Ethene gas can be prepared from ethanol in a school laboratory - Leaving Cert Chemistry - Question 2 - 2012

A 'suck-back' is most likely to occur when the heating is stopped and ethanol vapors condense back into the flask, creating a vacuum that pulls liquid back into the heating apparatus.

One possible consequence of a 'suck-back' is the risk of injury due to the glass apparatus cracking or breaking due to the sudden intake of liquid into the hot flask.

To avoid a 'suck-back,' a safety device such as a one-way valve or a water trap can be included in the setup to prevent backflow of liquid into the hot flask.

To test for unsaturation, bubble the ethene gas through bromine water. If the color of the bromine water changes from orange to colorless, it indicates the presence of unsaturation (double bond).

The balanced equation for the reaction is:

$C_2H_5OH \xrightarrow{Al_2O_3 / Heat} C_2H_4 + H_2O$

To find the maximum number of test tubes, first calculate the amount of ethanol in grams:

1. Calculate the mass of ethanol:

$2.4 \, cm^3 \times 0.8 \, g/cm^3 = 1.92 \, g$

2. Knowing that the molar mass of ethanol (C₂H₅OH) is 46 g/mol:

$\frac{1.92 \, g}{46 \, g/mol} = 0.0417 \, mol$

3. Theoretical yield of ethene (assuming full conversion) is:

each mole of ethanol produces one mole of ethene, therefore, the theoretical yield of ethene is also 0.0417 mol.

4. Given a 60% yield:

$0.0417 \, mol \times 0.60 = 0.0250 \, mol$

5. The volume of one mole of gas under RTP is approximately 24 dm³ (or 24000 cm³), making the volume of ethene produced:

$0.0250 \, mol \times 24000 \, cm^3/mol = 600 \, cm^3$

6. Finally, divide the total volume of ethene by the volume of one test tube:

$\frac{600 \, cm^3}{75 \, cm^3/test \, tube} = 8 \, test \, tubes$.