Kp

Question 1

For this reaction at 1050 K the equilibrium constant, Kp = 7.62 x 105 Pa For this reaction at 500 K the equilibrium constant, Kp = 3.94 x 104 Pa
Explain how this information can be used to deduce that the forward reaction is
endothermic.

Answer 1

• Kp is higher at higher temperature or converse
• At higher temperature more dissociation occurs / more products are formed / equilibrium shifts to the right/forward direction

Question 2

2 SO2(g) + O2(g) ⇌ 2 SO3(g)

State and explain the effect, if any, of a decrease in overall pressure on the
equilibrium yield of SO3

Answer 2

• decreases yield
• So equilibrium shifts to side with more moles/molecules or more moles/molecules on LHS
• So equilibrium shifts (to left side) to oppose decrease in pressure OR to increase pressure

Question 3

State the effect, if any, of the addition of a catalyst on the value of Kp for this equilibrium.
Explain your answer.

Answer 3

• No effect
• Increases rate of forward and backward reaction equally

Question 4

The melting point of XeF4 is higher than the melting point of PF3 Explain why the melting points of these two compounds are different.
In your answer you should give the shape of each molecule, explain why each molecule has that shape and how the shape influences the forces that affect the
melting point.

Answer 4

1a XeF4 4BP and 2LP around Xe
1b PF3 3BP and 1LP around P
2a XeF4 is square planar
2b PF3 is pyramidal (allow tetrahedral)
2c Electron pairs repel as far as possible or Lone pair repels more than bonding pairs
3a XeF4 has vdw forces and PF3 has dipole- dipole forces (and vdw)
3b Stronger/more intermolecular forces in XeF4 3c Due to larger Mr or more electrons