Calorimetry

Introduction 

This article will discuss the concept of calorimetry. We will focus on the calorimeter principle, calorimetry formula, and calorimetry problems in the following segments. Calorimetry is a very important aspect to be considered and worked upon in many industrial processes as well. It is with the help of calorimetric and thermodynamic principles we have been able to establish economic methods for industrial and production processes.  

Calorimeter Principle 

• The concept of calorimetry refers to the measurement of heat. And, the device used to measure heat is called a calorimeter. 
• The calorimeter principle states that in an isolated system with two substances with different temperatures, there is a transfer of heat energy from the substance at a higher temperature to the substance at a lower temperature. Therefore, the heat energy lost by the substance at a higher temperature is equal to the heat gained by the substance at a lower temperature, provided that no heat is escaped to the surroundings. 
• However, it is worth noting that the calorimeter principle also states that the flow of heat energy continues between the substances in isolation until thermal equilibrium is achieved. 
• A calorimeter is an instrument used to determine specific heat, latent heat, heat content, and other thermal properties of the substances in question. 

Calorimetry Formula 

As we all know, the basic concept of calorimetry is as follows- 

The heat released from a substance with higher temperature = Heat absorbed by a substance with a lower temperature. 

The concept of calorimetry uses the following formula to determine the transfer of heat- 

Q = m C △ T

In the above equation, Q is the unit of measure for heat transfer;

M is the mass of the substance;

C is the specific heat capacity; and

△ T is the temperature change. 

Calorimetry Problems 

Problem 1 – If the specific heat of water is 4.2 J/gm K, then what is the amount of heat needed to change the temperature of 3g of water by 60 ° C? 

Solution – From the above statement, we can gather the following information- 

Specific heat (C) = 4.2 J/gm K, mass (m) = 3g, and change in temperature (△ T) = 60 ° C 

To this, we apply the following formula: Q = m C △ T

Q = 3 4.2 60 = 756 Joules

Therefore, the amount of heat required to change 3g of water by 60 ° C with the specific heat of water being 4.2 J/gm K is 756 Joules. 

Problem 2- If the amount of heat required to change the temperature of a 0.5 kg mass of lead from 60 ° C to 100 ° C is 1,500 Joules, then determine the specific heat. 

Solution – From the above statement, we can gather the following information- 

mass (m) = 0.5 kg 

change in temperature (△ T) = 100 ° C – 60 ° C = 40 ° C

The amount of heat required (Q) = 1500 Joules 

We know that Q = m C △ T 

Therefore, specific heat (C) = Qm×△ T  

C = 15000.5100040     = 0.075 J/gm K     ( 1kg = 1000 grams )

Therefore, the specific heat or heat capacity is 0.075 J/gm K. 

Problem 3- If the amount of heat required to change the temperature of a 0.9 kg mass of lead from 35 ° C to 75 ° C is 1,800 Joules. Then, determine the specific heat. 

Solution – From the above statement we can gather the following information- 

mass (m) = 0.9 kg 

change in temperature (△ T) = 75 ° C – 35 ° C = 40 ° C

The amount of heat required (Q) = 1,800 Joules 

We know that Q = m C △ T 

Therefore, specific heat (C) = Qm×△ T  

C = 1,8000.9100040 = 0.05 J/gm K        ( 1kg = 1000 grams )

Therefore, the specific heat or heat capacity is 0.05 J/gm K. 

Different types of calorimeters 

• Bomb Calorimeters- A bomb calorimeter is a device that measures the internal energy change between products and reactants. While the reaction is being measured, bomb calorimeters can withstand large pressures. Bomb calorimeters are a type of constant-volume calorimeter. 

• Differential scanning calorimeter- Differential scanning calorimeters are based on the thermo-analytical method. Therefore, these calorimeters measure the difference in the amount of heating required to increase the temperature of a substance and its reference as a function of temperature. Differential scanning calorimeters are of two types, modulated temperature differential scanning calorimeter and heat flux differential scanning calorimeter. 

• Isothermal calorimeter- An isothermal calorimeter is a calorimeter where the sample chamber is kept at a constant temperature, and heat is lost directly to the surroundings.  

• Heat flow calorimeter- A heat flow calorimeter is a calorimeter that is designed to keep the thermal resistance between the sample chamber and the heat sink moderate. 

• Under/Overbridge calorimeters- The over/under bridge calorimeter is a transportable calorimeter that requires less floor space and is taller. The over/under bridge calorimeter is cheaper than a twin-bridge calorimeter; however, it is not as accurate. 

• Gradient Bridge Calorimeter- The gradient bridge calorimeter is a calorimeter whose design includes a series of concentric cylinders. The sensitivity of the calorimeter is determined by the gap between the sample cell cylinder and the reference cell cylinder. The gradient bridge calorimeter design is best suited for glove box installations. 

Conclusion 

With this, the article on calorimetry is concluded. We hope that we were able to clarify and add to your knowledge on the calorimeter principle and calorimeter problems. Calorimetric formulas and concepts can be used to find out the heat transfer in ideal conditions. Several types of calorimeters are used based upon the requirements. As things get further complex, we introduce more thermodynamic principles to cater for those problems.