Calorimetry

A thermometer tells you how hot something is, but it doesn't tell you how much heat energy an object contains or exchanges. To answer that question, scientists developed a technique called calorimetry and a device called a calorimeter.

Calorimetry is the science of measuring heat exchange during physical or chemical processes. The word comes from the Latin calor (heat) + Greek metron (measure).

OR

An experimental technique for the quantitative measurement of heat exchange is called calorimetry.

A calorimeter is a cylindrical vessel which is used to measure the amount of heat gained (or lost) by a body when it is mixed with another body or substance.

A calorimeter is an insulated device used to measure the heat gained or lost by a substance during a thermal process. It works on a simple idea: trap all the heat inside the device so nothing escapes, and then measure the temperature change.

Fig.: Cross-section of a simple water calorimeter showing all key components

Fig.: A calorimeter works just like a thermos flask — both trap heat inside using insulation

Statement: When different parts of an isolated system are at different temperatures, heat transfers from the part at higher temperature to the part at lower temperature. The heat lost by the hot object is equal to the heat gained by the cold object, provided no heat is allowed to escape to the surroundings.

(For liquid in calorimeter: m₁c₁Δθ + m_cc_cΔθ)

Key Points:

• A system is said to be isolated if no exchange of heat occurs between the system and its surroundings.
• Calorimetry literally means measurement of heat.
• Energy supplied by heater = VIt (voltage × current × time).
• This principle is based on the Law of Conservation of Energy.

The method of mixtures is the standard experimental technique for determining the specific heat capacity of an unknown substance using a calorimeter.

Procedure:

Let's build the master formula step by step. First, define all our variables clearly.

Variable Definitions

Step-by-Step Derivation

Problem: A 0.06 kg aluminium sphere is heated to 100°C and placed in a 0.12 kg copper calorimeter containing 0.30 kg of water at 25°C. The mixture reaches a steady temperature of 28°C. Find the specific heat capacity of aluminium.

Solution:

Given:
Mass of aluminium sphere (m₁) = 0.06 kg
Mass of copper calorimeter (m₂) = 0.12 kg
Mass of water (m₃) = 0.30 kg
Specific heat of copper (s₂) = 387 J kg⁻¹ K⁻¹
Specific heat of water (s₃) = 4180 J kg⁻¹ K⁻¹
Initial temp of sphere (T₁) = 100°C
Initial temp of water + calorimeter (T₂) = 25°C
Final equilibrium temp (T) = 28°C

Step 1: Calculate temperature changes

• Temperature drop of sphere: T₁ − T = 100 − 28 = 72°C
• Temperature rise of water & calorimeter: T − T₂ = 28 − 25 = 3°C

Step 2: Apply Eq.

Step 3: Substitute values

Step 4: Compute the numerator

• 0.12 × 387 = 46.44
• 0.30 × 4180 = 1254
• Sum = 46.44 + 1254 = 1300.44
• Numerator = 1300.44 × 3 = 3901.32

Step 5: Compute the denominator

• 0.06 × 72 = 4.32

Step 6: Final answer

• A calorimeter is an insulated device used to measure heat transfer; measurement of specific heat of a substance is carried out using it.
• Principle of Calorimetry: Heat lost by hot body = Heat gained by cold body, which represents the law of conservation of heat energy.
• In the method of mixtures, a heated sample is placed in the calorimeter and the temperature change is measured to calculate specific heat using the formula Q = msΔt.
• Specific heat of a substance depends on the nature of the substance; water is preferred in calorimetry due to its high specific heat, allowing it to absorb large amounts of heat with minimal temperature change.
• For accurate results, the sample must be transferred quickly into the calorimeter and stirred well to ensure uniform heat distribution.