Principle Methods and Experimental Procedure for Determining Boiling Point of Organic Compounds
The Meaning of Boiling Point
There are some topics in Chemistry that are very important from the point of view of the final examinations. The topic of determination of the boiling point of an organic compound is one such topic.
The determination of the boiling point of organic compounds is a topic that can also come up during the practice exam of students. This is why it is advised that students should pay special attention to this topic.
But what if a student missed the class during which the teacher was giving a lecture on the determination of the boiling point of organic compounds? If that is the case, then today is a lucky day for all students as we are going to discuss every major point that students need to learn about the determination of the boiling point of liquid organic compounds. However, before we get into that, students need to start with the basics.
Let’s begin with understanding the meaning of the boiling point. In the simplest terms, the boiling point of a substance can be defined as a particular temperature at which that substance would achieve a vapour pressure that is equal to the pressure around the liquid. This is also the temperature at which the liquid will change into a vapour.
To a large extent, the boiling point of a liquid will be dependent on the environmental pressure that is surrounding that liquid. This means that if a liquid is in a partial vacuum, then it would have a lower boiling point in comparison to the boiling point of the same liquid in atmospheric pressure.
Following the same logic, it can also be said that a liquid at a higher pressure will also have a higher boiling point in comparison to a liquid that is present at atmospheric pressure. To illustrate these points, an example can be taken. For example, if water boils at 1000 C at sea level, then it would boil at around 93.40 C at 1,905 metres altitude.
It is also important for students to note that different liquids will boil at different temperatures at a given pressure. These are all the basic points that students should know about the topic of boiling points.
Organic Compounds
Organic compounds are a wide range of specific compounds related to life processes. The constituent elements of these compounds are carbon and hydrogen or in other words known carbon-containing compounds.
How to determine the Boiling Point of an Organic Compound?
In this section, students will be able to learn about the process that should be followed for the determination of the boiling point of liquid organic compounds. Students can also use this process to determine the boiling point of a given organic compound.
Students must be aware of the fact that organic chemistry is a branch of chemistry that mainly deals with the subject of composition and synthesis of organic chemical compounds. For students who are not familiar with the term, organic compounds are compounds that contain a carbon atom in their composition.
Now, let’s move on to discuss the process of identifying the boiling point of an organic compound. For ease of understanding, we will look at this entire process as a large experiment.
Aim: This experiment aims to find out the boiling point of various organic compounds. To narrow things down, for this experiment, two organic compounds called Benzene and Benzaldehyde are selected.
The Materials Required: To perform this process, an individual will require a stand with clamps, capillary tubes, tripod, thermometer, kerosene burner, Benzene, Benzaldehyde, fusion tube, and an aluminium block.
The Experimental Setup: It is rather difficult to explain the experimental setup without any visual aid. This is why an image of the ideal experiment setup is attached below.
(Image will be Uploaded soon)
The Procedure for Determining the Boiling Point of Benzene
The student should begin by taking the capillary tube and closing its end by holding the end in the flame. Rotating the tube for 2-3 minutes should do the trick.
A few millilitres of benzene should be transferred to the fusion tube.
Proceed to dip the capillary tube in the liquid in the fusion tube. Do not forget to keep the sealed end of the tube up.
The tube should be inserted in one of the holes of the aluminium block. After that, a thermometer should also be inserted into the same block.
After everything is placed inside the hole, one should make sure that the liquid is completely visible in the fusion tube.
Put the aluminium block on the tripod.
Use the kerosene burner to slowly heat the aluminium block.
Make a note of the temperature as soon as regular streams of bubbles appear in the liquid present in the fusion tube.
The Procedure for Determining the Boiling Point of Benzaldehyde
Begin the process similarly by closing one end of the capillary tube by holding that end in the flame and rotating it for 2-3 minutes.
Place a few drops of benzaldehyde in the fusion tube.
Keep in mind to put the sealed side up and dip the capillary tube in the benzaldehyde liquid filled in the fusion tube.
Insert the tube and the thermometer into two separate holes made on the aluminium block.
Make sure that you can observe the liquid present in the fusion tube clearly
Put the aluminium block on the tripod and start heating the block with the help of the kerosene burner.
Make a note of the temperature as soon as a regular stream of bubbles appears on the liquid present inside the fusion tube.
Precautions to take During the Experiment
To date, we have looked at almost everything related to the process that one needs to follow to determine the boiling point of a given organic compound. However, the work of a student is still not done.
No task or experiment is complete or should even be allowed to carry out unless and until the students understand the precautions related to that experiment or procedure.
In this section, we will discuss some of the most prominent precautions students should take while finding out the boiling point of an organic compound. We have prepared a list of those precautions, and that list is mentioned below.
Make sure that the capillary tubes are completely sealed.
It is also important to ensure that the seal point of the capillary tubes should be within the liquid.
Use proper lab equipment and follow all safety precautions.
Fun Facts about Determining the Boiling Point of an Organic Compound
Did you know that the element with the lowest boiling point in the world is helium? Further, the boiling points of Tungsten and Rhenium are said to exceed 5000 K, but nobody has been able to find out the exact boiling point as it is difficult to measure extreme temperatures.
Also, if a student has carried out the experience that was mentioned above, then he or she would have observed that the boiling point of benzene is around 780C and the boiling point of benzaldehyde is 1780 C.
Did You Know?
The boiling point of an organic molecule is the temperature at which an attractive force (such as Vanderwaal attractive force) or an intermolecular bond (such as a hydrogen bond) breaks and the molecule moves. On the other hand, intramolecular covalent bonds are not broken at their boiling point. The boiling point of organic compounds depends on their molecular weight. As the molecular weight increases, so does the boiling point.
For two compounds of the same molecular weight, various factors determine the boiling point of an organic compound.
The most important factor is the shape of the molecule, whether or not the molecule is branched. If the molecule is more branched, it means a sphere, which causes a smaller surface area and a weaker Vanderwaal attraction between the molecules, thus having a lower boiling point. If the organic compound is straight or has few branches, the surface area will be large and the intramolecular Vanderwaal attraction will be strong, resulting in a high boiling point.
Another important factor is the functional groups of organic molecules. If the functional groups are polar and capable of forming hydrogen bonds between the molecules, higher temperatures are required to break these hydrogen bonds with the Vanderwaal attractive force between the molecules, thereby causing these types of compounds. The boiling point rises.
Difference between Boiling Point and Melting Point of an Organic Compound
Both boiling point and melting point are properties of an organic compound. They play a very important role in explaining the properties of the compound. The main key difference between boiling point and melting point is that boiling point is the temperature at which a liquid state converts to a gaseous state, and melting point is the temperature at which solid-state converts to a liquid state.
A liquid having high pressure has a higher boiling point and a liquid at lower pressure has a low boiling point. Therefore, the boiling point is the definition for the liquid state and the melting point is the definition for the solid-state. At the boiling point, the vapour pressure of the liquid becomes equal to the pressure around it whereas the melting point has no connection with the pressure around it.
Importance of Knowing Boiling Point of an Organic Compound
The boiling points of organic compounds can provide important information about their physical and structural properties. Boiling points help identify and characterise compounds. A liquid starts to boil when its vapour pressure becomes equal to the atmospheric pressure. Vapour pressure is determined by the kinetic energy of the molecule.
Kinetic energy depends on the temperature, mass and velocity of the molecule. As the temperature rises, so does the average kinetic energy of the particles. When the temperature reaches the boiling point, the average kinetic energy is sufficient to overcome the attraction between the liquid particles. When the attractive force subsides, the liquid molecules escape from the surface and become gas.
The boiling point of a liquid depends on the ambient atmospheric pressure. A liquid with high pressure has a higher boiling point than a liquid with low atmospheric pressure. The normal boiling point of a compound is an indicator of its volatility. If the boiling point is high, the compound will be less volatile. Conversely, the lesser the boiling point, the more volatile the compound will be. In general, a compound will exist as a gas at atmospheric pressure if the normal boiling point of the compound is low at a particular temperature. If the compound has a high boiling point, it will be in the form of liquid or solid.
FAQs on Determination of Boiling Point of an Organic Compound
1. What is the boiling point of an organic compound?
The boiling point of an organic compound is the temperature at which its vapour pressure becomes equal to the external atmospheric pressure. At this temperature, the liquid changes into vapour throughout the bulk of the liquid.
- It is usually measured at 1 atm (760 mmHg) pressure.
- Boiling point is a characteristic physical property used for identification and purity testing of organic compounds.
- Pure compounds have a sharp and constant boiling point, while impure ones show a range.
2. How do you determine the boiling point of an organic compound in the laboratory?
The boiling point of an organic compound is determined by heating the liquid and recording the constant temperature at which it boils under atmospheric pressure.
- Take a small amount of the organic liquid in a boiling tube.
- Insert a thermometer so that its bulb is just above the liquid level.
- Heat gently using a water bath or oil bath.
- Note the temperature when rapid and continuous boiling occurs and remains constant.
3. What is the Siwoloboff method for determining boiling point?
The Siwoloboff method is a capillary tube method used to determine the boiling point of small quantities of an organic liquid.
- A small amount of liquid is taken in a fusion tube.
- A sealed capillary tube (sealed at one end) is inserted upside down into the liquid.
- The tube is heated in a water or oil bath.
- The temperature at which a continuous stream of bubbles emerges is noted.
- The boiling point is taken as the temperature when bubbling stops on cooling.
4. Why is boiling point important in organic chemistry?
Boiling point is important in organic chemistry because it helps in the identification, characterization, and purification of organic compounds.
- Each pure organic compound has a characteristic boiling point.
- It helps in checking purity, as impurities usually raise or lower and broaden the boiling range.
- It is used in distillation techniques to separate mixtures based on differences in boiling points.
5. What factors affect the boiling point of organic compounds?
The boiling point of organic compounds is mainly affected by intermolecular forces, molecular mass, and molecular structure.
- Intermolecular forces: Hydrogen bonding > dipole–dipole > London dispersion forces.
- Molecular mass: Higher molar mass generally increases boiling point due to stronger dispersion forces.
- Branching: Increased branching lowers boiling point because of reduced surface area.
- External pressure: Lower pressure decreases boiling point.
6. How does atmospheric pressure affect the boiling point of an organic liquid?
Boiling point decreases when atmospheric pressure decreases and increases when atmospheric pressure increases.
- At high altitudes, atmospheric pressure is lower, so liquids boil at lower temperatures.
- Under reduced pressure (vacuum distillation), organic liquids boil at temperatures lower than their normal boiling point.
- The normal boiling point is defined at 1 atm (760 mmHg).
7. What is the difference between normal boiling point and boiling point?
The normal boiling point is the temperature at which a liquid boils at 1 atm pressure, while boiling point can refer to the temperature at any given external pressure.
- Normal boiling point is a standard reference value.
- Boiling point at reduced or elevated pressure will differ from the normal value.
- For example, water boils at 100°C at 1 atm, which is its normal boiling point.
8. How can boiling point be used to check the purity of an organic compound?
A pure organic compound has a sharp and constant boiling point, while an impure compound shows a changed and broadened boiling range.
- Pure liquid: temperature remains constant during boiling.
- Impure liquid: boiling occurs over a range of temperatures.
- Impurities usually cause either elevation or depression of boiling point.
9. What are common errors in determining the boiling point of an organic compound?
Common errors in boiling point determination include incorrect thermometer placement, overheating, and presence of impurities.
- Thermometer bulb not at vapour level gives inaccurate readings.
- Rapid heating may cause superheating and false temperature values.
- Impurities lead to a wide boiling range.
- Failure to correct for atmospheric pressure variations can affect accuracy.
10. What is the difference between boiling point and melting point in organic compounds?
The boiling point is the temperature at which a liquid changes to vapour, while the melting point is the temperature at which a solid changes to liquid.
- Boiling point involves liquid → gas transition.
- Melting point involves solid → liquid transition.
- Both are physical properties used to determine purity and identity.
- Melting point is mainly used for solids, while boiling point is used for liquids.
