School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Durban
Chemical Reactivity – CHEM120
Tutorial 2 – 7 August 2013
1.
Eugenol, the active ingredient in cloves, has a formula, C10H12O2. What is the boiling
point of a solution containing 0.154 g of this compound dissolved in 11.3 g of
benzene? (Kbp = 2.53 °C m-1 and boiling point of benzene = 80.1 °C)
Moles of eugenol = 0.154 g/164.2 g mol-1 = 9.38 × 10-4 mol
Molality = 9.38 × 10-4 mol/1.13 × 10-2 kg = 8.30 × 10-2 m
Tbp = Kbp × m  Tbp = 2.53 °C m-1 × 0.083 m = 0.210 °C
Boiling point of solution = 80.1 °C + 0.210°C = 80.310 °C
2.
What mass of ethylene glycol must be added to 5.50 kg of water to lower the freezing
point of the water from 0.0 °C to -10.0 °C? (Kfp = -1.86 °C m-1)
From Tfp = Kfp × m
m = Tfp/ Kfp  m = -10 °C/-1.86 °C m-1 = 5.38 m
m = nethylene glycol/mwater
nethylene glycol = 5.38 mol kg-1 × 5.50 kg = 29.59 mol
methylene glycol = 29.59 mol × 62.1 g mol-1 = 1838 g = 1.84 kg
3.
Write the equilibrium constant expressions Kc for each of the following reactions:
(a)
2H2S(g) ⇌ 2H2(g) + S2(g)
(b)
PCl3(g) + 3NH3(g) ⇌ P(NH2)3(g) + 3HCl(g)
(c)
Na2CO3(s) + SO2(g) + ½O2(g) ⇌ Na2SO4(s) + CO2(g)
Kc 
[H 2 ]2 [S2 ]
[H 2S] 2
[P(NH2 ) 3 ][HCl]3
Kc 
[PCl3 ][NH3 ]3
Kc 
[CO 2 ]
[SO 2 ][O 2 ]
4.
1
2
The equilibrium constant Kc for the equation
2HI(g) ⇌ H2(g) + I2(g)
At 425 °C is 1.84. What is the value of Kc for the following equation?
H2(g) + I2(g) ⇌ 2HI(g)
Kc 
[H 2 ][I 2 ]
 1.84
[HI]2
[HI]2
1
Kc 

 0.543
[H 2 ][I 2 ] 1.84
5.
Consider the system
4NH3(g) + 3O2(g) ⇌ 2N2(g) + 6H2O(l)
ΔH = -1530.4 kJ
How will the amount of ammonia at equilibrium be affected by
(a)
(b)
(c)
(d)
(e)
removing O2(g)?
adding N2(g)?
adding water?
expanding the container at constant pressure?
increasing the temperature?
(a)
(b)
(c)
(d)
(e)
Increase
Increase
Increase
Increase
increase