1. Hydrogen Gas from the Reaction of Magnesium Metal With Acid
When magnesium metal is treated with acid evolution
of hydrogen gas results

2. Purpose To observe a chemical reaction between a metal and an acid
To measure the amount of hydrogen gas produced

3. Demonstration
Hydrogen evolution began as soon as the magnesium metal was added to the water
The hydrogen was less dense than water so it rose through it
The hydrogen gas was able to be collected in the buret

4. Concepts 1. Single Replacement Reactions 2. Redox Reactions
3. Oxidation Numbers
4. Gases

5. 1. Single Replacement Reactions
A single replacement reaction is of the general form
A + BC AC + B
In order for a single replacement reaction
to occur, A must be more reactive than B
in order to combine with C
In this demonstration
Mg (s) + HCl (g) MgCl2 (aq)+ H2(g)
Since the reaction occurs, we must conclude that magnesium is more reactive than hydrogen

6. 2. Redox Reactions
In a redox reaction, one substance loses electrons while another substance gains electrons
“Redox” is a conventional term which is a combination of the words “reduction” AND “oxidation”
The substance that loses electrons is said to be oxidized and/or the reducing agent
The substance that gains electrons is said to be reduced and/or the oxidizing agent

7. 3. Oxidation Numbers (I)
Oxidation numbers are used to follow electron change and determine what is oxidized and what is reduced during a chemical reaction
Mg (s) + 2HCl (g) MgCl2 (aq)+ H2(g) 1+ 2+
oxidation
reduction
Magnesium undergoes oxidation
Hydrogen undergoes reduction

8. Oxidation Numbers (II)
It is important to understand that magnesium gives (loses) its two electrons to the H+ of the acid
Mg:
Mg 2+
Starting
Materials
Products
2H+
H--H
Magnesium passes its electrons to 2H+ to form H2

9. 4. Gases
The approximate volume of the gas can be calculated as follows
g Mg/Atomic Weight Mg = mol Mg
mol Mg/22.4 mol/L X 1000 = mL hydrogen gas

10. Conclusions
The reaction in this demonstration was a single replacement reaction AND a redox reaction
The reaction was a quiet reaction
Simple gas law theory was able to predict the approximate volume of gas which formed

11. Comments Many metals will react with acid as does magnesium
The hydrogen also contains water vapor and is therefore too wet to burn