— Physical Chemistry Chemistry Question
Structure and Chemistry of Sodium Chloride
The unit cell of NaCl is shown below:
[VISUAL]
(i) What is the coordination number of Na+ and Cl-?
(ii) What is the number of formula units in the unit cell?
(iii) If the length of the unit cell of NaCl is 560 pm and the mass of NaCl is 58.5 g mol-1, what is the density of NaCl?
Model Answer
(i) 6 and 6
(ii) 4
(iii) 2.21 g cm-3
Density = m/V
V = a^3 = (560 pm)^3 = 1.76×10^-22 cm^3
m = (4 × 58.5 g) / (6.022×10^23) = 3.89×10^-22 g
Density = 3.89×10^-22 g / 1.76×10^-22 cm^3 = 2.21 g cm-3
NaCl can be prepared by a reaction between Na(s) and Cl2(g):
Na(s) + ½ Cl2(g) → NaCl ΔHformation = -411 kJ
(i) What are the values of n and l for the valence electrons of a Na atom?
(ii) Compare the size of Na vs. Cl and Na+ vs. Cl-.
(iii) Draw the Lewis structure of Cl2.
(iv) Calculate the lattice energy of NaCl.
Given data:
ΔHsublimation for Na = 107 kJ mol-1
IE1 for Na = 496 kJ mol-1
Cl-Cl dissociation = 244 kJ mol-1
electron affinity of Cl = -349 kJ mol-1
Model Answer
(i) n = 3, l = 0
(ii) Na is larger than Cl, but Cl- is larger than Na+.
(iii) [VISUAL]
(iv) -787 kJ mol-1
ΔHoverall = -411 kJ mol-1 = ΔHsublimation for Na + IE1 for Na + ½(Cl-Cl dissociation) + electron affinity of Cl + lattice energy
-411 kJ mol-1 = 107 kJ mol-1 + ½ (224) kJ mol-1 + 496 kJ mol-1 - 349 kJ mol-1 + lattice energy
lattice energy = -787 kJ mol-1
Chemistry of NaCl:
(i) Write the reaction between NaCl(aq) and Br2(l).
(ii) Write the ionic equation between NaCl(aq) and AgNO3(aq).
(iii) What is the flame color when NaCl is placed in the flame?
Model Answer
(i) Br2(l) + Cl-(aq) → no reaction
(ii) Cl-(aq) + Ag+(aq) → AgCl(s)
(iii) Yellow