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 $\mathrm{N_2(g)+3H_2(g)\rightleftharpoons 2NH_3(g)}$ become constant while all three gases remain present. Which account correctly describes the reaction rates at this point and gives the expression for $K_c$

Show answer and explanation

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: $K_c=\dfrac{[\mathrm{NH_3}]^2}{[\mathrm{N_2}][\mathrm{H_2}]^3}$

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 $\mathrm{NH_3}$ so concentrations would change.

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

Question 2

Paper 2Hard12 marks

At 298 K, dinitrogen tetroxide gas dissociates to nitrogen dioxide gas in two sealed, rigid vessels. Each vessel initially contains only dinitrogen tetroxide. Figure 1 shows the structure of dinitrogen tetroxide.

The concentrations were measured over time. The final readings are treated as equilibrium concentrations for calculating $K_\mathrm{c}$

Time / s	Run A [N₂O₄] / mol dm⁻³	Run A [NO₂] / mol dm⁻³	Run B [N₂O₄] / mol dm⁻³	Run B [NO₂] / mol dm⁻³
0	0.0800	0.0000	0.1200	0.0000
20	0.0650	0.0300	0.0960	0.0480
40	0.0560	0.0480	0.0850	0.0700
60	0.0510	0.0580	0.0820	0.0760
80	0.0500	0.0600	0.0817	0.0767
100	0.0500	0.0600	0.0817	0.0767
120	0.0500	0.0600	0.0817	0.0767

(a)

Use Figure 1 to write a balanced equation, including state symbols, for the dissociation of dinitrogen tetroxide gas into nitrogen dioxide gas.

[2]

(b)

For Run A, calculate the ratio of the increase in [NO₂] to the decrease in [N₂O₄] between $t=0$ and $t=80\ \mathrm{s}$

[1]

(c)

Write the equilibrium law expression for $K_\mathrm{c}$ for the reaction in (a).

[2]

(d)

A researcher claims that $K_\mathrm{c}$ is greater in Run B because its equilibrium concentration of NO₂ is greater. Determine whether the data support this claim. Include calculations for both runs and a conclusion.

[5]

(e)

A student argues that Run B cannot be at equilibrium because the concentrations of N₂O₄ and NO₂ are unequal. Evaluate this argument using the definition of dynamic equilibrium and the time data.

[2]

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