IB Chemistry SL R2.3 How far? The extent of chemical change questions

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Question 1

Multiple choiceHard1 mark

In a sealed vessel, the concentrations in the reversible system N2(g)+3H2(g)⇌2NH3(g)\mathrm{N_2(g)+3H_2(g)\rightleftharpoons 2NH_3(g)} become constant while all three gases remain present. Which account correctly describes the reaction rates at this point and gives the expression for KcK_c

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Answer: B. Constant concentrations mean there is no net change, so the forward and backward rates are equal. At dynamic equilibrium, both reactions continue, making these rates non-zero. The equilibrium expression uses each concentration raised to its coefficient: Kc=[NH3]2[N2][H2]3K_c=\dfrac{[\mathrm{NH_3}]^2}{[\mathrm{N_2}][\mathrm{H_2}]^3}

A: takes the reciprocal, placing reactants over products.
C: treats equilibrium as static and sets both reaction rates to zero.
D: unequal rates would cause net formation of NH3\mathrm{NH_3} so concentrations would change.

Remember: At equilibrium, concentrations are constant; the rates, not the concentrations, are equal.

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