(c) doc bDoc Brown's Chemistry KS4 science GCSE/IGCSE/O Level Revision Notes

Factors affecting the Speed-Rates of Chemical Reactions

3f Light initiated chemical reactions

Can light start chemical reactions? These revision notes are suitable for GCSE IGCSE O Level KS4 science chemistry students studying the effect of light on certain chemical reactions. The descriptions of experiments to do with light initiated reactions and theoretical explanations should help with homework, coursework assignments, laboratory experiment investigations 'labs' on the chemical effects of light. These notes on light induced chemical reactions are designed to meet the highest standards of knowledge and understanding required for students/pupils doing GCSE chemistry, IGCSE chemistry, O Level chemistry, KS4 science courses and can be useful primer for A Level chemistry courses. These revision notes on light catalysed chemical reactions should prove useful for the new AQA GCSE chemistry, Edexcel GCSE chemistry & OCR GCSE chemistry (Gateway & 21st Century) GCSE (91), (9-5) & (5-1) science courses.


RATES of REACTION INDEX: 1. What do we mean by rate/speed of reaction? and how can we measure the speed?  2. Collision theory of chemical reactions  3. Factors affecting the speed of a chemical reaction: 3a Effect of changing concentration  3b Effect of changing pressure  3c Effect of changing particle size/surface area & stirring of a solid reactant  3d Effect of changing temperature  3e Effect of using a catalyst  3f Light initiated reactions 4. More examples of graphs and their interpretation  *  A Level pages on rates - chemical kinetics


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 reuired energy (activation energy) and react, so the reaction speed increases.

    • 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 INDEX: 1. What do we mean by rate/speed of reaction? and how can we measure the speed?  2. Collision theory of chemical reactions  3. Factors affecting the speed of a chemical reaction: 3a Effect of changing concentration  3b Effect of changing pressure  3c Effect of changing particle size/surface area & stirring of a solid reactant  3d Effect of changing temperature  3e Effect of using a catalyst  3f Light initiated reactions 4. More examples of graphs and their interpretation  *  A Level pages on rates - chemical kinetics



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