SOLUTION: dilute hydrochloric acid in a conical flask

CHEMISTRY
CHEMISTRY LAB 1

Rates of reaction – how is this affected by the concentration of acid?

Initial Planning

Magnesium reacts with dilute hydrochloric acid in a conical flask which is connected to a gas syringe. The volume of hydrogen gas produced is measured over a few minutes, and the results are used to plot a graph

Work in pairs – one student can add the magnesium ribbon to the acid and stopper the flask, while the other starts the stopclock.

During the experiment, one student can take the readings while the other records them.

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Read these instructions carefully before you start work.

Measure 50 cm³ of 1.0 M hydrochloric acid using one of the measuring cylinders. Pour the acid into the 100 cm³ conical flask.

Set up the apparatus.

When you are ready, add a 3 cm strip of magnesium ribbon to the flask, put the bung back into the flask as quickly as you can, and start the stopclock.

Record the volume of hydrogen gas given off at suitable intervals (eg 10 seconds) in a table such as the one below.

Continue timing until no more gas appears to be given off.

Time in seconds	Volume of gas produced for 2.0 M hydrochloric acid in cm³
10
20
30
40
50
60
70
80
90
100

Repeat steps 1-5 using 0.5 M hydrochloric acid.

Plot a graph with:

‘Volume of gas produced in cm³ (for 1.0 M hydrochloric acid)’ on the y-axis
‘Time in seconds’ on the x-axis

Draw a smooth curved line of best fit.

Plot a curve for 0.5 M hydrochloric acid on the same graph.

Health, safety and technical notes

Wear eye protection throughout. Ensure that there are no naked flames.
Magnesium ribbon, Mg(s) – see CLEAPSS Hazcard HC59a. The magnesium ribbon should be clean and free from obvious corrosion or oxidation. Clean if necessary by rubbing lengths of the ribbon with fine sandpaper to remove the layer of oxidation.
Hydrochloric acid, HCl(aq) – see CLEAPSS Hazcard HC47a and CLEAPSS Recipe Book RB043. The hydrochloric acid should be about 1M for a reasonable rate of reaction. Each experiment run will need 50 cm³. Though low hazard, eye protection is necessary as you may get a spray as tiny bubbles burst.
Hydrogen gas, H₂(g) (EXTREMELY FLAMMABLE) – see CLEAPSS Hazcard HC048. Ensure that all naked flames are extinguished, and that there are no other sources of ignition available to students.