The reaction between magnesium metal and hydrochloric acid is encountered by most school students when learning about the rate of reactions.

Magnesium is a solid whilst hydrochloric acid is in solution with hydrogen ions (protons) and chloride ions free to more around.

In this example we will need to consider only the movement of the hydrogen ions.

Look at the animation below.

[INS Coll_CI_02_ANI]

In order for this reaction to occur what to happens at the surface of the magnesium metal?

Two protons must collide with the magnesium metal so that electrons can be transfered.

What happens to the magnesium?

The magnesium atoms loose electrons to the protons so that hydrogen atoms are formed. The newly formed magnesium ions are now soluble and dissolve into the water.

What happens to the hydrogen ions?

Having gained electrons the newly formed hydrogen atoms join together forming a covalently bonded molecule of hydrogen. This gaseous molecule now floats upwards with other molecules to form bubbles of hydrogen gas.

How is the rate of this reaction often measured?

The rate of this reactions is often measured by the rate at which bubbles of hydrogen are evolved.

Can you suggest 3 ways in  which the rate of this reaction could be altered?

Increase temperature, increase concentration of hydrogen ions (HCl) and increase surface area by grinding up the magnesium to turnings or powder.