Module 4.2 Common Reaction Types
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Chemical Equations
Common Reaction Types
Net Ionic Equations

Combination Reactions

A combination reaction is a reaction in which two reactants combine to form one product.

Oxygen and the halogens are very reactive elements and are likely to undergo combination reactions with other elements. When a metal undergos a combination reaction with oxygen, a metal oxide is formed (similarily, a metal halide is formed if reacted with one of the halogens). You see the products of this type of reaction whenever you see rust. Rust is the product of a combination reaction of iron and oxygen:

Since this new compound is formed from a metal and a non-metal, it is an ionic compound. The product will be electrically neutral and since you can predict what charge most metal ions will have based on their location on the periodic table and oxygen will have a charge of -2, you can determine the molecular formula for the product.

Review charges of ions

Aluminum also reacts with oxygen to form aluminum oxide. What is the molecular formula for aluminum oxide?

Al O

The oxygen ion will have a charge of -2 and the aluminum ion will have a charge of +3. How can these two ions combine to form a neutral compound?

While the compound you have suggested is electrically neutral, it is not the simplest way aluminum and oxygen can combine.

Good!

Oxygen and the halogens can also undergo combination reactions with non-metals resulting in molecular compounds.

An example of this is the reaction occuring in the fuel cells that power space shuttles:

The molecular formula for these are more difficult to predict and are not discussed here.