Question

A student records the mass of a piece of iron. The iron is then left outside until it rusts. What type of reaction occurs, and how does the mass of the object after rusting compare with its original mass?

Answer 1

The Iron combines with Oxygen to form Rust ( Iron Oxide) This is an Oxidation reaction. It is also an Oxidation Reduction reaction. The Iron Oxide has more mass than the original Iron. (This can also be considered a synthesis reaction.

Explanation:

Iron atomic number 26 is not stable in its ground state of
`1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^6`
The valance electrons `4s^2 3d^6` can become somewhat stable by losing the two `4s^2` electrons resulting in a plus two charge(+2)
or by losing the two `4s^2` electrons and 1 of the `3d` electrons leaving a half filled subshell of `3d^5` electrons. resulting in a plus three charge ( +3)

Oxygen is a strong oxidizer it will take the electrons from the Iron resulting in Oxygen having a negative two charge (-2)

One equation for the Oxidation of Iron is below ( there are more than this possibility)

`4Fe +3 O_2 ==== 2Fe_2O_3`

the three Oxygen atoms that have been adding for every two Iron atoms add 48 grams to the compound. This increases the mass of the compound over the original mass of the Iron.