The work of Dr. Mahmoud Marsafy

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REMEMBER: you must do your own experiment risk assessment, ideas are illustrated but health and safety details are NOT included

21. The reaction between magnesium and dilute hydrochloric acid

Comments on the reaction of Magnesium metal (Mg) and Hydrochloric acid (HCl_(aq)

The reaction of Mg and HCl is an Exothermic,  Redox , Single displacement reaction. H_2(g)  is produced and can be detected by the POP burning test with a lighted match.   Mg metal is sufficiently reactive to react with cold water but very slowly, releasing also H_2(g).

All of these reactions are very basic and important at various levels in teaching chemistry. As presented these reactions were explored qualitatively by inserting a polished Mg strip to 3ml  of a dilute HCl solution to which 2 drops of Universal Indicator (UI) is added. Color changes to the indicator occurred as depicted in the Chart. The same vial was photographed as soon as a significant change in colour happened.

This very simple, and very safe technique, and highly economical ,can be extended for investigation of other related concepts: Order of reactivity of Zn, Fe and may be other metals with HCl. Patterns of reaction with different acids (H₂SO₄, Ethanoic Acid (Acetic acid, Citric acid, Nitric acid) , Effects of the concentration of any one acid, Effects of variable temperature, and other factors.

This technique can also be applied quantitatively, by recording the mass of the Mg strip at a given time. It is very easy to take out the strip, and wipe it dry, and precisely finding its mass.

The decolorisation of the acidic red colour of the Universal Indicator (in vial 3 above), is probably due to reduction of the indicator dyes. More universal indicator was added (in vial 4) after the rapid stream of hydrogen gas, H₂ gas bubbles, was terminated. The somewhat long time elapsed before observing an alkaline reaction , revealed by the blue colour of the indicator occurred only after one hour (in vial 5). This is interpreted as the reaction of magnesium with a nearly neural aqueous solution, producing H₂ gas bubbles and also magnesium hydroxide, Mg(OH)₂ probably adhering to the Mg surface. Mg(OH)₂ is almost insoluble. However there must have been enough dissolved to affect the Universal Indicator revealing the distinctly alkaline blue/ pink color.

H₂ gas is readily repeatedly tested by a lighted match applied directly to the top vial opening (Vial 3).But it could not be detected in the reactions occurring in vials,5, 6, & 7, although very slight steady slow bubbles could be observed. The gas must be collected before it could be tested. However, the steady production of the gas can be revealed by stoppering the vial and keeping it under an inverted glass container. After some long time the stopper will be ejected, due to accumulation of the H2gas. This can be heared loudly, and very safely. It can also be observed later, and the ejection of the stopper is thus easily confined.

A very quick survey of various acids has confirmed the same trend of the reaction pattern with HCl. But the rate of the reaction, after decolorisation varies greatly. It seems to be quickest in presence of Cl- anions. This is apparent from an experiment comparing the reaction of Mg and water Only, in presence of UI, with a similar reaction after addition of NaCl. The basic colour change occurred much more rapidly after the addition of NaCl.

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