A cooling curve is a graph that shows the change in temperature of a substance over time as it cools down from a melted state to a solid state. This type of experiment can be useful for studying the cooling behaviour of substances and can also provide information about the purity and crystalline structure of the substance. In this experiment, we will produce a cooling curve of stearic acid using a water bath set to 80 degrees Celsius and a 250 ml beaker.
Materials:
- Stearic acid (25 g)
- 100 ml beaker
- Thermometer
- Hotplate
- Stirring rod
- Water bath
- Ice bath
- Stopwatch or timer
Safety Precautions:
- Wear appropriate protective goggles.
- Be cautious when handling hot materials and equipment.
- Make sure to dispose of all chemicals properly.
Experimental Procedure:
- Set up the water bath by filling a large beaker with water and placing it on a hotplate. Heat the water to 80 degrees Celsius.
- Add 25 grams of stearic acid to a 100 ml beaker.
- Place the beaker in the water bath and melt the stearic acid by stirring with a stirring rod.
- Once the stearic acid has melted completely, remove the beaker from the water bath and start the cooling process.
- Record the temperature of the stearic acid every 10 seconds using a thermometer until it solidifies completely.
- Plot the temperature versus time data on a graph to produce the cooling curve.
Data Analysis:
- Observe the cooling curve and identify any plateau regions. These plateau regions indicate that the stearic acid is undergoing a phase change and solidifying.
- Determine the melting point of the stearic acid from the cooling curve. The melting point is the temperature at which the stearic acid changes from a solid to a liquid.
- Calculate the cooling rate of the stearic acid by dividing the temperature change by the time interval for each section of the cooling curve.
- Analyse the graph to determine the purity and crystalline structure of the stearic acid. A pure substance will exhibit a sharp melting point and a crystalline structure will have a characteristic pattern in the cooling curve.
Data Processing:
Calculate how much heat energy was released by stearic acid during freezing using this formula:
Q = m x Lf
Where Q is heat energy in joules (J), m is mass in grams (g), and Lf is latent heat of fusion in J/g. The value of Lf for stearic acid is 247 J/g.
Conclusion:
In this experiment, we have produced a cooling curve of stearic acid using a water bath set to 80 degrees Celsius and a 250 ml beaker. The cooling curve provides information about the cooling behaviour, melting point, purity, and crystalline structure of the stearic acid. The experimental procedure outlined above can be used by IB DP chemistry students to gain practical experience in the laboratory and to better understand the behaviour of materials during phase changes.
Practical Science 