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Thermodynamics is the study of the relationship between heat and work which explains the properties of heat and reaction of heat energy with different chemicals. Enthalpy is the crucial and central factor that determines whether a reaction can happen or not. The heat that passes into the system or out of the system during a reaction is called enthalpy change. When acetone is mixed with chloroform, it results in evolving heat because of the formation of hydrogen bonds between chloroform and acetone. The heat developed during this interaction can be measured by using a calorimeter to mix these two liquids and measure the enthalpy change.
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Key Terms: Enthalpy change, enthalpy, enthalpy change during interaction between acetone and chloroform, hydrogen bond formation, heat change, thermodynamics, chemistry
Aim
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To determine the enthalpy change between acetone and chloroform during interaction (hydrogen bond formation).
Theory
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Liquid pairs show departure from their ideal behavior on mixing. Acetone and chloroform form a non-ideal liquid pair system showing negative deviation from Raoult’s law. This negative deviation shows that the two components are strongly held together in liquid state on mixing due to hydrogen bonds.
This hydrogen bonding reduces the escaping tendency of both molecules and because of that; the released vapor pressure of the solution is less than that expected from Raoult’s law. In the pure state, only weak forces of Van der waal hold molecules of chloroform and acetone. The chemical equation for hydrogen bonding formation between the molecules of acetone and chloroform is shown below:
Hydrogen Bonding Between Chloroform And Acetone
In this process, enthalpy change takes place because of the hydrogen bond formation. The enthalpy change is an extensive thermodynamic property. Therefore, the heat evolved from the experiment depends on the amount of the liquid components mixed. Reason behind this is that the heat change is reported only for specific amount. Therefore, enthalpy change while mixing 1 mol chloroform with 1 mol acetone is reported. Equation for the enthalpy change is given below:
((ΔH1) Heat gained by calorimeter, thermometer and stirrer) + ((ΔH2) Enthalpy change for chloroform) + ((ΔH3) Enthalpy change for acetone) + ((ΔH4) Enthalpy change for interaction) = 0
∴ ΔH4 = (- ΔH1 + ΔH2 + ΔH3)
Materials Required
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- Beaker (250 mL)
- Measuring cylinder (250 mL)
- Thermometer (110°C)
- Boiling tube
- Cotton wool
- Glass rod
- Stirrer
- Piece of cardboard
- Chloroform
- Acetone
Procedure
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- Determination of calorimeter constant of beaker.
- Take 100 ml of water in a 250 ml beaker and marked it as A.
- Place this beaker on a wooden block and kept it in a larger beaker of capacity 500 ml as shown in figure.
Determination of calorimeter constant of a beaker.
- Pack the empty space with cotton wool, cover the beaker with cardboard and insert the thermometer and stirrer in the beaker.
- Note the temperature of the water (tc°C).
- Take 100 ml of hot water (50-60°C) in another beaker of 250 ml capacity.
- Note the exact temperature of this hot water. (th°C)
- Pour the hot water from the beaker B to Beaker A. stir the mixed water and read the temperature attained. (tm°C)
- Calculate the calorimeter constant from the expression given below:
W = \(\frac{((4.184)[V_c (t_m-t_c) +V_h (t_m-t_h)}{(t_m-t_c )}JK^{-1}\)
Where, Vc=volume of cold water
Vh=volume of hot water
- Determination of enthalpy change during interaction between acetone and chloroform
- Transfer 0.1 mol (≈8.14 mL) chloroform into the insulated boiling tube as shown in figure. Take the mass of chloroform as m1.
- Note the temperature of chloroform. (t1°C).
- Transfer 0.1 mol (≈7.34 mL) acetone in a measuring cylinder. Take the mass of chloroform as m2.
- Note the temperature of chloroform. (t2°C).
- Pour acetone into the chloroform contained in the boiling tube.
- Stir the mixture of acetone and chloroform carefully and gently.
- Note the temperature of the mixture of acetone and chloroform. (t3°C).
Observations and Conclusions
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| Observations | Details |
|---|---|
| Initial temperature of acetone and chloroform | t1°C |
| Final temperature of acetone and chloroform | t2°C |
| Change in temperature | (t1- t2)°C |
| Water level equivalent of calorimeter | W g |
| Density of acetone | 0.787 g/cm3 |
| Density of chloroform | 1.499 g/cm3 |
| Specific heat of acetone (S2) | 2.18 J/g |
| Specific heat of chloroform (S1) | 0.96 J/g |
Calculation of Enthalpy of Interaction
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From the observations, enthalpy change during interaction between acetone and chloroform is given by:
Heat change= W × 4.184 × (t2 – t1) + [100 × 1.499 × S1 + 100 × 0.787 × S2] (t2 – t1)
Heat change= X Joules
Precautions
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Necessary precautions to avoid risks and effects of chemicals during an experiment are listed below.
- Measure the quantity of chloroform and acetone carefully.
- Be careful while stirring the mixture of acetone and chloroform.
- Use cotton wool for thermal insulation as it is a great thermal insulator.
- Record the changing temperature carefully with a thermometer graduated up to 0.1°C.
Things to Remember
- Metallic calorimeters are not used for measuring thermodynamic changes as the metal may react with substances.
- During measurement of heat changes, the calorimeter, thermometer and stirrer also absorb some heat called calorimeter constant.
- Enthalpy change for the interaction between acetone and chloroform has negative sign, since t2>t1 in this experiment.
- Mixture of acetone and chloroform is a prominent example of a system with a marked negative deviation from the behavior of the ideal solution.
- While determining the enthalpy change, take care that the total volume of acetone and chloroform is equal to the volume of water for which the water equivalent of the calorimeter has been calculated.
Sample Questions
Ques. What is the enthalpy of the solution? (1 Mark)
Ans: The enthalpy of a solution (ΔH) or heat of solution is the heat released or absorbed during the dissolving process of a substance in solvent at a constant pressure resulting in infinite dilution. Enthalpy of solution is measured in kJ/mol at constant temperature.
Ques. Does the enthalpy of an ideal gas depend on its temperature? (1 Mark)
Ans: Yes, the enthalpy of an ideal gas depends only on its temperature because internal energy of an ideal gas depends only on the temperature and internal energy is a part of enthalpy change.
Ques. Which device is used to measure the dissolution enthalpy of a particular salt? (1 Mark)
Ans: Calorimeter is the device used to measure the dissolution enthalpy of a particular salt. It is also used to measure the amount of heat involved in a chemical reaction, mechanical, electrical or physical process and for calculating the heat capacity of materials.
Ques. What is meant by Thermochemistry? (1 Mark)
Ans: Thermochemistry is the branch of thermodynamics that focuses on changes occurring during chemical reactions or a physical transformation. Thermochemistry concentrates on the energy exchange between a system and its surroundings in the form of heat.
Ques. How is the vapor pressure of the liquids related to the interaction pattern between the molecules of the components of a liquid mixture? (1 Mark)
Ans: A pure liquid experiences a great amount of vapor pressure against a liquid’s solution. It is inversely proportional to the forces of attraction existing between the molecules of liquid which increases with a rise in temperature.
Ques. How can we correlate the heat evolved from the system with the strength of the hydrogen bond? (1 Mark)
Ans: If the hydrogen bond is stronger, then the heat debited from the system will be more and if the hydrogen bond is weaker, then the heat debited from the system will be lower.
Ques. Define Raoult’s law. (3 Mark)
Ans: Proposed by French chemist Francois-Marie Raoult in 1887, Raoult’s law is a relation of physical chemistry with implications in thermodynamics.
Raoult’s law states that, the mole fraction of any solvent is directly proportional to the vapor pressure of an ideal solution at given temperature which means if two liquid pairs interact with each other, they behave ideally.
Equation for the Raoult’s law is given by:
Psolution=XsolventPsolvent
Where, Psolution=vapor pressure of the solution
Psolvent= vapor pressure of the pure solvent
Xsolvent= mole fraction of the solvent
Ques. Explain how the enthalpy of a system will change, if heat is transferred at constant pressure? (2 Mark)
Ans: Enthalpy of a system increases if heat is transferred at a constant pressure.
The enthalpy at constant pressure is given by:
ΔH=Δu+pΔv
As heat is being given to the system, the internal energy(u) and the volume increases. Therefore,
Δu=+ve , Δv=+ve
∴ ΔH=+ve
Change in enthalpy is positive. So, the enthalpy of the system will increase.
Ques. What is the importance of enthalpy change in thermodynamics? (2 Mark)
Ans: Enthalpy change plays a significant role in thermodynamics and is very important in chemistry. Some of its uses are listed below:
- It is used to determine the type of reaction, whether it is endothermic reaction or exothermic reaction. If the enthalpy change is positive, then the reaction is endothermic; and if it is negative, then the reaction is exothermic.
- Enthalpy change also helps in the Joule-Thomson expansion.
- It is used to calculate the flow of heat and its reactions.
- It also helps in measuring the power of the compressor.
Ques. Give two examples of each of the liquids pairs for which enthalpy change on mixing (ΔHmixing) is negative and positive respectively. (2 Mark)
Ans: Examples of the liquids pairs for which ΔHmixing is negative are:
- Chloroform+Benzene
- Acetone+Aniline
Examples of the liquids pairs for which ΔHmixing is positive are:
- Acetone+Benzene
- Methyl alcohol+Water
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