Q2. * Preparation and properties of ethyne Flashcards
From what reaction is ethyne prepared?
Ethyne is prepared by the reaction of water with calcium carbide, CaC2
What is the formula for calcium carbide?
CaC2
What is CaC2?
Calcium carbide
What can calcium carbide also be called?
Calcium dicarbide
What can calcium dicarbide also be called?
Calcium carbide
How are the gases that contaminate hydrogen removed?
By bubbling the gas through acidified copper (II) sulfate
What chemicals are needed for this experiment?
5
Calcium carbide
Acidified copper (II) sulfate solution
Limewater
Bromine water
Acidified potassium permanganate solution
What is the first step of this experiment?
3
Set up the apparatus
Make sure to use a spatula when handling the calcium carbide and not to touch it with your fingers
Use only a few lumps of calcium carbide
Describe how you set up the apparatus
8
Place some calcium carbide lumps in the Buchner flask
Put a one-hole rubber stopper in the top of the Buchner flask
Connect the stopper to the dropping funnel and fill the funnel with water
Using a delivery tube connect the Buchner flask to the gas-washing bottle
Fill the gas-washing bottle with acidified copper sulfate solution
Stopper the gas-washing bottle with a two-hole rubber stopper
Using a delivery tube connect the gas-washing bottle to the has jar
Cover the gas jar with a gas-jar cover and place in a trough of water
Where is the calcium carbide placed?
In the Buchner flask
What is placed inside the Buchner flask?
Calcium carbide
What is attached to the Buchner flask?
2
A dropping funnel is attached to a one-hole rubber stopper at the mouth of the Buchner flask
A delivery tube leads from the upper side of the Buchner flask to the gas-washing bottle
What is the gas-washing tube filled with?
Acidified copper (II) sulfate solution
What is filled with acidified copper (II) sulfate solution?
The gas-washing bottle
What is attached to the gas-washing tube?
3
Two delivery tubes are connected
One delivery tube comes from the Buchner flask containing the calcium carbide - this tube must be underneath the surface of the acidified copper (II) sulfate solution
The second tube is connected to the gas jar and it must be above the surface of the acidified copper (II) sulfate solution
Where is the ethyne gathered?
At the space at the top of the gas jar
Where is the gas jar placed?
It is filled with water and placed in a trough of water
What gathers in the space at the top of the gas jar?
Ethyne
Why should no naked flame be brought anywhere near the gas preparation apparatus?
Because ethyne forms an explosive mixture with air
Why should you avoid handling the calcium carbide with your hands?
Because the calcium carbide could start a reaction with the moisture from your hands
What is the second step of this experiment, the apparatus has just been set up?
Double check that you have connected the gas-washing bottle correctly
What is the third step of this experiment, the gas-washing bottle connections have just been double checked?
Using the dropping funnel, add water slowly to the calcium carbide to maintain an even evolution of ethyne
What colour is the calcium carbide?
It is a grey-black or grey-brown colour
What four observations can be made when the water is added to the calcium carbide?
Fizzing takes place in the Buchner flask
A white solid is formed
Bubbles of as start to collect in the gas jar
The Buchner flask becomes warm
What causes the fizzing in the Buchner flask?
This is caused by ethyne gas being given off by the reaction in the flask
What reaction takes place in the Buchner flask?
CaC2 + 2H2O –> Ca(OH)2 + C2H2
What is the name of the white substance formed in the Buchner flask?
Calcium hydroxide
What colour is calcium hydroxide?
White
Why is the Buchner flask warm to the touch during the experiment?
Because the reaction that takes place between calcium carbide and water is exothermic
What is the fourth step of the experiment, ethyne gas is now bubbling in the gas jar?
(5)
Collect a few test-tubes of the gas
Use a lighted taper to test for the presence of ethyne gas in the fume cupboard
The first few will be simply displaced air
The next few will be a mixture of ethyne and air and might burn with a slight ‘pop’
When the gas burns quietly with a smoky flame, collect four test-tubes and a gas jar of the gas
How do you test for the presence of ethyne gas?
Use a lighted taper in the fume cupboard to test for ethyne gas
What may happen if there is only displaced air in the test tube?
The flame wont burn or ‘pop’
What may happen if there is a mixture of air and ethyne in the test tube?
There flame may burn with a ‘pop’
What may happen if there is a mixture of ethyne only?
The gas will burn quietly with a smoky flame
What does it mean if there is a ‘pop’ from the test tube?
This means there is a mixture of air and ethyne in the tube
What does it mean if there is a smoky flame burning quietly?
This means you have successfully prepared ethyne gas
How many test tubes of ethyne should you collect?
4 test tubes and a gas jar of ethyne
What is the fifth step of this experiment, you have just gathered four test tubes and a gas jar of ethyne?
(2)
Dismantle the apparatus when finished collecting ethyne
Dispose of the contents of the Buchner flask as directed by your teacher
Why should you dismantle the apparatus as soon as you have finished collecting ethyne?
As you don’t want there to be ethyne being generated when you test for ethyne’s properties as one test involves a lighted taper and ethyne forms an explosive mixture with air
What is the sixth step of this experiment, the apparatus has just been dismantled?
(7)
Note the physical properties of the gas;
- colourless gas
- sweetish type of smell
- won’t smell nice if there are still some impurities left
- Insoluble in eater
- soluble in organic solvents e.g. dissolves easily in propanone (acetone)
- Explosive nature (forms explosive mixture with air), stored in steel cylinders as a solution in propanone
List the physical properties of ethyne.
4
Colourless gas
Sweetish smell
Insoluble in water
Soluble in organic solvents e.g. propanone (acetone)
Why mightn’t ethyne smell nice?
If there are some impurities left after its production
How should pure ethyne smell?
Sweet
What is ethyne insoluble in?
Water
What is ethyne soluble in?
Organic solvents
Give an example of an organic solvent that ethyne is soluble in.
Dissolves easily in propanone (acetone)
How should ethyne be stored?
In steel cylinders as a solution in propanone
Why does ethyne need to be stored in steel cylinders as a solution in propanone?
Because of its explosive nature, it forms an explosive mixture with air
What is step seven of this experiment, you have just noted the physical properties of ethyne?
(6)
Combustion test
Bring the gas jar of ethyne to the fume cupboard, remove the gas jar cover and apply a lighted taper to the mouth of the gas jar
Note ethyne burns with a luminous smoky flame and produces a great deal of soot
The soot is formed as a result of unburnt carbon
You will need two and a half gas jars of oxygen of you want to burn the gas jar of ethyne to completion - to form carbon dioxide and water
Wash out the jar when finished
What is the name given to step seven?
The combustion test
How do you set up for the combustion test?
3
Bring the gas jar of ethyne to the fume cupboard
Remove the gas jar cover
Apply a lighted taper to the mouth of the gas jar
What happened when you opened the gas jar of ethyne with the lightened taper held at its mouth?
(2)
The ethyne burned with a luminous smoky flame
It produced a great deal of soot
What caused the soot produced from the burning of ethyne?
It is the result of unburnt carbon
How could you prevent the soot forming when burning ethyne?
You would need two and a half gas jars of oxygen to burn the gas jar of ethyne to completion - to form carbon dioxide and water
What is the formula for the combustion of ethyne?
2C2H2 + 5O2 –> 4CO2 + 2H2O
What is the eight step of this experiment, you have just carried out the combustion test?
(5)
Addition of bromine to test for unsaturation
Add approx. one-tenth of a test-tube of bromine water to a test-tube of ethyne
Replace the stopper and shake the test-tube a few times
Note the red colour changes to colourless/orange to colourless/yellow to colourless depending on bromine water concentration
The very unsaturated triple bond of ethyne reacts readily with the bromine
How do you test for unsaturation in ethyne?
3
Add one-tenth of a test-tube of bromine water to a test-tube of ethyne
Stopper and shake a few times
Note colour change
What colour change takes place when bromine is added to ethyne?
Red/orange/yellow to colourless
What does the colour of the bromine water depend on?
How concentrated it
What is the ninth step of this experiment, you have just tested for unsaturation?
(5)
Addition of acidified potassium permanganate solution (KMnO4) - test for unsaturation
Add about one-tenth of a test tube of very dilute potassium permanganate (pale purple colour) to a test-tube of ethyne
Replace the stopper and shake a few times
Note the colour change from purple to colourless
The very reactive (unsaturated) carbon carbon triple bond of ethyne reacts readily with the potassium permanganate
What is KMnO4?
Potassium permanganate
What is the formula for potassium permanganate?
KMnO4
What can potassium permanganate also be called?
Potassium manganate (VII)
Other than the addition of bromine how do you test for unsaturation?
Add about one-tenth of a test tube of very dilute potassium permanganate (pale purple colour) to a test-tube of ethyne
Replace the stopper and shake a few times
Note the colour change from purple to colourless
What colour change takes place when potassium permanganate is added to ethyne gas?
Pale purple to colourless
Why does potassium permanganate turn colourless in the presence of ethyne?
Because the very reactive (unsaturated) carbon carbon triple bond of ethyne reacts readily with the potassium permanganate
