Equilibria (OCR GCSE Chemistry A (Gateway)): Flashcards
Exam code: J248
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Define reversible reaction.
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Cards in this collection (19)
Define reversible reaction.
A reversible reaction is one in which the products can react to reform the original reactants — the reaction can proceed in both the forward and backward directions.
What does the symbol ⇌ indicate in a chemical equation?
The ⇌ symbol shows that the reaction is reversible — both the forward reaction (left to right) and the backward reaction (right to left) can occur.
Dynamic equilibrium is reached when the rate of the .......... reaction equals the rate of the .......... reaction in a closed system.
Dynamic equilibrium is reached when the rate of the forward reaction equals the rate of the backward reaction in a closed system.
True or False?
Dynamic equilibrium can be reached in an open system where gases can escape.
False.
Equilibrium can only be reached in a closed system, where no substances can enter or leave the reaction vessel.
If the forward reaction in a reversible reaction is exothermic, what can you say about the backward reaction?
The backward reaction is endothermic.
In reversible reactions, the energy change is equal and opposite in each direction.
What remains constant at dynamic equilibrium?
The concentrations of reactants and products remain constant at dynamic equilibrium.
The reactions are still occurring, but at the same rate in both directions so there is no net change.
At the start of the Haber process, only nitrogen and hydrogen are present, so the rate of the .......... reaction is highest. As ammonia builds up, the rate of the .......... reaction increases.
At the start of the Haber process, only nitrogen and hydrogen are present, so the rate of the forward reaction is highest. As ammonia builds up, the rate of the backward reaction increases.
True or False?
At dynamic equilibrium, the forward and backward reactions have stopped.
False.
At dynamic equilibrium, both reactions are still occurring — they just happen at the same rate, so there is no overall change in concentrations.
Why must equilibrium be established in a closed system?
In a closed system, no substances can enter or leave.
If the system were open, reactants or products would escape, preventing the concentrations from reaching a stable, constant balance.
Define Le Chatelier's Principle.
(Higher Tier Only)
Le Chatelier's Principle states that when a change is made to the conditions of a system at equilibrium, the system responds to oppose that change.
What happens to the position of equilibrium if the concentration of a reactant is increased?
(Higher Tier Only)
The equilibrium shifts to the right (towards the products).
The system opposes the change by using up the extra reactant to make more product.
If the concentration of a product is .........., the equilibrium shifts to the .......... to produce more product.
(Higher Tier Only)
If the concentration of a product is decreased, the equilibrium shifts to the right to produce more product.
True or False?
Increasing temperature always shifts the equilibrium position to the right.
(Higher Tier Only)
False.
Increasing temperature favours the endothermic direction. If the forward reaction is exothermic, equilibrium shifts left; if endothermic, it shifts right.
How does increasing pressure affect a gaseous equilibrium where there are more moles of gas on the left?
(Higher Tier Only)
Increasing pressure shifts the equilibrium to the right — towards the side with fewer moles of gas.
The system opposes the pressure increase by reducing the total number of gas molecules.
The forward reaction of a reversible equilibrium is exothermic. What happens to the equilibrium position when temperature is increased?
(Higher Tier Only)
The equilibrium shifts to the left (towards reactants).
The system opposes the temperature increase by favouring the endothermic backward reaction, which absorbs heat.
Increasing pressure shifts equilibrium towards the side with .......... moles of gas, because this .......... the pressure.
(Higher Tier Only)
Increasing pressure shifts equilibrium towards the side with fewer moles of gas, because this reduces the pressure.
True or False?
If there are equal moles of gas on both sides of an equilibrium, changing pressure has no effect on the position of equilibrium.
(Higher Tier Only)
True.
Pressure only shifts equilibrium when there are different numbers of moles of gas on each side. Equal moles means no net shift.
Why is Le Chatelier's Principle important to chemical engineers?
(Higher Tier Only)
It allows engineers to select conditions (temperature, pressure, concentration) that shift equilibrium towards the product side.
This increases percentage yield and reduces the cost of manufacturing chemicals.
What happens to the position of equilibrium if a product is continuously removed from the reaction vessel?
(Higher Tier Only)
The equilibrium shifts to the right to replace the product that has been removed.
This can be used to increase yield, as the system keeps producing more product to restore equilibrium.
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