WC3_ 10_04

Which among the following is the formula for Gibbs free energy?

• H-T∆S

• H-S

• T∆S

• H-T

• The sign of Δ G predicts whether the formation of the products in a reaction is favored. If the value is positive, the formation of the products in a reaction is

• • not favored

• • favored

• • favored some of the time

Which of the following statements is NOT true in relation to the triple point on a single component phase diagram?

• The point at which the solid, liquid and gaseous phases for a substance co-exist

• The triple point exists for a substance occurs at a specific temperature and pressure

• The triple point exists at a single temperature and is independent of pressure

• The system must be enclosed so that no vapour can escape

Answer explanation

The triple point for a pure substance occurs in a closed system at a set temperature and pressure (i.e. one cannot be varied with the other).

Which of the following statements is NOT true in relation to a system that has reached the critical temperature?

• The system must a closed system

• The system can be described as a homogenous fluid

• Substances can have more than one critical temperature.

• A surface between the liquid and vapour phases is no longer present.

Answer explanation

The critical temperature is reached by heating a liquid in a closed system until the point where the density of the liquid is equal to the density of the vapour. At this point the interface between the liquid and vapour disappears resulting in a homogeneous fluid. This can only occur in a closed system (otherwise the vapour would escape into the surroundings) and each substance has a single critical temperature.

In which of the following case the process of sublimation can occur?

• 
  Solid nitrogen

• 
  Solid carbondioxide

• Solid oxygen

• Sodium