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GCSE Chemistry Notes: Reactions catalysed by light

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(c) doc b3f Light initiated chemical reactions

Doc Brown's Chemistry KS4 science GCSE/IGCSE/O Level Revision Notes - Factors affecting the Speed-Rates of Chemical Reactions - Doc Brown's chemistry revision notes: basic school chemistry science GCSE chemistry, IGCSE  chemistry, O level & ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old science students for national examinations in chemistry

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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!

      • (i) hydrogen + chlorine ==> hydrogen chloride

        • H2 + Cl2 == uv ==> 2HCl

      • (ii) methane + chlorine ==> chloromethane

        • CH4 + Cl2 == uv ==> CH3Cl + HCl

  • 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

Rates of reaction notes INDEX

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