ or Le Chatelier’s Principle to explain what happens to the percentage yield when temperature is increased.
The enthalpy value is negative- exothermic reaction
Increasing temperature favours the endothermic reaction
This reaction is endothermic in the reverse direction
equilibrium shift to the left
percentage yield decreases.
Black circles = reactant
White circles = product
How do the series of diagrams show that the reaction in the reaction vessel has reached dynamic equilibirum at t = 10 min
The concentration (or number of ) white and black substances is constant after t = 10 min.
(5 black and 4 white particles in the reaction vessel)
Use the ‘teenage rule’ or Le Chatelier’s Principle to explain what happens to the percentage yield if the pressure is decreased.
Decreasing pressure favours the side with more moles
There are more moles on the left side of the reaction ( 4 moles –> 2 moles)
Equilibrium shifts left
percentage yield decreases
Use the ‘teenage rule’ or Le Chatelier’s Principle to explain what happens to the percentage yield if the pressure is increased.
Increasing pressure favours the side with fewer moles
There are fewer moles on the right ( 4 moles –> 2 moles)
Equilibrium shifts right
percentage yield increases
Reactant (A) is placed in a reaction vessel, A is converted into B and some time later dynamic equilibrium is established.
What would happen to the percentage yield if some reactant A is removed from the reaction vessel?
The equilibrium would shift left to make more reactant
The percentage yield would decrease
Hydrogen iodide decomposes to form hydrogen and iodine.
This is a reversible reaction
2HI <–> H2 + I2
Explain what would happen to the concentration of hydrogen iodide and iodine if hydrogen was added to the reaction vessel
The equilibrium would shift left to decrease the concentration of hydrogen
The concentration of hydrogen iodide would increase.
The concentration of iodine would decrease
