Oxidation Reactions

Burning magnesium to make magnesium oxide

Theory behind the observations

Formation of water

NC State University

Magnesium is a metal. It conducts electricity and itCan be bent to form shapes like other metals. Unlike many metals we know (iron, copper, silver

etc.), magnesium will burn in air.

Burning magnesium metal

Magnesium strip

We can light magnesium strip with a match.Burning is combustion: chemical combination with

oxygen.Mg + O MgO

Burning magnesium metal

When magnesium burns it forms an oxide. Since it combines with oxygen gas, which is O2 we really need to write:

2 Mg + O2 2 MgO

Chemical reaction

Atoms are most stable if they have a filled shell of electrons. The filled shells occur at the following numbers of electrons 2, 10, 18, 36.

Where do these numbers come from?

Why does magnesium burn?

The Periodic Table of the Elements

Atomic orbitals

s

p

d

Atomic Energy LevelsThe atomic energy levels come from the quantumMechanical solution for an atom.

1

2

3

Hydrogen, H

The Periodic Table of the Elements

Atomic Energy LevelsHelium has a filled shell. It is non-reactive.

1

2

3

Helium, He

The Periodic Table of the Elements

Atomic Energy LevelsWe start filling the second shell. Lithium can obtain a filled shell by giving up one electron.Li Li+ + e-

1

2

3

Lithium, Li

The Periodic Table of the Elements

Atomic Energy LevelsBeryllium can obtain a filled shell by giving up two electrons.Be Be2+ + 2 e-

1

2

3

Beryllium, Be

The Periodic Table of the Elements

Atomic Energy LevelsAt boron we begin filling the p-orbitals. Boroncan obtain a filled shell by giving away threeelectrons.

1

2

3

Boron, B

The Periodic Table of the Elements

Atomic Energy Levels

1

2

3

Carbon, C

Carbon has two p-electrons. It can obtain afilled shell by bonding to four other atoms.

The Periodic Table of the Elements

Atomic Energy LevelsNitrogen can either give up three electrons orgain three electrons to get a filled shell.

1

2

3

Nitrogen, N

The Periodic Table of the Elements

Atomic Energy LevelsOxygen can gain two electrons to obtain a filledshell.O + 2 e- O2-

1

2

3

Oxygen, O

The Periodic Table of the Elements

Atomic Energy LevelsOxygen can gain two electrons to obtain a filledshell.F + e- F-

1

2

3

Fluorine, F

The Periodic Table of the Elements

Atomic Energy LevelsNeon has a filled shell and it is non-reactive.It is a gas (also called a noble gas).Note that neon has 10 electrons total.

1

2

3

Neon, Ne

The Periodic Table of the Elements

Atomic Energy LevelsJumping ahead we can see that the next filled shell will occur for argon with 18 electrons.

1

2

3Argon, Ar

The Periodic Table of the Elements

Atomic Energy LevelsSodium is in the same column as lithium. It hasone electron more than a filled shell. It will easilygive up an electron. Na Na+ + e-

1

2

3Sodium, Na

The Periodic Table of the Elements

Atomic Energy LevelsMagnesium will be obtain a filled shell if it givesaway two electrons. Mg Mg2+ + 2 e-

1

2

3Magnesium, Mg

The Periodic Table of the Elements

Chemical ReactivityBased on the shell concept for atoms we can seethat magnesium and oxygen can both reach a filled shell if Mg gives electrons to O.

Mg Mg2+ + 2 e-

O + 2 e- O2-

Mg + O Mg2+ + O2- MgO

When this sharing of electrons occurs a chemicalbond is made and the atoms combine to form amolecule. In this case the bond is called ionic.

To form a filled shell we can think of magnesiumgiving two electrons to oxygen. Magnesium startswith ten outershell electrons and oxygen startswith six. When Mg gives 2 electrons to O we endup with 8 electrons on each.

Ionic Bonding

Mg O+ Mg O

10 6 8 8

Covalent BondsHydrogen wants to gain one electron to form a filled shell. Hydrogen can also give away anelectron to form the ion H+.

Lose electron: H H+ + e- (hydrogen ion) Gain electron: H + e- H- (hydride ion)

Hydrogen can also share an electron.

The combination of two hydrogen atoms makesdiatomic hydrogen gas.

H H+ H H

Covalent BondsThe combination of two hydrogen atoms iswritten as:

2 H H2

Hydrogen can combine with oxygen to make water.

2 H + O H2Oor2 H2 + O2 2 H2O

H H+ H H+ O O

Making Hydrogen (H2)We can make hydrogen using water and magnesium.

Mg + H2O MgO + H2

We can break this up into steps.

Mg Mg2+ + 2 e-

H2O O2- + 2 H+

2 H+ + 2 e- H2

Using the dot formula

H H+H H + OO

Mg + O

H H+

Mg O

H H

The experiment

Pear-shaped flask

Stop cock

Empty balloon

The experiment

Add magnesium and water

The experiment

Bubbles form and the solution heats.These are signs that a reaction isoccuring.

The experiment

Bubbles form and the solution heats.These are signs that a reaction isoccurring.

The balloon begins to inflate.

H2

The experiment

The balloon fills with hydrogen gas.

H2

The experiment

Close the stop cock to keepthe gas in the balloon. Thenseparate the reaction flask.

H2

The experiment

Close the stop cock to keepthe gas in the balloon. Thenseparate the reaction flask.

H2

The experiment

Bring up a match.

The experiment

The hydrogen gas explodes.

Boom

What happened to the H2?Hydrogen can combine with oxygen. This is anoxidation reaction also known as combustion of H2.

The chemical reaction is:

2 H2 + O2 2 H2O

We have already seen that hydrogen will share Electrons with oxygen. Both atoms move to a lowerenergy state by sharing electrons.

When hydrogen and oxygen combine a lot of heat isreleased.