What does the reaction of magnesium with hydrochloric acid produce

A small piece of magnesium ribbon is placed in a Petri dish containing 1M hydrochloric acid. For a large audience presentation, this demonstration should be viewed via a camera or document camera. Students observe bubbles of hydrogen gas forming.

Table of Contents Show

Apparatus notes
Health, safety and technical notes
Teaching notes
Additional information

Curriculum Notes

There are many places in the curriculum where this demonstration could be used. it is most commonly used when discussing the characteristic reactions of either metals or acids. It's also a great example of a single replacement reaction (an oxidation-reduction reaction) when discussing types of reactions. This demonstration may also be performed using a document projector if a dark background is used. Allow about 5 minutes for this demo. A computer simulation is available to compliment and enhance this demonstration.

Learning Objectives

1. Magnesium metal is an active metal. Magnesium will react with hydrochloric acid.

2. Illustration of a single displacement reaction. Illustration of an oxidation-reduction reaction. Illustration of a reaction that produces a gas.

3. Write a balanced chemical equation and a net ionic equation.

Lead Time

One day of lead time is required for this project.

Discussion

Magnesium reacts with hydrochloric acid according to the equation: Mg(s) + 2 HCl(aq) --> MgCl2(aq) + H2(g) This demonstration can be used to illustrate the characteristic reaction of metals with acid, a single replacement reaction, or to demonstrate the generation of hydrogen gas. The flammability of hydrogen gas can be demonstrated by carefully holding a match or fireplace lighter up to the popping hydrogen bubbles. An audible crackling sound is heard as the hydrogen burns.

The net ionic equation for this reaction is: Mg(s) + 2H+(aq) --> Mg2+(aq) + H2(g)

Materials

Petri dish
1cm strip of magnesium ribbon
wash bottle containing 1M HCl
forceps
overhead projector
(Optional) butane fireplace lighter

Procedure

Mechanics of presenting this demonstration

Turn on the camera and focus it. Place the Petri dish on the overhead projector. Pour hydrochloric acid into the Petri dish to a depth of about 5 mm. Using forceps, place the magnesium ribbon into the Petri dish. The magnesium reacts with the acid, producing visible bubbles of hydrogen gas.

(Optional) If the flame of a butane fireplace lighter is held above the bursting bubbles, they will produce audible pops as the hydrogen ignites.

Safety Precautions

Magnesium ribbon is a flammable solid. Hydrochloric acid is a corrosive liquid. Hydrogen gas is explosive. However, the very small quantities and low dilutions used in or produced by this demo present little hazard. Wear safety goggles. Keep flammables and open flame away from the vicinity of the demo.

Footnotes

1. B.Z. Shakhashiri; Chemical Demonstrations: A Handbook for Teachers of Chemistry; Wisconsin; 1985; Volume 1; p. 25-26.

Schedule This Demo

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

This is intended as a class practical. It is best if the students work in pairs because setting up and starting the experiment requires more than one pair of hands. 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. The experiment itself takes only a few minutes. But allow at least 30 minutes to give students time to set up, take readings and draw graph.

Hydrogen gas (extremely flammable) is generated in the experiment. Students should not have access to any source of ignition.

Equipment

Apparatus

Eye protection
Conical flask (100 cm3)
Single-holed rubber bung and delivery tube to fit conical flask (note 1)
Trough or plastic washing-up bowl (note 2)
Measuring cylinders (100 cm3), x2
Clamp stand, boss and clamp
Stopwatch
Graph paper

Apparatus notes

The bungs in the flasks need to be rubber. Corks are too porous and will leak. The tube through the bung should be a short section of glass, and then a flexible rubber tube can be connected.
Gas syringes can be used instead of troughs of water and measuring cylinders. But these are very expensive and are probably best used by the teacher in a demonstration. Syringes should not be allowed to become wet, or the plungers will stick inside the barrels.

Chemicals

Magnesium ribbon cut into 3 cm lengths
Dilute hydrochloric acid, 1M

Health, safety and technical notes

Read our standard health and safety guidance
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.

Procedure

Measure 50 cm3 of 1M hydrochloric acid using one of the measuring cylinders. Pour the acid into the 100 cm3 conical flask.
Set up the apparatus as shown in the diagram. Half fill the trough or bowl with water.
Fill the other measuring cylinder with water, and make sure that it stays filled with water when you turn it upside down.
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 stopwatch.
Record the volume of hydrogen gas given off at suitable intervals (eg 10 seconds). Continue timing until no more gas appears to be given off.

Teaching notes

The equation for the reaction is: magnesium + hydrochloric acid → magnesium chloride + hydrogen

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

Students follow the rate of reaction between magnesium and the acid, by measuring the amount of gas produced at 10 second intervals.

3 cm of magnesium ribbon typically has a mass of 0.04 g and yields 40 cm3 of hydrogen when reacted with excess acid. 50 cm3 of 1M hydrochloric acid is a six-fold excess of acid.

In this reaction, the magnesium and acid are gradually used up. However the acid is in excess, so it is mainly the loss of magnesium (surface area becomes smaller) that causes the change in the rate.

If a graph of volume (y-axis) against time (x-axis) is drawn, the slope of the graph is steepest at the beginning. This shows that the reaction is fastest at the start. As the magnesium is used up, the rate falls. This can be seen on the graph, as the slope becomes less steep and then levels out when the reaction has stopped (when no more gas is produced).

The reaction is exothermic, but the dilute acid is in excess and the rise in temperature is only of the order of 3.5˚C. There is some acceleration of the reaction rate due to the rise in temperature. Some students might notice the flask becoming slightly warm and they could be asked how this would affect the rate of reaction, and how they might adapt the experiment to make it a ‘fair test’.

Additional information

This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology.

Health & Safety checked, 2016