GCSE level rates notes 3f. Light initiated chemical reactions

GCSE Level Chemistry Notes: Reactions catalysed by light

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.

  • These may be referred to as photochemical 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 of the induced reaction.

  • 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 because the photons of energy are used up in the chemical changes they induce, the photons cannot be recycled as with a true catalyst!

• Examples of light initiated chemical reactions:

  • 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.

    • The more intense the light, the greater the chemical change induced on the photographic paper, i.e. the greater the exposure of the photograph.

  • Photosynthesis in green plants:

    • The conversion of water + carbon dioxide ==> glucose + oxygen

      • 6H₂O_(l) + 6CO_2(g) ==> C₆H₁₂O_6(aq) + 6O_2(g)

      • Photosynthesis requires the input of sunlight energy and the green chlorophyll molecules absorb the photon energy packets of light and initiate the chemical changes summarised above.

      • The greater the intensity of light, the greater the rate of photosynthesis.

      • Photosynthesis explained - factors affecting the rate - light, temperature & CO₂ concentration

      • e.g., the faster the rate of photosynthesis. the greater the concentration of carbon dioxide in the air

  • Photochemical Smog:

    • This is very complex chemistry involving hydrocarbons, carbon monoxide, ozone, and nitrogen oxides.

    • 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 to make hydrogen chloride for PVC polymers and hydrochloric acid!

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

      • H₂ + Cl₂ == uv ==> 2HCl

    • (ii) methane + chlorine ==> chloromethane

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

  • Cancer

    • The photons of ultraviolet light have more energy than visible light photons.

    • The energy of uv photons is sufficient to break bonds in molecules in the skin.

    • This can sometimes cause mutations in the genetic material and lead to the formation of cancer cells, whose uncontrolled growth is NOT good for you, so take care in sunlight!

• 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

Key revision points for light initiated chemical reactions

A structured set of brief summary revision notes tailored to GCSE/IGCSE chemistry exam specifications across AQA, Edexcel, OCR (Gateway & 21st Century), WJEC, CCEA, and CIE Cambridge.

They are aligned the content to syllabus expectations, added exam tips, and highlighted common misconceptions.

Revision Notes: Rates of Chemical Reactions (Light-Dependent Investigations)

1. Core Concept: Rate of Reaction

• Definition: The rate (speed) of a chemical reaction is how quickly reactants are converted into products.
• Measurement: Change in concentration, mass, volume, or other measurable property per unit time.

2. Experiments: Light Intensity & Light-Initiated Reactions

Chemistry Examples

• Photochemical reactions: e.g. decomposition of silver halides in photography.
• Rate affected by light intensity: Higher intensity → more photons → more particles activated → faster reaction.

Biology Examples

• Photosynthesis: 6CO₂ + 6H₂O ==> C₆H₁₂O₆ + 6O₂
• Light intensity directly affects the rate of photosynthesis (until limited by other factors like CO₂ or temperature).

3. Designing a Fair Test

• Control variables: Keep temperature, concentration, volume, and type of reactants constant etc. all constant apart from light intensity.
• Independent variable: Light intensity (distance from lamp, use of filters).
• Dependent variable: Rate of reaction (e.g. oxygen volume produced in photosynthesis, time for colour change).
• Repeat & average: Improves reliability.

4. Analysing Results

• Graphs: Plot rate versus light intensity.
• Patterns: Often show a curve (rate increases with intensity until plateau).
• Interpretation: Identify limiting factors (collision frequency, photon availability, enzyme saturation in biology).

5. Collision Theory Explanation

• Particles must collide with sufficient energy (≥ activation energy).
• Light provides energy: Excites particles, increases chance of successful collisions.
• Higher intensity: More photons → more collisions with enough energy → faster reaction.

6. Typical Exam Board Content - may apply to biology and/or chemistry

7. Student Exam Tips

• Always state variables clearly: Independent, dependent, controlled.
• Use collision theory in explanations: Examiners expect this terminology.
• Graph skills: Label axes correctly, identify plateaus, explain limiting factors.
• Link biology & chemistry: Photosynthesis is a common crossover example.
• Units matter: Rate expressed correctly (e.g. cm³/s, mol/dm³/s).

8. Typical Misconceptions

•  “More light always increases rate indefinitely” → Wrong. Rate plateaus when another factor becomes limiting, you see this in photosynthesis e.g. lack of carbon dioxide or water.
•  “Collisions always lead to reactions” → Wrong. Only collisions with sufficient energy and correct orientation succeed.
•  “Fair test means one variable only” → In fact, it means controlling all other variables while changing one.
•  “Rate is the same as yield” → Rate = speed, yield = amount produced.

GCSE level 'Rates of Reaction' multiple choice quiz