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Date	November 2019	Marks available	1	Reference code	19N.2.sl.TZ0.6
Level	SL	Paper	2	Time zone	TZ0
Command term	Calculate	Question number	6	Adapted from	N/A

Question

Automobile air bags inflate by a rapid decomposition reaction. One typical compound used is guanidinium nitrate, C(NH₂)₃NO₃, which decomposes very rapidly to form nitrogen, water vapour and carbon.

Deduce the equation for the decomposition of guanidinium nitrate.

[1]

a(i).

Calculate the total number of moles of gas produced from the decomposition of 10.0 g of guanidinium nitrate.

[1]

a(ii).

Calculate the pressure, in kPa, of this gas in a 10.0 dm³ air bag at 127°C, assuming no gas escapes.

[1]

a(iii).

Suggest why water vapour deviates significantly from ideal behaviour when the gases are cooled, while nitrogen does not.

[2]

a(iv).

Another airbag reactant produces nitrogen gas and sodium.

Suggest, including an equation, why the products of this reactant present a safety hazard.

[2]

Markscheme

C(NH₂)₃NO₃(s) → 2N₂(g) + 3H₂O (g) + C (s) ✔

a(i).

moles of gas = « $\frac{{10.0{\text{g}}}}{{122.11{\text{g mo}}{{\text{l}}^{ - 1}}}}$ » 0.409 «mol» ✔

a(ii).

« $p = \frac{{0.409{\text{mol}} \times {\text{8}}{\text{.31 J }}{{\text{K}}^{ - 1}}{\text{mo}}{{\text{l}}^{ - 1}} \times \left( {127 + 273} \right){\text{K}}}}{{{\text{10}}{\text{.0 d}}{{\text{m}}^3}}}$ » = 136 «kPa» ✔

a(iii).

Any two of:
nitrogen non-polar/London/dispersion forces AND water polar/H-bonding ✔
water has «much» stronger intermolecular forces ✔
water molecules attract/condense/occupy smaller volume «and therefore deviate from ideal behaviour» ✔

a(iv).

2Na (s) + 2H₂O (l) → 2NaOH (aq) + H₂(g) ✔

hydrogen explosive
OR
highly exothermic reaction
OR
sodium reacts violently with water
OR
forms strong alkali ✔

NOTE: Accept the equation of combustion of hydrogen.
Do not accept just “sodium is reactive/dangerous”.