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Measurement of Enthalpy Change

Last Updated : 24 Feb, 2022
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The experimental technique known as calorimetry is used to assess enthalpy and internal energy in most cases. Calorimetry is based on thermometric methods that are carried out in a calorimeter that is immersed in a known volume of liquid. The heat released during the process is typically computed using known heat capacities of the liquid and temperature changes measured by the calorimeter.

What is Enthalpy?

Enthalpy, or heat energy, is released or absorbed during chemical reactions. The H indicates how much energy is released or absorbed. H represents the change in enthalpy, where delta represents the change and joules or kilojoules are used as units.

In chemistry, enthalpy is defined as the sum of the system’s internal energy. During a chemical reaction, there is a change of internal energy, and this change can be quantified as enthalpy. 

To determine it, use the following expression:

H = U + PV

where,

  • H = Enthalpy
  • U = Internal Energy
  • P = Pressure of a system
  • V = Volume of a system

A system’s energetics refers to its internal energy and the energy needed to maintain a constant volume at a given pressure. PV refers to the work required to make room for the system in the environment.

Measurement of Enthalpy Change

Calculations utilizing a calorimeter of constant pressure (an instrument used to detect changes in enthalpy during chemical processes at constant pressure) offer direct measurement of delta h enthalpy since H is expressed as the flow of heat at constant pressure. This setup is well suited to studying reactions that take place in a solution at a constant atmospheric pressure. A “student” version known as the coffee-cup calorimeter is regularly encountered in general chemistry laboratories. Commercial calorimeters work on a similar concept as well. They may still be used with smaller-volume solutions since they have higher thermal insulation and can detect temperature changes as tiny as 10-6 degrees Celsius.

The relationship between ΔHrn and heat is:

ΔHr×n = qr×n = -qcalorimeter = -mCsΔT

∴ ΔHr×n = -mCsΔT

The coffee-cup calorimeter is a simple variant of a constant-pressure calorimeter that is constructed up of two nested Styrofoam cups that are sealed with an insulated stopper to keep the instrument thermally isolated from the environment. One of the stopper holes will be used for the stirrer, which will blend the reactants, and the other will be used for the thermometer, which will be used to calculate the temperature.

For a better understanding of how to calculate enthalpy change, look at the example below.

In a coffee-cup calorimeter, 5.03 g of solid potassium hydroxide is dissolved in 100.0 mL of distilled water, and the liquid temperature rises from 23.0 to 34.7 degrees Celsius. The average density of water in this temperature range is 0.9969 g / cm3. What is the delta h enthalpy in kilojoules per mole? Assume that the calorimeter absorbs just a small quantity of heat and that the specific heat of the solution is the same as pure water due to the enormous volume of water.

Reaction enthalpy

  • Bond dissociation enthalpy: The amount of energy required to break the bonds between one-mole molecules is known as the enthalpy of bond dissociation. It is detailed on a mole-by-mole basis.
  • Combustion enthalpy: When a mole of a substance burns in the presence of oxygen, the enthalpy of combustion is the quantity of heat emitted or absorbed.
  • Formation enthalpy: When a mole of a compound is generated from its component elements in their usual elemental forms, the enthalpy of production is the amount of heat emitted or absorbed.
  • Atomization enthalpy: The amount of energy required to convert any material into gaseous atoms is known as the enthalpy of atomization. It is expressed as a number of moles of gaseous atoms.
  • Sublimation enthalpy: The amount of heat required to convert a mole of a substance from a solid to a gaseous state at STP is called enthalpy of sublimation.
  • Phase transition enthalpy: When a phase transition happens from one phase to another, the enthalpy of phase transition is a standard enthalpy that is released or absorbed.
  • Ionization enthalpy: The amount of energy necessary for an isolated gaseous atom to lose an electron in its ground state is known as the enthalpy of ionization.
  • The solution’s enthalpy: When a mole of a substance dissolves in excess of a solvent, the quantity of heat generated or absorbed is called enthalpy of solution (usually water).
  • Diluted enthalpy: The enthalpy change associated with the dilution of a component in a solution under constant pressure is called enthalpy of dilution. It is defined as energy per unit mass or amount of substance.

Detailed technique for determining the enthalpy change of an anhydrous copper (II) sulphate solution-

  • Weigh between 3.90 g and 4.10 g of anhydrous copper(II) sulphate in a dry weighing container. It is critical to keep track of the precise weight.
  • Place 25 cm3 deionized water in a polystyrene cup with a volumetric pipette and record the temperature at the beginning (t=0), then start the timer and record the temperature every minute while swirling the liquid constantly.
  • During the fourth minute, immediately mix the powdered anhydrous copper(II) sulphate into the water in the polystyrene cup, but do not record the temperature. Reweigh the weighing bottle after filling it.
  • Every minute, stir the solution in the polystyrene cup and record the temperature until 15 minutes have passed.
  • Make a graph of temperature (on the y-axis) vs. time. Draw two independent best fit lines, one that connects the points before the addition and one that joins the points after the addition, to extrapolate both lines to the fourth minute.
  • Calculate the temperature change at the fourth minute, which, according to your graph, should have occurred immediately after the solid was added.
  • Using q = m × cp × ΔT Calculate energy change = 20 × 4.18 × ΔT
  • Divide q by the number of moles of anhydrous copper(II) sulphate in mass added to get ΔHsolution.

Sample Questions

Question 1: How to measure Enthalpy Change?

Answer:

It is impossible to calculate a system’s absolute enthalpy directly. As a result, the enthalpy around a reference point is calculated. As a result, we usually calculate the change in enthalpy. Endothermic reactions have a positive enthalpy change, while exothermic reactions have a negative enthalpy change. In the laboratory, calorimetry techniques are used to measure enthalpy change. Enthalpy change is defined as the heat change at constant pressure, which is ΔH= qp. The enthalpy change is frequently calculated using a “coffee-cup calorimeter.” The cup is partially filled with a known volume of water, and a thermometer is put through the cup’s lid such that its bulb is below the water’s surface.

ΔH= qp = m × cp × ΔT

where,

  • m = mass of water,
  • cp = water’s specific heat capacity at constant pressure
  • ΔT = Temperature difference

Question 2: What is a change in enthalpy?

Answer:

The quantity of heat emitted or absorbed in a reaction under constant pressure is referred to as the enthalpy change.

Question 3: What Does Enthalpy Mean In Real Life?

Answer:

Hand warmers and refrigerators both employ enthalpy change. Refrigerators evaporate refrigerants like Freon. The enthalpy of vaporization is equal to the coolness of the food.

Question 4: Explain Flame Calorimetry for Measuring Combustion Enthalpies in brief.

Answer:

Calorimetry can be used to determine the enthalpies of combustion. Typically, the fuel is burned, and the flame is used to warm water in a metal cup.

  • Before and after mass of the spirit burner
  • The water’s temperature fluctuates.
  • The amount of liquid in the cup

Question 5: Explain three Enthalpy of Reactions.

Answer:

  1. Bond dissociation enthalpy : The amount of energy required to break the bonds between one-mole molecules is known as the enthalpy of bond dissociation. It is detailed on a mole-by-mole basis.
  2. Atomization enthalpy : The amount of energy required to convert any material into gaseous atoms is known as the enthalpy of atomization. It is expressed as a number of moles of gaseous atoms.
  3. Sublimation enthalpy : The amount of heat required to convert a mole of a substance from a solid to a gaseous state at STP is called enthalpy of sublimation.

Question 6: What is the relationship between Enthalpy and Internal Energy?

Answer:

The internal energy change of a system is the sum of heat transmission and work done. Internal energy and heat flow are related to a system’s enthalpy at constant pressure.



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