3f Light initiated
chemical reactions
Doc Brown's
Chemistry KS4 science GCSE/IGCSE/O Level Revision Notes - Factors
affecting the Speed-Rates of
Chemical Reactions -
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3.
The Factors affecting the Rate of Chemical Reactions
3f
The
Effect of Light
-
CAN LIGHT AFFECT THE SPEED OF ANY REACTIONS?
-
IF IT DOES, HOW DOES CHANGE THE SPEED OF A
CHEMICAL REACTION?
-
Why does increasing light intensity sometimes
increase the speed of a reaction?
-
Light energy
(uv or visible
radiation) can initiate or catalyse particular chemical reactions.
-
As well as acting as an
electromagnetic wave, light can be considered as
an energy 'bullets' called photons and they have sufficient 'impact
energy' to
break chemical bonds, that is, enough energy to overcome the activation
energy.
-
The greater the
intensity of light (visible or ultra-violet) the more reactant molecules
are likely to gain the required energy (activation energy) and react, so the reaction speed increases
- greater frequency of initiation.
-
Strictly speaking the light
(visible or uv) is NOT a catalyst in the sense that the photons of
energy are used up in the chemical changes they induce, the photons
cannot be recycled!
-
Examples:
-
Silver salts
are converted
to silver in the chemistry of photographic exposure of the film.
-
Silver
chloride (AgCl), silver bromide (AgBr) and silver iodide (AgI)
are all sensitive to light ('photosensitive'), and all three are
used in the production of various types of photographic film to
detect visible light and beta and gamma radiation from radioactive
materials.
-
Each silver halide salt
has a different sensitivity to light.
-
When radiation hits the
film the silver ions in the salt are reduced by electron gain to
silver
-
Ag+ + e-
===> Ag (X = halogen atom, Cl, Br or I)
-
and the halide ion is
oxidised to the halogen molecule by electron loss
-
2X- ===>
X2
+ 2e-
-
so overall
the change via light energy is: 2AgX ==> 2Ag + X2
-
AgI is the least
sensitive and used in X-ray radiography, AgCl is the most sensitive
and used in 'fast' film for cameras.
-
Photosynthesis
in
green plants:
-
Photochemical Smog:
-
This is very complex
chemistry involving hydrocarbons, carbon monoxide, ozone, nitrogen
oxides etc.
-
Many of the reactions
to produce harmful chemicals are catalysed or promoted by light energy.
-
The
very fast reactions
between (i) hydrogen and chlorine AND (ii) between chlorine and methane
are both initiated by uv light. At room temperature nothing
happens until violet or uv light is shone onto the mixture, then it
explodes into action BUT well controlled under industrial manufacturing
conditions!
- More details of laboratory investigations
('labs') involving 'rates of reaction' i.e. experimental methods for
observing the speed of a reaction are given in
the INTRODUCTION
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