Reversible reactions & equilibria (Edexcel IGCSE Chemistry (Modular): Unit 2): Flashcards
Exam code: 4XCH1
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Define reversible reaction.
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Cards in this collection (27)
Define reversible reaction.
A reversible reaction is one in which the products can react to re-form the original reactants, meaning the reaction can proceed in both the forward and reverse directions.
True or False?
The forward reaction in a reversible reaction forms the products.
True.
The forward reaction converts reactants into products. In a reversible reaction, the reverse reaction converts those products back into reactants.
In a reversible reaction, the symbol .......... is used in the equation to show that the reaction can go in .......... directions.
In a reversible reaction, the symbol ⇌ is used in the equation to show that the reaction can go in both directions.
What happens when ammonium chloride is heated?
When ammonium chloride (NH4Cl) is heated, it decomposes to form ammonia (NH3) and hydrogen chloride (HCl) gases. When the gases cool, they recombine to reform solid ammonium chloride.
What is water of crystallisation?
Water of crystallisation is water that is incorporated into the crystal structure of some salts, affecting their shape and colour.
What is the colour change when hydrated copper(II) sulfate is heated?
When hydrated copper(II) sulfate (CuSO4•5H2O) is heated, it loses its water of crystallisation and turns from blue to white, forming anhydrous copper(II) sulfate.
True or False?
Anhydrous copper(II) sulfate is blue.
False.
Anhydrous copper(II) sulfate is white (a white powder). It is the hydrated form, copper(II) sulfate pentahydrate (CuSO4•5H2O), that appears blue.
Complete the table about copper(II) sulfate.
Form
Formula
Colour
Process to form it
Hydrated copper(II) sulfate
CuSO4•5H2O
Dissolve anhydrous form in water (exothermic)
Anhydrous copper(II) sulfate
CuSO4
Heat hydrated form (endothermic)
Complete the table about copper(II) sulfate.
Form
Formula
Colour
Process to form it
Hydrated copper(II) sulfate
CuSO4•5H2O
Blue
Dissolve anhydrous form in water (exothermic)
Anhydrous copper(II) sulfate
CuSO4
White
Heat hydrated form (endothermic)
Why do reversible reactions use the ⇌ symbol rather than a single arrow?
Reversible reactions use the ⇌ symbol to show that the reaction proceeds in both directions simultaneously. A single arrow would only indicate a one-way reaction going to completion.
Define dynamic equilibrium.
Dynamic equilibrium is the state reached in a reversible reaction when the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products remain constant.
What are the two key features of a system at dynamic equilibrium?
The two key features of a system at dynamic equilibrium are:
The rate of the forward reaction equals the rate of the reverse reaction
The concentrations of reactants and products remain constant
True or False?
Dynamic equilibrium can only be reached in a closed system.
True.
Dynamic equilibrium requires a closed system so that none of the reacting chemical species can escape and nothing else can enter the reaction vessel.
At the start of the Haber process, only nitrogen and hydrogen are present, so the rate of the .......... reaction is at its 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 at its highest. As ammonia builds up, the rate of the reverse reaction increases.
Why does the rate of the forward reaction decrease as a reversible reaction proceeds?
As the reversible reaction proceeds, the concentrations of the reactants decrease. Since reaction rate depends on concentration, a lower concentration of reactants means the forward reaction slows down.
What is a closed system?
A closed system is one in which no substances can enter or leave the reaction vessel, ensuring that all the reacting species remain present throughout the reaction.
True or False?
At dynamic equilibrium, reactants are completely used up.
False.
At dynamic equilibrium, the concentrations of both reactants and products remain constant — neither is fully used up. Both forward and reverse reactions continue at equal rates.
Why is equilibrium described as "dynamic"?
Equilibrium is described as dynamic because the forward and reverse reactions are still occurring continuously, even though the overall concentrations of reactants and products appear unchanged.
How does a system at equilibrium respond to a change in conditions?
A system at equilibrium responds to oppose any change made to its conditions. The equilibrium shifts in the direction that counteracts the imposed change.
How does increasing the temperature affect an equilibrium reaction where the forward reaction is exothermic?
Increasing the temperature shifts the equilibrium in the direction of the endothermic reaction to oppose the change. If the forward reaction is exothermic, the equilibrium shifts to the left, favouring the reverse reaction and producing more reactants.
True or False?
Increasing pressure always shifts the equilibrium to the right.
False.
Increasing pressure shifts the equilibrium towards the side with fewer moles of gas. If both sides have equal numbers of gas molecules, there is no effect on the position of equilibrium.
An increase in pressure shifts the equilibrium towards the side with the .......... number of gaseous molecules. A decrease in pressure shifts it towards the side with the .......... number of gaseous molecules.
An increase in pressure shifts the equilibrium towards the side with the smallest number of gaseous molecules. A decrease in pressure shifts it towards the side with the greatest number of gaseous molecules.
What does it mean when the position of equilibrium shifts to the right?
When the position of equilibrium shifts to the right, the forward reaction is favoured, meaning more products are formed and the concentration of reactants decreases.
What is meant by the position of equilibrium?
The position of equilibrium describes the relative amounts of reactants and products present in a system at equilibrium. It indicates whether the equilibrium lies more to the left (more reactants) or to the right (more products).
How does a catalyst affect the position of equilibrium?
A catalyst does not alter the position of equilibrium. It speeds up both the forward and reverse reactions by the same amount, so equilibrium is reached more quickly but the concentrations at equilibrium remain unchanged.
True or False?
A catalyst shifts the position of equilibrium towards the products.
False.
A catalyst does not shift the position of equilibrium. It increases the rate of both the forward and reverse reactions equally, so the same equilibrium concentrations are reached, just more quickly.
For a reaction at equilibrium, complete the table to show how temperature changes affect the position of equilibrium.
Temperature change
Direction equilibrium shifts
Reaction favoured
Increase
Decrease
For a reaction at equilibrium, complete the table to show how temperature changes affect the position of equilibrium.
Temperature change
Direction equilibrium shifts
Reaction favoured
Increase
To the left
Endothermic
Decrease
To the right
Exothermic
True or False?
Decreasing the temperature of an equilibrium reaction favours the exothermic direction.
True.
Decreasing the temperature shifts the equilibrium to oppose the change by favouring the exothermic reaction, which releases heat.
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