Rate of Reactions Study Notes

LEARNING OUTCOMES

  • Explain what is meant by "rate of reaction": The rate of reaction indicates how fast or slow a reaction is occurring.
  • Interpret graphical diagrammatic presentation of data: Understand how to read graphs that present data on reaction rates.
  • Identify the factors affecting the rate of reaction: Recognize variables such as temperature, concentration, particle size, and pressure.
  • Predict the effect of factors on rates of reaction from given data: Learn how to analyze data and infer the impact of changes on reaction speed.

RATES OF REACTIONS

Collision Theory

  • A chemical reaction occurs when two particles (atoms or molecules) collide and bond through chemical forces.
  • For bonding to happen, the force of collision must surpass initial repulsive forces, referred to as activation energy.
  • Collision theory helps understand how temperature and concentration influence reaction rates.

Measuring the Speed of Reaction

  • Different reactions vary in speed:
    • Fast reactions: e.g., explosions of gases.
    • Slow reactions: e.g., rusting of iron and fermentation of sugar.

Defining Reaction Rate

  • The rate of a reaction indicates the speed at which it occurs.
  • We can measure the rate of a reaction in different ways:
    1. Timing how long it takes for a reaction to finish.
    2. Measuring the amount of product formed over time.
    3. Determining the decrease in mass of reactants over time.

Experimental Measures

Experiment 1 & 2: Magnesium and Hydrochloric Acid

  • Conducted to compare reaction rates between magnesium and dilute hydrochloric acid solutions:
    • Experiment I: 2 mol/dm³ hydrochloric acid with magnesium.
    • Experiment II: 1 mol/dm³ hydrochloric acid with magnesium.

Measurement of Reaction Completion Time

  • Example: Time taken for magnesium to dissolve in hydrochloric acid:
    • Experiment I: 60 s to complete.
    • Experiment II: 30 s to complete.
  • Hence, the reaction rate in Experiment II is two times faster than in Experiment I.
  • Conclusion: Speed of a reaction is inversely proportional to time taken.
    • ext{Reaction Rate (speed)} = rac{1}{ ext{Time taken}}

Measuring Product Formed

  • Experiment using calcium carbonate and hydrochloric acid:
    • CO₂ gas is collected in a gas syringe to measure reaction speed.
    • Plotting a graph of gas volume against time reveals:
    • Gradient becomes zero at 2.5 minutes, indicating the reaction has stopped.
    • The rate is fastest initially and decreases over time.

Rate Calculation Formula

  • Rate (Speed) of Reaction: ext{Rate of Reaction} = rac{ ext{Quantity of product formed}}{ ext{Time taken}}.
    • Average rate over 2.5 minutes can be calculated:
    • ext{Average Rate} = rac{70 - 0 ext{ cm}^3}{2.5 ext{ min}} = 28 ext{ cm}^3/ ext{min}.

Measuring Decrease in Reaction Mass

  • Weighing the reaction mixture using an electronic balance to plot mass versus time:
    • The graph shows that the reaction speed is greatest at the beginning and decreases over time.
    • The reaction stops after approximately 4.2 minutes when a reactant is exhausted.

Summary of Key Experimental Methods

  • Measuring rate from graphical data:
    • Rate of reaction is defined as how fast the reaction occurs and is inversely related to completion time.
    • Reaction rates can be measured by the quantity of product formed or reactant consumed.
  • Speed of chemical reactions can be illustrated with examples:
    • E.g., hydrogen production with magnesium in acid can be analyzed for reaction speed by measuring produced gas volume over time.

Factors Affecting Speed of Reactions

Effect of Temperature

  • Increased Temperature: Food cooks faster due to higher energy levels.
    • Pressure cookers cook quicker (30 min at 120ºC versus 2 hours at 100ºC).
  • Generally, reaction rates double with a 10ºC increase in temperature.
  • Effect: Particles move faster, collide more frequently, and with greater force due to enhanced kinetic energy.

Effect of Particle Size

  • Smaller pieces of food cook faster.
  • Smaller particle size increases surface area, allowing more collisions with reactants and thus speeding up reactions.

Effect of Concentration

  • Concentrated acids react more aggressively than diluted solutions:
    • Experiment II shows a higher speed of reaction due to increased concentration of hydrochloric acid compared to Experiment I.
  • Increased concentration means more reactants per volume => more chances of collision and reaction.

Effect of Pressure

  • Pressure influences gas reactions significantly but has little effect on liquids and solids.
  • Increased pressure reduces gas volume, leading to greater concentration and increased reaction rates.

Everyday Applications

  • Cooking food involves cutting for smaller particle sizes and higher temperatures.
  • Low temperatures in refrigerators slow food decay.
  • Fine powdered medicines with warm water work more efficiently.
  • Safety measures in industries to prevent dust explosions include maintaining low particle sizes.

Catalysts and Their Importance

Definition and Function

  • A catalyst is a substance that accelerates a chemical reaction while remaining unchanged after the reaction.
  • Catalysts are crucial in many slow chemical processes.
  • A catalyst can:
    • Provide an alternative pathway with lower activation energy (more particles can react).
    • Offer a large surface area to enhance reactant contact.

Industrial Applications of Catalysts

  • Manufacturing processes rely heavily on catalysts:
    • Ammonia: Iron as catalyst.
    • Sulfuric Acid: Vanadium(V) oxide as catalyst.
    • Margarine: Nickel catalyst.
    • Catalytic converters in automobiles use platinum.

Enzymes

  • Enzymes: Biological catalysts consisting primarily of proteins.
    • Essential for bodily functions like digestion.
    • E.g., Amylase converts starch into sugars; yeast enzymes are used in bread and wine making.

Example: Catalytic Decomposition

  • Hydrogen Peroxide: Demonstrates catalyst effect:
    • Decomposed rapidly by adding manganese(IV) oxide.
    • Reaction: 2H2O2
      ightarrow 2H2O + O2 used often in laboratory oxygen preparation.

Quick Checks

Quick Check 1

  • Rate of Reaction: Indicates how fast a reaction occurs, inversely related to time taken.
  • Measured by:
    1. Quantity of product formed over time.
    2. Quantity of reactant used over time.
  • Example calculation of hydrogen production:
    • ext{Average Rate} = rac{32 ext{ cm}^3}{80 ext{ s}} = 0.4 ext{ cm}^3/s.

Quick Check 2

Questions

  • Factors affecting rate: Temperature, Concentration, Particle Size.
  • Identify faster reactions through graph analysis comparing marble forms with dilute hydrochloric acid.
    • Graph comparisons and outcome explanations.

Quick Check 3

Questions

  • Catalyst: Introductory definition and example, such as iron in ammonia production.
  • Catalytic decomposition of hydrogen peroxide examined with varying speeds and methods to enhance reaction rates.
  • Volume of oxygen produced and calculation of hydrogen peroxide mass concludes the section.