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

Exam-style multiple choice and written questions with full markschemes. Check multiple choice answers straight away, or type, photograph or draw a written answer and get it marked.

Type
Multiple choiceWritten
Difficulty
EasyMediumHard

Question 1

Multiple choiceMedium1 mark

For the equilibrium 2A(g)+B(g)⇌2C(g)+D(g)2\text{A}(g)+\text{B}(g)\rightleftharpoons2\text{C}(g)+\text{D}(g) the equilibrium concentrations at a fixed temperature are [A]=0.50[\text{A}]=0.50 [B]=0.20[\text{B}]=0.20 [C]=0.10[\text{C}]=0.10 and [D]=0.10 mol dm−3[\text{D}]=0.10\ \text{mol dm}^{-3} What is KcK_c and which side is favoured?

Show answer and explanation

Answer: D. The stoichiometric coefficients are the powers in the equilibrium expression:

Kc=[C]2[D][A]2[B]=(0.10)2(0.10)(0.50)2(0.20)=0.020K_c=\frac{[\text{C}]^2[\text{D}]}{[\text{A}]^2[\text{B}]}=\frac{(0.10)^2(0.10)}{(0.50)^2(0.20)}=0.020

(dimensionless). Since Kc<1K_c<1 reactants are favoured at equilibrium.

A: calculates KcK_c correctly but reverses the implication of Kc<1K_c<1
B: omits the square on [C][\text{C}] obtaining 0.200.20 this is still less than 1.
C: uses the reciprocal expression, which gives the equilibrium constant for the reverse reaction.

Remember: a value of Kc<1K_c<1 indicates that reactants predominate at equilibrium.

Question 2

Paper 2Hard9 marks

In a sealed, fixed-volume vessel, nitrogen and oxygen form nitrogen monoxide at 1500 K:

N₂(g) + O₂(g) ⇌ 2NO(g)

At this temperature, Kc = 4.00 × 10⁻⁶. At a particular instant, the concentrations are [N₂] = 0.0500 mol dm⁻³, [O₂] = 0.0400 mol dm⁻³ and [NO] = 1.00 × 10⁻⁵ mol dm⁻³.

(a)

Calculate Qc for this mixture and predict the direction in which the reaction will proceed to reach equilibrium.

[2]
(b)

Determine the equilibrium concentrations of the three gases. Show how you check that any approximation used is justified.

[4]
(c)

After the mixture reaches equilibrium, an injection increases [NO] by 2.00 × 10⁻⁴ mol dm⁻³. Immediately after the injection, before further reaction occurs, calculate Qc and predict the direction in which the reaction will proceed.

[3]

The best questions are still waiting

Unlock every question in this topic, then type, photograph or draw your answer and have it marked against the markscheme. Find out where you are dropping marks.

Unlock all questionsCompare Student and Student Plus