US - USING SUNLIGHT

Topic Overview

Redox Chemistry

Modern Methods

Introduction

Where does Colour come from?

Fluorescence

Photosynthesis

The Role of Chlorophyll

Introduction

Without the Sun, life on Earth could not exist.  The Sun's energy can bring about photochemical reactions.  In the stratosphere it produces oxygenatoms from oxygen molecules which then form ozone.  In the troposphere it brings about photosynthesis.

Most of our energy needs today are met by products of past photosynthesis, e.g. coal, oil, natural gas and wood.  Electricity can also be produced fromsunlight in photovoltaic cells.

Where does Colour come from?

The visible light from the Sun is often called white light, but it can be split into light of different colours.  If all incident sunlight is reflected off a surface, the object appears white.  Most objects appear coloured because wavelengths corresponding to different colours are absorbed.

When absorption occurs, wavelengths corresponding to one colour are removed from the white light and you see the complementary colour.

Colours are set as a guide only

For example if blue is absorbed, the colour which is seen is orange.

Fluorescence

If an atom absorbs ultra-violet radiation, an electron moves to a higher energy level.  The same thing happens with molecules.

E.g.

X₂ X₂*

X₂* represents a molecule in an electronically excited state.  The bonds in a molecule can also vibrate.  This represents a much smallerenergy change.

Fluorescent substances are able to absorb invisible uv radiation and re-emit it as visible radiation.  A molecule of the substance first absorbs uvradiation which gives it both a high electronic and a high vibrational energy.  The relatively small quantities of vibrational energy are rapidly lost toother molecules during collisions, and so the molecule drops to a lower vibrational energy.  The electronic energy change is larger, and this energy isre-emitted as visible radiation.

The radiation emitted has a lower energy than the radiation absorbed.  This explains why clothes which have been washed in soap powders containingfluorescent compounds glow only when the ultra-violet disco lights are on.

Photosynthesis

Water and carbon dioxide react together in the presence of sunlight and chlorophyll in green plants to produce oxygen and glucose:

6H₂O   +   6CO₂     C₆H₁₂O₆   +   6O₂

Glucose is a carbohydrate.  It is stored in green plants in the form of starch molecules.  Starch is made up of long chains of glucose units.

The overall reaction can be written:

nCO₂   +   nH₂O     (CH₂O)_n   +   nO₂

The sunlight which reaches the Earth has a mixture of wavelengths, including those which we perceive as colours.  It is the visible radiations that areneeded for photosynthesis.  However, all the colours of visible light are not equally good at helping photosynthesis to occur.

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The Role of Chlorophyll

Chlorophyll appears green because it absorbs wavelengths that correspond to other colours, i.e. red and violet.  In plants absorbed energy istransferred from excited chlorophyll molecules and used to start a chain of chemical reactions.  The most important pigment for photosynthesis ischlorophyll a which is a complex compound similar to hæmoglobin.

Photosynthesis can be divided into three stages:

1. light harvesting
2. using light energy to start photochemical redox reactions
3. reducing carbon dioxide to (CH₂O)_n.

Chlorophyll a is involved in the first two stages.  The molecules operate in teams.  They can be involved in one of two ways. Most act as light harvesters where they absorb light and pass the energy on from one chlorophyll a molecule to another until it reaches the reactioncentre.  This consists of carefully positioned pair of chlorophyll a molecules which start the reactions of photosynthesis.  The processes occurin chloroplasts.

Photosynthesis is a redox reaction.  Two half-equations can be written to summarise the reactions of water and carbon dioxide:

	2H2O      O2   +  4H+   +   4e-	(1)
	4e-   +   4H+   +   CO2     (CH2O)   +   H2O	(2)

Reaction 1 is an oxidation, reaction 2 is a reduction.  Combining the two gives the overall equation for photosynthesis.

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