Illustrated Guide to Home Chemistry Experiments

274 DIY Science: Illustrated Guide to Home Chemistry Experiments

If you add heat to a mixed-phase system

that comprises a substance in its solid and

liquid phases, both at the melting point of the

solid, the temperature of the system does

not increase until all of the solid is converted

to liquid. The additional heat energy that is

absorbed without causing a temperature

increase is required to give the atoms or

molecules that make up the solid suffi cient

energy to break the attraction that holds them

in solid form. The additional heat energy that

is required to change a solid to a liquid at its

melting point is called heat of fusion, enthalpy

of fusion, or specifi c melting heat.

REQUIRED EQUIPMENT AND SUPPLIES

goggles, gloves, and protective clothing

calorimeter

thermometer

graduated cylinder, 100 mL

ice bucket or similar wide-mouth container

beaker, 600 mL

beaker tongs

hotplate

strainer

ice, crushed or chipped

hot tap water

£ £ £ £ £ £ £ £ £ £ £

In this laboratory, we’ll measure the heat of fusion of ice, which

has a high heat of fusion because of the strong intermolecular

attraction of hydrogen atoms. Heat of fusion is (offi cially)

expressed as Joules per mole (J/mol) in the SI system, but

many older and alternate forms are still used. Among the most

common of those are calories per gram (cal/g), kiloJoules per

kilogram (kJ/kg), and even British Thermal Units per pound

(BTU/lb). In particular, cal/g is still very widely used, and is the

format we use in this laboratory.

LABORATORY 15.2:

DETERMINE THE HEAT OF FUSION OF ICE

SUBSTITUTIONS AND MODIFICATIONS

You may substitute a sauce pan or similar container

for the 600 mL beaker.

You may substitute any heat source for the hotplate.

•

•

CAUTIONS

Take care with the hot water and hotplate to avoid burns.

Wear splash goggles, gloves, and protective clothing.

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PROCEDURE

If you have not already done so, put on your splash

goggles, gloves, and protective clothing.

Fill one of the 600 mL beakers about half full of crushed

or chipped ice and allow the ice to begin melting while you

perform the following steps.

1.

2.

Add about 400 mL of hot tap water to the second 600 mL

beaker and heat the water to 65°C ± 5°C.

Using the beaker tongs, pour about 100 mL of the heated

water into the 100 mL graduated cylinder to preheat

it. Wait 30 seconds and then dump the water from the

graduated cylinder into the sink.

3.

4.

COLD AS ICE

Ice in a typical home freezer is typically at a temperature

of –15°C to –20°C, well below the melting point of ice. To

determine the heat of fusion of ice accurately, we need

to start with ice at 0°C. Otherwise, some of the heat

absorbed will be needed to warm the ice to 0°C, giving a

false high value for the heat of fusion of ice. You can avoid

this by crushing or chipping the ice into small pieces,

allowing it to melt partially into slush, and using the

strainer to separate solid ice from the water.

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