Delivery Tube and Gas Syringe

Description

Delivery tube

A delivery tube is a bent glass tube or a length of flexible rubber tubing that connects the reaction vessel to the gas collection apparatus. It provides a sealed pathway for gas to travel from the reaction to the collection point.

Glass delivery tubes are typically L-shaped or have a longer bend. Rubber tubing is more flexible and can connect glass components that are not in a straight line. Both are fitted to the reaction flask using a bung (rubber stopper) with a hole.

Gas syringe

A gas syringe is a large syringe (typically 100 cm3 capacity) with a plunger that moves outward as gas fills the barrel. It provides a direct volume reading.

Key features:

• Capacity: usually 100 cm3
• Resolution: 1 cm3 (sometimes 0.5 cm3)
• The plunger must move freely for accurate readings
• Connected to the reaction vessel via a delivery tube and a side-arm flask

Setup for gas collection

Using a gas syringe

1. Clamp the gas syringe horizontally in a retort stand. The plunger must face outward and move freely.
2. Connect the syringe to a side-arm flask or a flask with a bung and delivery tube.
3. Check that all connections are airtight. Loose connections allow gas to escape, giving low readings.
4. Place the reactants in the flask (or add through the top while the connection is sealed quickly).
5. As gas is produced, the plunger moves outward. Read the volume at regular time intervals for rate experiments, or at the end for total volume.

Using a delivery tube with collection over water

1. Fill a trough with water. Fill a measuring cylinder or gas jar with water and invert it in the trough.
2. Connect the delivery tube from the reaction flask to the end that sits under the water surface, inside the inverted container.
3. As gas is produced, it bubbles through the water and collects at the top of the inverted container, displacing water.
4. Read the volume from the markings on the measuring cylinder.

Using a delivery tube with displacement of air

1. Connect the delivery tube from the reaction flask to a gas jar (upright for dense gases, inverted for light gases).
2. Direct the tube to the bottom of the jar (for upward displacement) or into the mouth of the inverted jar (for downward displacement).
3. The gas fills the jar, displacing the air.

Common errors

Gas syringe plunger sticking: If the plunger does not move freely, the reading is too low. Check before starting by pulling and releasing the plunger; it should slide smoothly.

Leaky connections: Gas escaping through loose bungs or tubing gives a measured volume that is too low. Check all seals.

Starting the syringe above zero: The plunger should begin at 0 cm3. If it starts at a higher reading, subtract the initial value or reset it.

Soluble gas dissolving when collected over water: Gases like HCl, NH3, and SO2 dissolve in water. Use a gas syringe or displacement of air for these gases.

Not waiting for the reaction to finish: If you read the gas syringe before the reaction is complete, you record less than the total volume of gas produced. Wait until the reading stops increasing.