6.6 Dynamic equilibrium
5 exam questions worth 9 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What is meant by dynamic equilibrium?
Mark scheme
- rates of forward and reverse reactions are equal (1)
- concentrations / amounts of reactants and products stay constant (1)
Model answer
Dynamic equilibrium is when the forward and reverse reactions happen at the same rate, so the amounts of reactants and products stay constant.
Why can equilibrium only be reached in a closed system?
Mark scheme
so no reactants / products can escape (and the reverse reaction can occur)
Model answer
So that no products or reactants can escape, letting the reverse reaction happen as well.
A student says: "At equilibrium, the reaction has stopped." Explain why this is wrong.
Mark scheme
- forward and reverse reactions are still happening (1)
- at the same rate, so amounts do not change (1)
Model answer
Both reactions are still happening; they just happen at the same rate, so the amounts of substances do not change.
At equilibrium, which statement is always true?
Choose one answer.
Correct Not quite: the answer is The rates of the forward and reverse reactions are equal
Mark scheme
B
Model answer
B: the rates of the forward and reverse reactions are equal.
Describe how the rates of the forward and reverse reactions change from the moment reactants are mixed until equilibrium is reached.
Mark scheme
- forward rate starts fast and decreases (as reactants are used up) (1)
- reverse rate starts at zero and increases (as products form) (1)
- until the rates are equal (1)
Model answer
The forward reaction starts fast and slows down as reactants are used up. The reverse reaction starts at zero and speeds up as products build up, until both rates are equal.
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