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PHASE DIAGRAM: UREA Vs BENZOIC ACID

AIM

To construct a phase diagram for a binary system consisting of urea and benzoic acid and find the eutectic composition and temperature.

PRINCIPLE

The number of phases that exist together when the system is in equilibrium depend upon the conditions of temperature and composition. These conditions are determined experimentally and then plotted on a graph. The diagram so obtained giving the conditions of equilibria between various phases of substance is called phase diagram.

Eutectic point is where two curves AO and BO meet together. At this point three phases (solid urea, solid benzoic acid and their liquid solution) co-exist. Eutectic point ‘O’ represents the lowest possible temperature (70–75°C) at which liquid phase cannot be enriched in either component by freezing out the other component. The temperature (70–75°C) and composition of the components (50% urea, 50% benzoic acid) corresponding to the eutectic point are called the eutectic temperature and eutectic composition. According to reduced phase equation eutectic point has no degree of freedom. So it is invariant.

PROCEDURE

Fit a boiling test tube with a two-holed cork, and insert a stirrer and a thermometer into the holes. Take approximately 4 g of urea in the test tube. Heat this in a dry glycerine bath. The temperature remains constant for a range of time at the melting point. After complete melting, cool it and note the temperature at which it solidifies (freezing point). In a similar way, repeat the experiment with varying compositions of urea and benzoic acid from 0 to 100%. Note the freezing points. Similar way determine the freezing point of unknown composition. Plot them against the percentage composition. Calculate the eutectic composition, unknown composition, and temperature from the graph.

GRAPH

RESULT

The phase diagram is constructed for urea and benzoic acid.

From the graph

Eutectic temperature = _____ °C.

Eutectic composition = _____% of urea, _____% benzoic acid.

Unknown mixture composition = _____% of urea, _____% of benzoic acid.

VIVA QUESTIONS

1. Define eutectic point.
2. What is the melting point of urea?
3. What is the melting point of benzoic acid?
4. What is the necessity of using the glycerine bath?
5. What is the principle involved in the phase rule?
6. How many components are involved in this phase rule experiment?
7. Define freezing point.
8. What is the importance of eutectic mixtures?
9. Give the phase rule equation.
10. Give the phase rule equation for condensed systems.