Measurement of Enthalpy and Internal Energy Change

Enthalpy and internal energy change are measured using an experimental method known as calorimetry. These methods are based on thermometric processes performed in a vessel known as a calorimeter that is submerged in a certain liquid volume.

• The heat produced throughout the process is measured using the known heat capacities of the calorimeter and liquid and the temperature difference.
• Particularly, there are two separate requirements for measuring enthalpy and internal energy.
• These include constant pressure (enthalpy) and constant volume (internal energy). 

Key Terms: Enthalpy, Internal energy, Calorimetry, Calorimeter, Temperatures, Pressure, Volume, Heat formula, Heat capacity, Kinetic energy, Potential energy

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What is Enthalpy?

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The heat energy, which is absorbed or evolved during a chemical reaction, is called enthalpy. The enthalpy is represented by H, and here the letter H indicates the amount of energy.

• The change in enthalpy is given by ΔH, and here the delta (Δ) symbol represents the change and its unit is joule or kilojoules.
• In other words, it can be said that the sum of the internal energies of the system is enthalpy.
• The change in internal energy takes place at the time of chemical reaction, and this change of energy is calculated as enthalpy.

It can be given by the following expression:

Where

• H = enthalpy
• U = sum of internal energy
• P = pressure system
• V = Volume of system.

From the above expression, we can say that enthalpy is the addition of internal energy and the energy that is needed to conserve a system’s volume at a given pressure. PV shows the work that is required to be done on the environment to create space for the system.

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What is Internal Energy?

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The sum of the potential and kinetic energy of the particular system is referred to as internal energy. The energy which is released due to the movement of molecules is kinetic energy whereas the stored energy is potential energy.

• The internal energy is represented by U and the change in internal energy is given by ΔU.
• Internal energy and enthalpy are the same for a particular system but the condition is that it can only happen at constant pressure.
• The change in internal energy can take place in two ways.
• The first way is because of the transfer of heat and the second way is by doing work.

Hence, the change in internal energy can be expressed by the following equation:

Where

• ΔU = internal energy change
• q = transfer of heat
• w = work done by or on a system.

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Measurement of Enthalpy Change

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The absolute enthalpy of a system cannot be calculated directly. Therefore, we determine the enthalpy with respect to a reference point. Hence, we usually measure the change in enthalpy.

• Enthalpy change (ΔH) is positive for endothermic processes and negative for exothermic ones.
• Enthalpy change in the laboratory is measured using calorimetry techniques.
• Enthalpy change is defined as the heat change at constant pressure, ΔH = q_p.
• The "coffee-cup calorimeter" is commonly used to calculate enthalpy change.
• In this procedure, the cup is partially filled with a known volume of water.
• A thermometer is put through the cup's lid such that the bulb is below the surface of the water.
• When a chemical reaction happens, water absorbs the heat produced.
• The change in water temperature is used to determine how much heat has been absorbed or evolved.
• Since the cup is made of polystyrene foam, a highly effective insulator, very little heat energy escapes.

The energy or enthalpy change in this process is calculated as follows:

Where

• m is the mass of the water
• c_p is the specific heat capacity of water at constant pressure
• ΔT is the change in temperature or temperature difference

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Measurement of Internal Energy Change

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Internal energy change refers to the energy change at a constant volume.

• A bomb calorimeter is commonly used to measure the internal energy change.
• This technique involves immersing a steel vessel (also known as a bomb) in a water bath to ensure that no heat is lost to the surroundings.
• In the bomb, a combustible substance is burnt in oxygen.
• The heat produced is absorbed by the water surrounding the bomb, and the temperature difference is measured.
• The energy changes associated with the reaction are measured at constant volume since volume does not vary in the sealed bomb calorimeter.
• The system does not work since the volume remains constant.

The energy change or internal energy change in the process is calculated as follows:

Where

• m is the mass of water
• c_v is the specific heat capacity at constant volume

• ΔT is the temperature difference

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Things to Remember

• The heat energy, which is absorbed or evolved during a chemical reaction, is called enthalpy.
• The sum of the internal energies of the system is enthalpy.
• The sum of the potential and kinetic energy of the particular system is referred to as internal energy.
• The change in internal energy is given by, ΔU = q + w
• Enthalpy change (ΔH) is positive for endothermic processes and negative for exothermic ones.
• A bomb calorimeter is commonly used to measure the internal energy change.

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