What do you mean by Calorimetry?

Measuring the heat transferred during chemical reactions or other physical processes is known as calorimetry. It comes under the branch of thermochemistry.  

The universe is made up of precise matter and different forms of energy. Matter can further be divided into smaller particles called atoms and molecules. When these particles gain energy, they move to a state of motion that can either be vibrating to and fro in a fixed position or colliding with one another. Due to this motion, there is thermal energy (heat) is generated, which is one of the fundamentals of the principle of calorimetry.  

The temperature of an object determines the thermal energy it possesses. They are directly proportional; the more the amount of heat energy, the greater is the temperature of the object.  

If there is no exchange of energy between a system and its surroundings, the system is said to be an isolated system.  

Heat transfers from high-temperature region to low temperature region. This very basic principle. 

For example, let us take a cup of tea and a scoop of ice cream as an example. If left outside for long, the ice cream will melt and gain temperature whereas the tea will cool down. In terms of energy, the tea loses heat energy and the ice cream gains energy.  

But now take this deeper to calculate the exact amount of energy being transmitted from one system to another.  

Principle of Calorimetry  

When two bodies (preferably a solid and a liquid) of different temperatures are brought in physical contact, the heat lost by the body with a higher temperature is equal to the heat gained by the body with a lower temperature given that none of the energy is lost to the surroundings. This takes place till a state of thermal equilibrium is established. It is said to be in thermal equilibrium when two bodies in contact do not exchange heat energy with each other. In such a case both the bodies attain the same temperature. 

If observed this process indicates the law of conservation of energy: Heat lost = Heat gained 

The process of quantifying the transfer of thermal energy or change in thermal energy of an object is known as calorimetry. This process is carried out using a device called the calorimeter.  

Calorimeter  

A calorimeter is a device used to measure heat energy. 

It is mainly made up of a metallic vessel which is made up of materials that are good conductors of electricity such as copper and aluminum. Some also include a stirrer to stir the contents in the vessel and maintain a uniform temperature. The metallic vessel with or without the stirrer is enclosed in an insulating jacket that is needed to prevent any heat loss to the environment that will affect the data values of the experiment and not give accurate results. There is one small hole in the lid through which a thermometer can be inserted. It is needed to measure the change in temperature of the contents of the vessel.  

Bomb Calorimeter  

There are different types of calorimeters. One of which is a bomb calorimeter. In the bomb calorimeter, a sealed metal conductor is used for the reaction. This container is surrounded by water and is kept inside another container. The temperature of the water is measured to calculate the heat change. 

It is mainly used to measure heat flows in solids. This process gives very accurate results. 

Coffee cup calorimeter  

A coffee cup calorimeter is made up of a polystyrene cup with a lid and a hole in it for the thermometer.  The cup contains water filled in it and when the reaction occurs, the heat released is absorbed by the water. Thus there is a change in the temperature of the water and this temperature is measured. 

It is usually not used in gaseous reactions as the gas would escape from the system. Even for high-temperature reactions, this is not a good option since the temperature would melt the cup. 

Formula to Calculate the Heat Energy Transferred  

Q = mCDT    

Where Q = Heat energy (J), m (kg) is the mass of the object, C is the specific heat capacity (J/ (kg °C)) and DT (°C) is the temperature change. 

The sign (positive or negative) of the energy value depends if it is endothermic (absorbs energy) or exothermic (releases energy).  

During an endothermic process, the temperature change of the solution is positive so the energy change will also be positive. But during exothermic reactions, the temperature change is negative as it loses energy. This causes energy change (Q) to be negative. 

Specific Heat Capacity  

The specific heat capacity is given as the amount of energy needed to raise the temperature of one kilogram of a substance by one degree Celsius.  

Some basic values of specific heat capacity to remember:  

The formula to calculate heat energy can be rearranged to find the specific heat capacity of materials.  

C = Q / m DT    

Simple Calorimeter Experiment 

The below experiment can be used to measure the heat energy transferred between liquids when mixed.  

Procedure:  

• First, measure the weight of the empty calorimeter. 
• Now add cold water to the empty calorimeter and measure the total mass. 
• Now, cover the calorimeter with a lid having a small hole. Insert a thermometer through the hole till it touches the water. 
• The temperature is measured. 
• Now follow the same procedure for hot water. 
• Now mix the hot water into the cold water container. 
• Instantly cove it using the lid to prevent any loss of heat. 
• Read the temperature and measure it till it reaches a stable value. 
• Measure the total mass of both the hot and cold water. 
• Calculate the change in temperature in each of them. 
• Use the formula to calculate the heat energy. 

Repeat this experiment and take the average to reduce the error margin.  

You can extend this experiment by taking different masses of water or different starting temperatures to show that temperature and mass affect the transfer of heat energy.  

Energy Change During Reactions 

The concept of calorimetry and the experiment can be used to understand the concepts of energy change during a reaction. It is given by the following equation: 

$\Delta H=H_{\mathrm{Pr}oduct}-H_{\mathrm{Re}ac\tan t}$

Calorimetry for Latent Heat

Latent heat is the amount of energy absorbed by a substance to change states of matter (phase change). There are specific names for each change of state.  

Latent heat of fusion is the amount of heat needed to change the phase of 1 kg of the substance from solid to liquid phase.  

Latent heat of vaporization is the amount of heat needed to change the phase of 1 kg of the substance from liquid to gas phase.  

For example, for the phase change from ice to water:  

• Measure the weight of a cup before and after filling it with water. Now find the difference to measure the weight of water. 
• Then measure the initial temperature. 
• Insert an ice cube into the cup. 
• Find the weight of the cup with water and ice now. 
• Calculate the weight of the ice by finding the difference between the previous step mass and the mass of cup+water. 
• Find the final temperature after the ice has completely melted. 

The ice first absorbs energy from the surrounding water to change phase. The melted ice further takes in energy to establish thermal equilibrium.  

Q _water = - (Q _melting + Q _icewater)   

Heat lost by the water bath = the heat needed to melt the ice + the heat needed to raise the temperature of the melted ice to the equilibrium temperature 

where Q_melting = mL 

Q is the heat energy, m is the mass of the substance and L is the latent heat.  

 Context and Applications 

This topic is studied mainly in higher grades including 

• Bachelors in Science (Physics) 
• Masters in Science (Physics) 
• Bachelors in Science (Chemistry)