IB Chemistry HL 2.2 Rate of Chemical Change Questions

Practise IB Chemistry HL 2.2 with HL questions on rate graphs, energy profiles, mechanisms, rate equations and Arrhenius analysis.

Exam points

Read concentration, volume or mass graphs, using tangents to calculate rate at a chosen time.
Use Maxwell–Boltzmann distributions and energy profiles to show activation-energy and catalyst effects.
Deduce rate equations, reaction orders, units of k and activation energy from kinetic and Arrhenius data.

Covered syllabus points

2.2.1—Rate of reaction 2.2.2—Collision theory 2.2.3—Factors affecting rate 2.2.4—Activation energy (Ea)2.2.5—Catalysts 2.2.6 (HL)—Reaction mechanisms 2.2.7 (HL)—Energy profiles 2.2.8 (HL)—Molecularity 2.2.9 (HL)—Rate equations 2.2.10 (HL)—Reaction order 2.2.11 (HL)—Rate constant (k)2.2.12 (HL)—Arrhenius equation 2.2.13 (HL)—Arrhenius factor (A)

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Study 2.2 Rate of chemical change conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

[Maximum number: 11]

The reaction between hydrogen and nitrogen monoxide is thought to proceed by the mechanism shown below.

\begin{array}{ll} 2 \mathrm{NO}(\mathrm{~g}) \rightleftharpoons \mathrm{N}_{2} \mathrm{O}_{2}(\mathrm{~g}) & \text { fast equilibrium } \\ \mathrm{N}_{2} \mathrm{O}_{2}(\mathrm{~g})+\mathrm{H}_{2}(\mathrm{~g}) \rightarrow \mathrm{N}_{2} \mathrm{O}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) & \text { slow reaction } \\ \mathrm{N}_{2} \mathrm{O}(\mathrm{~g})+\mathrm{H}_{2}(\mathrm{~g}) \rightarrow \mathrm{N}_{2}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) & \text { fast reaction } \end{array}

(a)

Deduce the rate expression consistent with this mechanism.

[ 1 ]

rate $=k[\mathrm{NO}]^{2}\left[\mathrm{H}_{2}\right]$

(b)

Explain how you would attempt to confirm this rate expression, giving the results you would expect.

[ 3 ]

test the effect «on the reaction rate» of varying each concentration «independently»

OR

test the effect of varying [NO] «on rate», whilst keeping $\left[\mathrm{H}_{2}\right]$ constant AND test effect of varying $\left[\mathrm{H}_{2}\right]$ «on rate», whilst keeping [NO] constant

rate proportional to [NO] ${ }^{2}$

OR

doubling [NO] quadruples rate

rate proportional to $\left[\mathrm{H}_{2}\right]$

OR

doubling $\left[\mathrm{H}_{2}\right]$ doubles rate

Remember to refer back to a (ii) for ECF.

If only one species in rate expression, third mark can be awarded for zero order discussion.

(c)

State, giving your reason, whether confirmation of the rate expression would prove that the mechanism given is correct.

[ 1 ]

(d)

Suggest how the rate of this reaction could be measured experimentally.

[ 1 ]

(e)

The enthalpy change for the reaction between nitrogen monoxide and hydrogen is -664 kJ and its activation energy is 63 kJ .

[ 5 ]

(i)

Sketch the potential energy profile for the overall reaction, using the axes given, indicating both the enthalpy of reaction and activation energy.

[ 2 ]

(ii)

This reaction is normally carried out using a catalyst. Draw a dotted line labelled "Catalysed" on the diagram above to indicate the effect of the catalyst.

$(iii) Sketch and label a second Maxwell-Boltzmann energy distribution curve representing the same system but at a higher temperature, \(T_{\text {higher$

(iii) Sketch and label a second Maxwell-Boltzmann energy distribution curve representing the same system but at a higher temperature, \(T_{\text {higher

[ 1 ]

(iii)

Explain why an increase in temperature increases the rate of this reaction.

Figure for Question (iii) — IB Chemistry HL

[ 2 ]

[Maximum number: 4]

Sulfur trioxide is an important compound in industry.

(a)

$\quad \mathrm{SO}_{3}(\mathrm{~g})$ is made using the contact process.

2 \mathrm{SO}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{SO}_{3}(\mathrm{~g}) \quad \Delta H^{\ominus}<0

[ 4 ]

(i)

Explain why increasing the temperature increases the rate of reaction.

[ 2 ]

more molecules/collisions have energy $\geq E_{a}$

frequency/probability of «successful» collisions increases

Marking guidance:

Do not accept "more collisions" in M2
without reference to time or probability.

(ii)

Vanadium pentoxide, $\mathrm{V}_{2} \mathrm{O}_{5}$ , is used as a catalyst. Explain how a catalyst increases the rate of a reaction.

[ 2 ]

«provides» alternative reaction pathway/mechanism

lowers $E_{a}$

OR

more molecules/collisions have sufficient energy/energy $\geq E_{a}$

Marking guidance:

Accept description of how catalyst
lowers $E_{a}$ such as "reactants adsorb on
surface «of catalyst»", "reactant bonds
weaken «when adsorbed»" for M1.

[Maximum number: 5]

When nitrogen gas and hydrogen gas are allowed to react in a closed container the following equilibrium is established.

\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H=-92.6 \mathrm{~kJ}

(a)

Define the term activation energy, $E_{\mathrm{a}}$ .

[ 1 ]

(b)

Ammonia is manufactured by the Haber process in which iron is used as a catalyst. Explain the effect of a catalyst on the rate of reaction.

[ 2 ]

(c)

Typical conditions used in the Haber process are $500^{\circ} \mathrm{C}$ and 200 atm , resulting in approximately 15 % yield of ammonia.

[ 2 ]

(i)

Explain why a temperature lower than $500^{\circ} \mathrm{C}$ is not used.

[ 2 ]

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Question 2

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Question 3

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