| Date | May 2016 | Marks available | 4 | Reference code | 16M.2.hl.TZ0.3 |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Analyse and State | Question number | 3 | Adapted from | N/A |
Question
The reaction between hydrogen and nitrogen monoxide is thought to proceed by the mechanism shown below.
(i) State the equation for the overall reaction.
(ii) Deduce the rate expression consistent with this mechanism.
(iii) Explain how you would attempt to confirm this rate expression, giving the results you would expect.
(iv) State, giving your reason, whether confirmation of the rate expression would prove that the mechanism given is correct.
(v) Suggest how the rate of this reaction could be measured experimentally.
The enthalpy change for the reaction between nitrogen monoxide and hydrogen is −664 kJ and its activation energy is 63 kJ.
(i) Sketch the potential energy profile for the overall reaction, using the axes given, indicating both the enthalpy of reaction and activation energy.
(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, Thigher.
(iv) Explain why an increase in temperature increases the rate of this reaction.
One of the intermediates in the reaction between nitrogen monoxide and hydrogen is dinitrogen monoxide, N2O. This can be represented by the resonance structures below:
(i) Analyse the bonding in dinitrogen monoxide in terms of σ-bonds and Δ-bonds.
(ii) State what is meant by resonance.
Markscheme
(i)
2NO(g) + 2H2(g) → N2(g) + 2H2O(g)
(ii)
rate = k [NO]2[H2]
(iii)
test the effect «on the reaction rate» of varying each concentration «independently»
OR
test the effect of varying [NO] «on rate», whilst keeping [H2] constant AND test effect of varying [H2] «on rate», whilst keeping [NO] constant
rate proportional to [NO]2
OR
doubling [NO] quadruples rate
rate proportional to [H2]
OR
doubling [H2] 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.
(iv)
no AND different mechanisms could give the same rate expression
OR
no AND mechanisms can only be disproved
OR
no AND just suggest it is consistent with the mechanism given
OR
no AND does not give information about what occurs after RDS
(v)
change of pressure «at constant volume and temperature» with time
OR
change of volume «at constant pressure and temperature» with time
Accept other methods where rate can be monitored with time
(i)
products lower than reactants AND enthalpy of reaction correctly marked and labelled with name or value
activation energy correctly marked and labelled with name or value
Accept other clear ways of indicating energy/ enthalpy changes.
(ii)
lower dotted curve, between same reactants and products levels, labelled “Catalysed”
(iii)
second curve at a higher temperature is correctly drawn (maximum lower and to right of original)
(iv)
greater proportion of molecules have E ≥ Ea or E > Ea
OR
greater area under curve to the right of the Ea
greater frequency of collisions «between molecules»
OR
more collisions per unit time/second
Do not accept just particles have greater kinetic energy.
Do not accept just “more collisions”.
(i)
ALTERNATIVE 1:
σ-bond from N to N AND from N to O
π-bond from N to N
delocalized π-bond/π-electrons «extending over the oxygen and both nitrogens»
ALTERNATIVE 2:
both have 2 σ-bonds «from N to N and from N to O» AND π-bond from N to N
one structure has second π-bond from N to N and the other has π-bond from N to O
delocalized π-bond/π-electrons
Award [1 max] if candidate has identified both/either structure having 2 σ-bonds and 2 π-bonds
(ii)
more than one possible position for a multiple/π-/pi- bond
Accept “more than one possible Lewis structure”.
Accept reference to delocalisation if M3 not awarded in c (i).
Accept reference to fractional bond orders.
