A-LevelChemistryGroup 29701/43 · May/June 2019 · Q29 Marks

Group 2 nitrates decompose when heated.

(a)Write an equation for the decomposition of strontium nitrate.[1]
(b)Describe and explain how the thermal stability of Group 2 nitrates changes with increasing atomic number.[3]
(c)(i)The variation in the thermal stability of Group 2 amides is similar to that of Group 2 nitrates. Suggest whether calcium amide, Ca(NH2)2, will decompose more or less readily than barium amide, Ba(NH2)2. Explain your answer.[1]
(c)(ii)Ba(NH2)2 decomposes when heated to form barium nitride, Ba3N2, and ammonia as the only products. Write an equation for this reaction.[1]
(d)Ba(NH2)2 contains the NH2¯ ion. Predict the bond angle of NH2¯. Explain your answer using the qualitative model of electron-pair repulsion. bond angle explanation[3]

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[1] (a) Sr(NO3)2 → SrO + 2NO2 + 1/2O2
[1] (b) M1 increases
[1] (b) M2 cationic radius / ion size increases (down the group)
[1] (b) M3 less polarisation/distortion of anion / nitrate ion / NO3-/ nitrate group
[1] (c)(i) more readily and Ca2+ has a smaller ionic radius or more readily and Ca2+ has a greater charge density
[1] (c)(ii) 3Ba(NH2)2 → Ba3N2 + 4NH3
[1] (d) M1 bond angle 104–105°
[1] (d) M2 explanation two lone pairs and two bonding pairs
[1] (d) M3 lone pairs repel more

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