Activity 2: Energy Needed to Melt Ice

Major Chemical Concept

Heat flows from warmer to cooler bodies. In a closed system, the heat energy lost by the warmer body must equal the heat energy gained by the cooler body. The Styrofoam Ò cup reduces the heat flow between the water and the outside room to a minimum. The volume inside the cup can be considered to be a closed system where there is no inflow or outflow of heat or matter. Hence, the heat energy exchange can be considered to be only between the warm water in the cup and the ice which the students are adding to the cup.

By knowing the mass of warm water and the temperature change which it undergoes, one can determine the heat lost by the warm water when the ice is added. This is the same quantity of heat which is overcoming the attractions in the ice crystal and causing the ice to melt. The mass of ice melted can be determined. Since the mass of ice melted and the heat energy used to melt it are known, the molar heat of fusion is just a quick calculation away.

Level

Introductory, first-year high school chemistry.

Expected Student Background

See Related Concepts and Related Skills. Students must understand the concept of heat flow from warmer to cooler bodies. They must also be somewhat familiar with the concept of a Òclosed system.Ó It would be very helpful for the mathematical work in this laboratory if the students are proficient in direct proportion equations and the analysis of units (labels) on measurements.

Time

Teacher preparation: 15 min

Student laboratory time: Approximately 30 min per trial.

Safety

Read the Safety Considerations in the Student Version. Caution students to be careful with the heated objects. The burner, ring, and wire gauze are all possible sources of burns. Caution students not to allow chunks of ice to drop on the floor; slipping on ice chunks might lead to serious injury.

Materials (For 24 students working in pairs)

36 Beakers, 250-mL

12 Styrofoam cups,12-oz, with lid

12 Burners, support stands, rings, and wire gauze

12 Graduated cylinders,100-mL

12 Graduated cylinders, 250-mL

12 Thermometers, ­10 °C to 110 °C

Ice cubes

Paper towels

Hot pad

Advance Preparation

1. The ice cubes should be wet, that is, at melting temperature. They should not be used immediately after being removed from the refrigerator. Allow them to stand at room temperature a few minutes. It is instructive for many students to use a thermometer to measure the temperature of ice that has been freshly removed from a deep freeze. Many students do not believe that the temperature of ice can go below 0°C.

2. Styrofoam ice chests are very efficient in keeping ice throughout the day.

Pre-Laboratory Discussion

Even though the melting of ice and freezing of water are common occurrences, the concept of heat released during freezing seems counterintuitive. Citrus fruit growers spray their orchards during freeze warnings to prevent damage to the fruit trees. The environment is affected by the phase change involving water and ice. A melting snow cover will prevent the temperature from rising as high as it would without snow and the freezing of snow melt after sunset prevents the temperature from dropping as rapidly as it would otherwise.

In this particular activity we will determine the heat needed to melt 1.0 g of ice. This is the same amount of heat released to the surroundings during freezing.

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