When specially treated paper is exposed to light it turns blue

Blueprint Paper

Purpose To observe a photochemical

reaction

Demonstration The paper was pre-treated with a

mixture of potassium hexacyanoferrate(III) and iron(III) ammonium

This mixture formed the photosensitive iron(III)hexacyanoferrate(III)

Iron(III)hexacyanoferrate(III) was the source of the photosensitive Fe3+ ion that reacted with light

Background

1. Photochemical Reactions

2. Photoreduction Reactions

1. Photochemical Reactions A photochemical reaction is a

reaction that requires light to occur

“Photo” comes from the Greek word

“photos” which means “light”

light

2. Photoreduction Reactions In a photoreduction reaction light causes

a specie to gain electrons, thus becoming less positive

For example,

Fe3+ Fe2+

Iron(III) has been photoreduced to iron(II)

light

Reducing agent

Conclusions Where light hit the paper a new

compound was produced by a photoreduction reaction

The new compound was iron(III)hexacyanoferrate(II) which was blue

If the paper were left in the light it would turn completely blue

Comments Photography relies on

photoreduction reactions

Photosynthesis by plants is a photoreduction of carbon dioxide to form sugars