Cracking a hydrocarbon

Summary

This educational video demonstrates the industrial catalytic cracking of long-chain alkanes using liquid paraffin and a silica-based catalyst like pumice stone. The process involves heating in a boiling tube to produce shorter-chain alkanes and alkenes. Safety precautions against suckback are highlighted, and the experiment concludes with testing collected gases for flammable properties and the presence of carbon-carbon double bonds using bromine water and acidified potassium permanganate.

Takeaways

• 🔬 The experiment simulates the industrial catalytic cracking of long-chain alkanes to produce shorter-chain alkanes and alkenes.
• 🧪 Mineral wool is used at the bottom of the boiling tube instead of cotton wool to hold the liquid paraffin.
• 📐 Approximately 2 cm3 of liquid paraffin is added to the mineral wool to ensure it soaks in properly.
• 🔧 A pumice stone or pieces of broken plant pot serve as catalysts, mimicking industrial silica and alumina-based catalysts.
• 🔥 The pumice stone is heated using the hottest part of the Bunsen flame, just beyond the blue cone.
• 💨 Gas produced during the reaction is collected by an assistant in test tubes filled with water.
• ⚠️ It's crucial to prevent water from being sucked back into the boiling tube, which can cause it to crack.
• 🔥 Continuous heating of the catalyst is necessary to maintain the reaction and prevent cooling.
• 🌡️ Monitoring for signs of 'suckback' in the delivery tube is important to avoid damage to the apparatus.
• 🧪 The first two test tubes of gas are discarded as they contain air from the boiling tube.
• 🔥 The third test tube's gas can be smelled, indicating a similarity to camping gas, a mixture of propane and butane.
• 🔥 The fourth test tube's gas can be ignited to demonstrate the flammable nature of the produced gas.
• 🧪 Bromine water is added to the fifth test tube to test for the presence of alkenes, as the bromine decolorizes in their presence.
• 🧪 Acidified potassium permanganate is used in a similar test with the sixth tube to confirm the presence of carbon-carbon double bonds.

Q & A

What is the purpose of the experiment described in the script?

The experiment models the industrial catalytic cracking of long-chain alkanes to demonstrate that the process can produce shorter-chain alkanes and alkenes, which have carbon-carbon double bonds.

What material is used to mimic the industrial catalyst in this experiment?

Pumice stone or pieces of broken plant pot are used as catalysts in the experiment, both of which are silica-based and mimic the silica and alumina-based catalysts used in industry.

Why is mineral wool preferred over cotton wool in the boiling tube?

Mineral wool is used because it allows the liquid paraffin to soak in, providing a larger surface area for the reaction to take place, unlike cotton wool which may not absorb the paraffin as effectively.

How much liquid paraffin is needed for the experiment?

Approximately 2 cm3 of liquid paraffin is needed, enough to soak into the mineral wool at the bottom of the boiling tube.

What is the role of the bung with a delivery tube in the setup?

The bung with a delivery tube is attached to the mouth of the boiling tube to collect and direct the gases produced during the catalytic cracking process into separate test tubes filled with water.

Why is it important to heat the pumice stone before heating the paraffin-soaked mineral wool?

Heating the pumice stone first ensures that it is hot enough to catalyze the reaction when the paraffin is heated, promoting the cracking process effectively.

What is the significance of the 'suckback' phenomenon mentioned in the script?

Suckback occurs when water is sucked back into the boiling tube due to cooling, which can cause the tube to crack. It is crucial to avoid this by keeping the tube hot and removing it from the water if signs of suckback are observed.

How can you test for the presence of an alkene in the collected gases?

The presence of an alkene can be tested by adding a few drops of bromine water or acidified potassium permanganate to the gas in a test tube and observing a color change, which indicates the reaction with a carbon-carbon double bond.

Why are the first two test tubes of gas collected discarded?

The first two test tubes are discarded because they likely contain air from the boiling tube rather than the products of the catalytic cracking reaction.

How can you demonstrate that the gas collected is flammable?

The flammability of the gas can be demonstrated by igniting the gas in the fourth test tube, which should burn if it contains flammable alkanes or alkenes.

What safety precautions should be taken when heating the boiling tube?

Safety precautions include using the hottest part of the Bunsen flame, avoiding cooling the tube to prevent suckback, and ensuring that the experiment is conducted away from flammable materials or other hazards.

Outlines

00:00

🔬 Industrial Catalytic Cracking Experiment Setup

This paragraph outlines the setup for an experiment that simulates the industrial catalytic cracking of long-chain alkanes. It involves placing mineral wool at the bottom of a boiling tube, soaking it with approximately 2 cm3 of liquid paraffin, which serves as the source of alkanes. The catalyst, either pumice stone or broken plant pot pieces, is then added to mimic industrial silica and alumina-based catalysts. The experiment requires careful heating of the catalyst with a Bunsen flame, ensuring the paraffin-soaked mineral wool is heated briefly without allowing the setup to cool, which could cause the tube to crack. The escaping gases are collected in test tubes filled with water, with an assistant's help, while monitoring for 'suckback' to prevent water from entering the tube.

05:04

🔬 Testing and Conclusion of the Catalytic Cracking Experiment

The second paragraph details the conclusion and testing phase of the catalytic cracking experiment. After stopping the heating process, it emphasizes the importance of removing the setup from water first and continuing to heat briefly to ensure no water remains in the tube. The collected gases are then tested: the first two are discarded as they are merely air, while the third is smelled to confirm the presence of gases similar to camping gas. The fourth gas is ignited to demonstrate flammable properties. Bromine water is added to the fifth test tube, and its color change indicates the presence of alkenes due to the carbon-carbon double bonds. A similar test is conducted with acidified potassium permanganate, which also turns colorless or brown, further confirming the presence of alkenes.

Keywords

💡Catalytic Cracking

Catalytic cracking is a chemical process used to break down larger hydrocarbon molecules into smaller, more useful ones. In the context of the video, it refers to the industrial process of converting long-chain alkanes into shorter-chain alkanes and alkenes. The script describes how this is modeled in an experiment using a boiling tube, mineral wool, and a catalyst.

💡Alkanes

Alkanes are a class of hydrocarbons with the general formula CnH2n+2, characterized by single bonds between carbon atoms. They have the simplest structure among hydrocarbons. In the script, alkanes are the starting materials for the cracking process, with liquid paraffin mentioned as having about twenty carbons in its chain length.

💡Alkenes

Alkenes are unsaturated hydrocarbons containing carbon-carbon double bonds, with the general formula CnH2n. They are reactive and are often products of catalytic cracking, as indicated in the script. The presence of alkenes is confirmed through chemical tests with bromine water and acidified potassium permanganate.

💡Mineral Wool

Mineral wool is a type of insulation material made from natural rock or stone, spun into fibers. In the script, it is used at the bottom of the boiling tube to soak up the liquid paraffin, which is crucial for the experiment's setup.

💡Pumice Stone

Pumice stone is a volcanic rock that has a highly porous structure, making it an excellent catalyst in chemical reactions due to its large surface area. In the video, it is used to mimic industrial catalysts and facilitate the cracking of alkanes.

💡Catalyst

A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. In the script, both pumice stone and pieces of broken plant pot serve as catalysts to speed up the cracking of long-chain alkanes.

💡Bunsen Burner

A Bunsen burner is a laboratory device used for producing a single open flame, often used for heating substances. In the script, it is used to heat the pumice stone catalyst and the paraffin-soaked mineral wool to initiate the cracking reaction.

💡Suckback

Suckback refers to the phenomenon where water is drawn back into a heated tube, which can cause the glass to crack due to rapid cooling. The script emphasizes the importance of preventing suckback by keeping the boiling tube hot and removing it from the water bath before stopping the heat.

💡Bromine Water

Bromine water is a solution of bromine in water, commonly used in chemistry to test for the presence of alkenes. In the script, it is used to confirm the formation of alkenes by causing the bromine water to decolorize upon reaction.

💡Acidified Potassium Permanganate

Acidified potassium permanganate is a chemical reagent that reacts with alkenes, causing it to change color. In the script, it is used as a test to confirm the presence of alkenes in the collected gases.

💡Flammable Gas

A flammable gas is one that can ignite in the presence of oxygen and a source of ignition. In the script, the production of a flammable gas is demonstrated by attempting to ignite the collected gas, which is expected to have properties similar to propane and butane.

Highlights

The experiment models industrial catalytic cracking of long-chain alkanes.

Mineral wool is used instead of cotton wool at the bottom of the boiling tube.

Liquid paraffin with about twenty carbons in its chain length is used as the initial source of alkanes.

Approximately 2 cm3 of liquid paraffin is added to soak into the mineral wool.

Excess paraffin is removed to prevent it from going down the inside of the tube.

Pumice stone or broken plant pot pieces are used as catalysts, mimicking industrial silica and alumina-based catalysts.

The catalyst should be spread to prevent it from falling to the bottom of the tube.

A bung with a delivery tube is attached to collect gases produced during the reaction.

Heating is done using the hottest part of a Bunsen flame, just beyond the blue cone.

The pumice stone is heated before the paraffin soaked in mineral wool to initiate the reaction.

It is crucial to keep the boiling tube and its contents hot to prevent water suckback and tube cracking.

Signs of suckback are monitored, and if detected, the tube is heated with the hottest flame part and removed from water.

Avoid pushing the Bunsen closer or removing heat to prevent increased suckback.

Continue heating until six test tubes of gas have been collected.

After heating, the tube must be removed from water first and then heating stopped to ensure no water is left inside.

The first two gas samples are discarded as they are likely air from the boiling tube.

The third gas sample is identified by its smell, resembling camping gas.

The fourth gas sample is tested for flammability by attempting to ignite it.

Bromine water is added to the fifth gas sample to test for the presence of alkenes, which causes the bromine to lose color.

Acidified potassium permanganate is used to test for alkenes in the sixth gas sample, changing color upon reaction.

Transcripts

This experiment models the industrial catalytic cracking of long-chain alkanes

and shows that some of the products have shorter chains than the starting

material and

some are alkenes (they have carbon-carbon double bonds)

The experiment is carried out in a boiling tube

and you need to start by putting some mineral wool

not cotton wool into the bottom of the tube

the initial source of alkanes is liquid paraffin

which has about twenty carbons

in its chain length.

Use a dropping pipette to add liquid paraffin so that it soaks into the mineral wool. You will need about

2 cm3.

Just help that to soak in a bit.

and just invert it to check if there's any excess

and if there is remove it using a pipette

you don't want to get paraffin all the way down the inside of the tube

if you can avoid it. You put that into the clamp

and clamp it right near the mouth of the tube.

next you're going to put some catalyst

into the tube and we are using pumice stone

but you can also use pieces

of broken plant pot. Both of these are

silica based and they mimic the silica and alumina based catalysts

that are used in industry. Now you want this tilted

but not so much so that all your catalyst

falls to the bottom.

This has got quite an open structure which gives you

a large surface area for the reaction to take place on.

You attach a bung with a delivery tube

to the mouth of your boiling tube. I have already prepared a bowl of water

with about six test tubes also filled with water

and some bungs. Next you need an assistant

who could possibly be a competent student, although actually, I often do this experiment

with two students

and don't get too involve myself at all

and what you are going to do is to heat

using a roaring Bunsen flame. And the hottest part of the flame which

is just beyond

the blue cone. And you are going to heat

the pumice stone. As you do so

some gas will escape and your assistant needs to collect the gas

in a test tube. You want to get the pumice stone

good and hot before you go any further. Once you've been heating for about 10 seconds

then you can flick across and just briefly

heat your paraffin soaked in the mineral wool.

Then go back and keep the catalyst hot.

The whole time keep the catalyst hot and then just

flick across to start the paraffin

evaporating you want to heat the pumice stone for about 10 seconds

for every second that you're heating your paraffin

it is important to keep the boiling tube and its contents hot

if it is allowed to cool water may be sucked back into the boiling tube

and it can cause the boiling tube to crack. It's vital to watch the delivery

tube for signs of suckback. If this seems to be starting

the solution is to continue to heat the boiling tube with the hottest part of the flame

just beyond blue cone and to remove the whole thing from the water.

It is tempting to push to Bunsen closer to the boiling tube

or two remove the heat both of which will increase suckback,

and you can see there, there are some water drops

coming up the tube and if I just keep heating those

then they should come out.

Continue until six test tubes of gas have been collected.

Okay when you come to the end

and you need to remove it from the water and stop heating.

it's really important that you remove from the water first

and carry-on heating for a moment. When you are happy that there is no more water in the tube

you can stop heating. But I wouldn't turn off the Bunsen burner

until you're convinced at that everything has stopped.

Then you need to test the gases.

The first two you can discard because they will just be

air from your boiling tube. Okay the third one

you can pass around the class and they can have a gentle smell

and it should smell something like camping gas, which is a mixture of propane

and butane. The fourth one you can attempt to ignite

to prove that you have made a flammable gas.

To the 5th tube

you can add a few drops of bromine water, which is just bromine

dissolved in water

and shake it up and you can see that the bromine

goes colourless. This shows the presence of an alkene

or carbon-carbon double bond. A similar reaction you can do with acidified

potassium permanganate.

A few drops into the test tube again shake it up

and you can see that this has changed colour and it will go either to

brown or a colourless solution and again that shows the presence of an alkene

or a carbon-carbon double bond.