Solved: Automobile air bags inflate by a rapid decomposition reaction. One typical... [IB Chemistry (Old)]

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Question

SLPaper 2

Automobile air bags inflate by a rapid decomposition reaction. One typical compound used is guanidinium nitrate, $C(NH_2)_3NO_3$ , which decomposes very rapidly to form nitrogen, water vapour and carbon.

Deduce the equation for the decomposition of guanidinium nitrate.

[1]

Verified

Solution

$C(NH_2)_3NO_3 (s) → 2N_2 (g) + 3H_2O (g) + C (s)$ 1 mark

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

[2]

Verified

Solution

Calculate the moles of guanidinium nitrate:

n({C(NH_2)_3NO_3}) = \frac{10.0\,\mathrm{g}}{\mathrm{M_r}({C(NH_2)_3NO_3})} = \frac{10.0\,\mathrm{g}}{122.09\,\mathrm{g\,mol^{-1}}} = 0.0819\,\mathrm{mol}

From the balanced equation, 1 mol of ${C(NH_2)_3NO_3}$ produces 2 mol of ${N_2}$ and 3 mol of ${H_2O}$ :

n({N_2}) = 0.0819\,\mathrm{mol} \times 2 = 0.164\,\mathrm{mol}

n({H_2O}) = 0.0819\,\mathrm{mol} \times 3 = 0.246\,\mathrm{mol}

Total moles of gas produced = $n({N_2}) + n({H_2O})$ = 0.164 mol + 0.246 mol = 0.410 mol

Note

Award 1 mark for correct substitution of values and 1 mark for the correct final answer.

Calculate the pressure, in kPa, of this gas in a 10.0 $dm^3$ air bag at 127°C, assuming no gas escapes.

[2]

Verified

Solution

Convert the volume to SI units:
$V = 10.0\,\mathrm{dm^3} = 10.0 \times 10^{-3}\,\mathrm{m^3} = 0.010\,\mathrm{m^3}$
Convert the temperature to Kelvin:
$T = 127^\circ\mathrm{C} + 273.15 = 400.15\,\mathrm{K}$
Use the ideal gas equation to calculate the pressure:
$pV = nRT$
Rearranging for $p$ :
$p = \frac{nRT}{V}$
Substituting the values:
$p = \frac{(0.4095\,\mathrm{mol})(8.314\,\mathrm{J\,mol^{-1}\,K^{-1}})(400.15\,\mathrm{K})}{0.010\,\mathrm{m^3}}$ $p = 1.36 \times 10^5\,\mathrm{Pa} = 136\,\mathrm{kPa}$
Note
Award 1 mark for correct substitution of values and 1 mark for the correct final answer

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

[2]

Verified

Solution

Any two of:
nitrogen non-polar/London/dispersion forces AND water polar/H-bonding 1 mark

water has «much» stronger intermolecular forces 1 mark

water molecules attract/condense/occupy smaller volume «and therefore deviate from ideal behaviour» 1 mark

2 marks max

Another airbag reactant produces nitrogen gas and sodium.

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

[2]

Verified

Solution

$2 H_2 + O_2 \to 2 H_2O$

$4Na + 2 O_2 \to 2 Na_2O$ 1 mark

hydrogen explosive
OR
highly exothermic reaction
OR
sodium reacts violently with water
OR
forms strong alkali 1 mark

2 marks in total

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

Chemistry (Old) International Baccalaureate (IB) Practice Question: Automobile air bags inflate by a rapid decomposition reaction. One typical compound used is guanidinium nitrate, $C(NH2)...