Rates of Reaction and Factors Affecting Reaction Rates

5.10.1 Rates of Reaction

• Chemical reactions exhibit varying rates.
    - Some reactions occur in fractions of a second.
    - Others may take prolonged periods (days to years).

• Importance of understanding reaction rates:
    - Crucial for industrial applications due to financial implications of production speed and output.
    - Relevant in daily life (e.g., cooking times, medical effects of medications like antacids).

• Controlling reaction rates is significant:
    - Sometimes reactions need to be sped up (e.g., manufacturing).
    - Conversely, reactions may require slowing down (e.g., metal corrosion, food decay).

5.10.2 Collision Theory

• Collision theory explains how reaction rates are influenced by particle interactions:
    - For a reaction to occur, particles (atoms, ions, or molecules) must:
      - Collide with one another.
      - Possess sufficient energy (known as activation energy) to break bonds in the reactants; otherwise, particles will merely bounce off each other.
      - Have the correct orientation during the collision.

5.10.3 Temperature and Rate

• Effects of temperature on particle behavior:
    - Particles are constantly in motion and exhibit a range of kinetic energies.
    - Increasing the temperature leads to:
      - Higher average speeds of particles.
      - Increased average kinetic energy of particles.
      - More frequent collisions between particles.
      - Higher percentage of particles possessing enough energy to collide effectively.
    - Example scenario with hydrogen and oxygen:
      - At room temperature, no reaction occurs due to insufficient energy in the molecules.
      - Introducing a spark ignites the reaction:
        - $2H_2(g) + O_2(g) <br>ightarrow 2H_2O(g)$

• Lower temperatures reduce particle motion:
    - Example of food preservation with refrigeration.
      - Slows microbial activity, prolonging food shelf life.

5.10.4 Surface Area and Rate

• Importance of surface area in reactions:
    - As seen with bath bombs:
      - Solid bath bomb dissolves slowly when whole.
      - Crushing the bath bomb increases surface area, resulting in faster dissolution.
    - Key point:
      - When solid reactants are ground into powder, more reactive surface is available for collisions.
      - Larger chunks have limited surface area for collisions.

5.10.5 Concentration and Rate

• Higher concentration leads to increased reaction rates:
    - More particles per volume increases likelihood of collisions.
    - Example with magnesium and hydrochloric acid:
      - Use of concentrated hydrochloric acid results in a more vigorous reaction.

5.10.6 Catalyst and Rate

• Catalysts effectively increase reaction rates:
    - Remain unchanged post-reaction and can be reused.
    - Facilitate the breaking of bonds, reducing energy requirements for reactions.
    - Example applications of catalysts:
      - Use in catalytic converters in vehicles to reduce harmful emissions:
        - Convert nitrogen oxides into less harmful substances (reference to Figures 5.33 and 5.34).
      - Platinum and rhodium are common materials used in converters.

• Catalysts in industry:
    - Iron oxide for ammonia production, crucial for fertilizers and explosives.
    - Vanadium oxide used in sulfuric acid production:
      - Vital reaction with sulfur dioxide gas and oxygen occurs rapidly with a catalyst at 450 °C:
        - $2SO_2(g) + O_2(g) ext{ } ext{V}_2 ext{O}_5^{ ext{C° 450 } } <br>ightarrow 2SO_3(g)$
    - Zeolites in crude oil processing (cracking) for smaller molecule production such as octane.

• Catalytic processes in everyday life:
    - Catalysts used in contact lens cleaning (e.g., decomposing hydrogen peroxide with a platinum catalyst).
      - Kills microbes on lenses during decomposition:
        - $2H_2O_2 (aq) ext{ platinum } <br>ightarrow 2H_2O (l) + O_2 (g)$

5.10.7 Measuring Rate

• Rate of reaction quantified as change over time:
    - Can be experimentally determined by:
      1. Measuring the decrease in a reactant over time.
      2. Measuring the increase in a product over time.

• Experimental measurement strategies include:
    - Observing changes in mass, pH, color, or gas volume over time, illustrated with an acid and metal carbonate reaction.

• Example test apparatus:
    - Reactions conducted in a flask on a balance to observe mass changes.
    - Graphing mass changes over time shows reaction rate; steeper graph gradient indicates faster reaction.

Investigations and Experiments

• INVESTIGATION 5.5: Aim to observe the effect of a catalyst on a decomposition reaction.

5.10 Quick Quiz

• Quiz items are present but were not transcribed.