Rate Expression & Reaction Mechanism (DP IB Chemistry: HL)

For the reaction below, consider the following experimental data.
 

X (aq) + Y (aq) → Z (aq)

Deduce the order of reaction with respect to X.

Deduce the order of the reaction with respect to Y.

Write the rate expression for the reaction between X and Y.

Determine the rate constant, k, correct to three significant figures and state its units, using data from Experiment 2. 

Explain why the reaction represented below is a redox reaction. 

2ClO₂ (aq) + 2NaOH (aq) → NaClO₃ (aq) + NaClO₂ (aq) + H₂O (l) 

For the reaction below, consider the following experimental data. 

2ClO₂ (aq) + 2OH^- (aq) → ClO₃^- (aq) + ClO₂^- (aq) + H₂O (l)

Deduce the rate expression.

Determine the rate constant, k, and state its units, using data from Experiment 3. 

Calculate the rate when [ClO₂ (aq)] = 3.10 x 10^-2 mol dm^-3 and [OH^- (aq)] = 1.50 x 10^-2 mol dm^-3.

Sketch a graph to show how the rate constant, k, varies with temperature.

The following mechanism is proposed for the reaction where ethanal dimerises in dilute alkaline solution to form 3-hydroxybutanal: 

Step 1: CH₃CHO + :OH^- → :CH₂CHO + H₂O 
Step 2: CH₃CHO + :CH₂CHO → CH₃CH(O:^-)CH₂CHO
Step 3: CH₃CH(O:^-)CH₂CHO + H₂O → CH₃CH(OH)CH₂CHO + :OH^- 

Classify OH^-, CH₂CHO and CH₃CH(O:^-)CH₂CHO as reactant, product, catalyst or intermediate, based on the proposed mechanism.

Using the following information about the proposed mechanism, deduce the rate expression. 

Step 1: CH₃CHO + :OH^- → :CH₂CHO + H₂O                                                slow step
Step 2: CH₃CHO + :CH₂CHO → CH₃CH(O:^-)CH₂CHO                                fast step
Step 3: CH₃CH(O:^-)CH₂CHO + H₂O → CH₃CH(OH)CH₂CHO + :OH^-         fast step

Calculate the initial rate of reaction for experiment 2, if measured under the same conditions.

State the effect, if any, increasing the concentration of a reactant would have on the value of the rate constant, k.

Nitrogen dioxide and carbon monoxide react according to the following equation. 

            NO₂ (g) + CO (g) → NO (g) + CO₂ (g) 

Using the following graph, what is the order with respect to NO₂?

 

The rate expression for the reaction of nitrogen dioxide and carbon monoxide at T < 227 ^OC is: 

            Rate = k [NO₂]² 

Sketch a labelled graph of concentration against time for carbon monoxide.

A student proposed the following single step mechanism for the reaction of nitrogen dioxide and carbon monoxide. 

NO₂ + CO → NO + CO₂         slow 

Rate = k [NO₂]² 

Justify whether the student’s proposed mechanism is correct.

Another student proposed the following mechanism for the reaction of nitrogen dioxide and carbon monoxide. 

Step 1:             NO₂ + NO₂ → NO + NO₃
   Step 2:             NO₃ + 2CO → NO + 2CO₂ 

Rate = k [NO₂]² 

Explain which of the student’s proposed mechanism steps is the rate determining step.

Nitrogen(II) oxide is oxidised according to the following equation. 

2NO (g) + O₂ (g) → 2NO₂ (g) 

The following mechanism is proposed for the two-step oxidation of nitrogen(II) oxide. 

Step 1:             NO (g) + NO (g) → N₂O₂ (g)
   Step 2:             N₂O₂ (g) + O₂ (g) → 2NO₂ (g) 

The potential energy profile for this two-step reaction is shown.

Explain which step is the rate determining step.

Energy profile diagrams give evidence for or against a proposed mechanism or proposed rate expression.

Explain why the following reaction between iodide ions and peroxodisulfate ions has a high activation energy. 

S₂O₈^2- (aq) + 2I^- (aq) → 2SO₄^2- (aq) + I₂ (aq)

Sketch the potential energy diagram for the reaction of iodide ions with peroxodisulfate ions catalysed by iron(II) ions according to the following mechanism. 

2Fe²⁺ (aq) + S₂O₈^2- (aq) → 2Fe³⁺ (aq) + 2SO₄^2- (aq)             slow
2Fe³⁺ (aq) + 2I^- (aq) → 2Fe²⁺ (aq) + I₂ (aq)                             fast

Deduce the rate expression for the reaction of iodide ions with peroxodisulfate ions catalysed by iron(II) ions according to the following mechanism. 

2Fe²⁺ (aq) + S₂O₈^2- (aq) → 2Fe³⁺ (aq) + 2SO₄^2- (aq)             slow
2Fe³⁺ (aq) + 2I^- (aq) → 2Fe²⁺ (aq) + I₂ (aq)                               fast

The conversion of hydrogen and iodine into hydrogen iodide proceeds via a three step reaction mechanism:

1. I₂ (g)   2I (g)                   fast
2. H₂ (g) + I (g)  H₂I (g)      fast
3. H₂I (g) + I (g) → 2HI (g)    slow

Write the rate equation for this reaction and show how the mechanism is consistent with the stoichiometric equation.

An investigation into the rate of reaction between hydrogen and iodine was carried out at 298 K and the data obtained is shown below.

Determine the rate equation for the reaction and justify your answer.

Calculate the rate constant using Expt 2 data, including its units.

Using section 11 of the Data booklet, determine whether the forward reaction is favoured by an increase in temperature.

The reaction between iodide ions and persulfate ions is a 'clock' reaction and often used to study reaction kinetics.

2I^- (aq) + S₂O₈^2- (aq) → I₂ (aq) + 2SO₄^2- (aq)

Deduce the redox changes taking place in the reaction.

A persulfate-iodide clock reaction was studied and the following rate data obtained.

Deduce the order with respect to persulfate ions and iodide ions.

Determine the rate equation for the reaction and calculate rate constant, including the units.

Four mechanisms are proposed for the persulfate-iodide reaction. Deduce which mechanism(s) is/are consistent with the rate equation in part c) and justify your answer.

Mechanism 1:

1. I^- (aq) + I^- (aq) → I₂^2- (aq)                                         slow
2.  I₂^2- (aq) + S₂O₈^2- (aq) → I₂ (aq) + 2SO₄^2- (aq)    fast

Mechanism 2:

1. I^- (aq) + S₂O₈^2- (aq) → S₂O₈I^3- (aq)                     slow
2.  S₂O₈I^3- (aq) + I^- (aq) → I₂ (aq) + 2SO₄^2- (aq)    fast

Mechanism 3:

1. I^- (aq) + S₂O₈^2- (aq)  → S₂O₈I^3- (aq)                    fast
2.  S₂O₈I^3- (aq) + I^- (aq) → I₂ (aq) + 2SO₄^2- (aq)    slow

Mechanism 4:

1.  2I^- (aq) + S₂O₈^2- (aq) → I₂ (aq) + 2SO₄^2- (aq)    slow

The reaction between nitrogen monoxide and hydrogen produces nitrogen and water:

2NO (g) + 2H₂ (g) →  N₂ (g) + 2H₂O (g)

Rate data for this reaction is shown below.

What is the molecularity of the reaction?

Draw a sketch graphs of:

Suggest a possible mechanism for the reaction.

Suggest a Lewis structure for N₂O₂ and draw the shape of the molecule.

The rate of reaction between manganate(VII) ions and oxalate ions, C₂O₄^2-, can be investigated by measuring how the concentration of manganate(VII) varies with time.

2MnO₄^- (aq) + 16H⁺ (aq) + 5C₂O₄^2-  →  2Mn²⁺(aq) + 8H₂O(l) + 10CO₂(g) 

The rate is first order with respect to oxalate ions and the general rate equation for the reaction is:

rate = k [MnO₄^-]^p[C₂O₄^2-]^q[H⁺]^r

[1]

[2]

The student used an acid concentration of 1.0 mol dm^-3. She then varied it, keeping the other concentrations constant. She measured the rate of reaction and found the following results:

Identify the relationship between the relative rate of reaction and H⁺, and hence determine the order of reaction with respect to H⁺ ions.

The student varied the concentration of [MnO₄^-] and plotted the rate against time at three different concentrations:

[2]

The student then measured the reaction time for different concentrations of C₂O₄^2- and obtained a curve as follows:

Comment on the shape of the graph.

A reaction proceeds by a three step mechanism. The energy profile for the reaction is shown below:

Explain the difference between points A, C, E and B, D on the profile.

Deduce which step is the rate determining step of the reaction, giving a reason.