10.7. CALORIMETRY

A system is said to be isolated if no exchange or transfer of heat occurs between the system and its surroundings. When different parts of an isolated system are at different temperatures, a quantity of heat transfers from the part at higher temperature to the part at lower temperature. The heat lost by the part at higher temperature is equal to the heat gained by the part at lower temperature.

Calorimetry is the scientific measurement of heat. It relies on the principle that in an isolated system, heat will flow from the hotter part to the cooler part until both parts reach the same temperature. This principle is crucial in understanding how heat is transferred and helps in calculating the specific heat capacities of substances through careful measurement.

Think of an ice cube in a warm drink. The heat from the drink warms the ice cube (which is colder), causing it to melt. The drink loses some heat, while the ice cube gains heat until both reach the same temperature. This is like how a game of tag works—heat is 'tagged' from the warmer drink to the cooler ice.

Calorimetry means measurement of heat. When a body at higher temperature is brought in contact with another body at lower temperature, the heat lost by the hot body is equal to the heat gained by the colder body, provided no heat is allowed to escape to the surroundings. A device in which heat measurement can be done is called a calorimeter. It consists of a metallic vessel and stirrer of the same material, like copper or aluminium. The vessel is kept inside a wooden jacket, which contains heat insulating material, like glass wool etc. The outer jacket acts as a heat shield and reduces the heat loss from the inner vessel. There is an opening in the outer jacket through which a mercury thermometer can be inserted into the calorimeter.

A calorimeter is an essential tool for measuring heat changes during chemical reactions or physical changes like melting and boiling. The design of the calorimeter helps to ensure that heat does not escape to the environment, allowing accurate measurements of heat transfer. The use of a stirrer ensures even distribution of temperature within the calorimeter.

Imagine a thermal coffee cup designed to keep your coffee hot. Just like a calorimeter, it has insulating layers to maintain temperature. When you pour hot coffee inside, it stays hot longer because the heat doesn't escape as quickly.

The following example provides a method by which the specific heat capacity of a given solid can be determined by using the principle, heat gained is equal to the heat lost. Example 10.3: A sphere of 0.047 kg aluminium is placed for sufficient time in a vessel containing boiling water, so that the sphere is at 100 °C. It is then immediately transferred to 0.14 kg copper calorimeter containing 0.25 kg water at 20 °C. The temperature of water rises and attains a steady state at 23 °C. Calculate the specific heat capacity of aluminium.

In this example, the aluminum sphere, after being heated, is placed in a cooler environment (the calorimeter with water). Here, we assume that heat lost by the aluminum sphere equals the heat gained by the water and the calorimeter. Using the formula for heat transfer and substitution with known quantities, we can solve for the specific heat capacity of aluminum. This example illustrates the principle of conservation of energy in calorimetry.

Think of this like a game of hot potato, where the hot potato (aluminum sphere) is passed around. As it moves from one player (the boiling water) to another (the cooler water), it cools down while the other players warm up, all while keeping track of the temperature shifts.

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