Reversible Reactions and Dynamic Equilibrium

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Equilibria

Last updated 6:41 AM on 8/31/26
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29 Terms

1
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What is a reversible reaction?
A reaction in which the products can react to reform the reactants.
2
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What symbol is used to represent a reversible reaction?
⇌
3
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What is the forward reaction?
The reaction in which the reactants shown on the left-hand side of the equation form the products.
4
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What is the backward reaction?
The reaction in which the products react to reform the original reactants.
5
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What is an irreversible reaction?
A reaction that effectively proceeds to completion with negligible amounts of reactants remaining.
6
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Why are most combustion reactions considered irreversible?
They are highly exothermic and the reverse reaction is extremely unlikely under the same conditions.
7
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When is a reaction usually considered to have gone to completion in practical terms?
When it is more than 99% complete.
8
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How can you tell that a reaction is reversible?
Detectable amounts of both reactants and products remain because both the forward and backward reactions can occur.
9
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What is the reversible reaction between hydrogen and iodine?
H₂(g) + I₂(g) ⇌ 2HI(g)
10
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What happens when hydrogen and iodine are heated together in a closed container at 573 K?
They react to form hydrogen iodide, but some hydrogen and iodine remain unreacted when equilibrium is reached.
11
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What happens when hydrogen iodide is heated to 573 K in a closed container?
It partially decomposes into hydrogen and iodine until equilibrium is reached.
12
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What is equilibrium?
A state in which there is no further change in the concentrations of reactants and products.
13
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What is dynamic equilibrium?
A state in a closed system where the forward and backward reactions continue at equal rates, so the concentrations of reactants and products remain constant.
14
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Why is equilibrium described as dynamic?
The forward and backward reactions are still continuously occurring even though there is no overall change in concentrations.
15
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What two conditions are required for dynamic equilibrium to be established?
The reaction must be reversible and the reaction mixture must be in a closed system.
16
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Why is a closed system needed for dynamic equilibrium?
It prevents reactants or products from escaping, allowing both the forward and backward reactions to continue.
17
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What happens to the forward reaction rate as equilibrium is approached?
It decreases as the concentrations of the reactants decrease.
18
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What happens to the backward reaction rate as equilibrium is approached?
It increases as the concentration of the products increases.
19
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What happens to the forward and backward reaction rates at equilibrium?
They become equal.
20
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Do reactions stop when equilibrium is reached?
No. Both the forward and backward reactions continue.
21
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What happens to the concentrations of reactants and products at equilibrium?
They remain constant.
22
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Are the concentrations of reactants and products equal at equilibrium?
Not necessarily. They are constant, but they do not have to be equal.
23
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Are the rates of the forward and backward reactions equal at equilibrium?
Yes.
24
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At the start of a reversible reaction containing only reactants, how does the forward reaction rate compare with the backward reaction rate?
The forward reaction rate is high while the backward reaction rate is initially zero.
25
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Why does the forward reaction rate decrease with time?
Reactants are used up, so their concentrations decrease and there are fewer successful collisions per second.
26
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Why does the backward reaction rate increase with time?
Products accumulate, increasing their concentrations and therefore increasing the frequency of successful collisions.
27
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When is dynamic equilibrium established?
When the forward and backward reaction rates become equal.
28
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What happens after dynamic equilibrium has been established?
The forward and backward reactions continue at equal rates and the concentrations of all reactants and products remain constant.
29
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What would a rate–time graph show when equilibrium is reached?
The forward and backward reaction rate curves meet and then remain at the same constant rate.