Chapter 8: Rates of Reaction Study Notes

Chapter 8: Rates of Reaction

Table of Contents

  • 01 Measuring the rate of reaction

  • 02 Surface area and the rate of reaction

  • 03 Temperature and the rate of reaction

  • 04 Concentration and the rate of reaction

Measuring the Rate of Reaction 8.1

  • Overview:

    • Explore how the rate of reaction can change.

    • Measure the rate of reaction.

    • Use graphs to discuss and measure the rate of reaction.

    • Explain why the rate of reaction changes.

Fast and Slow Reactions

  • Types of Reactions:

    • Slow Reactions:

    • Cooking an egg.

    • Rusting of iron.

    • Forming of crystals.

    • Fast Reactions:

    • Burning paper.

    • Neutralising an acid with a base.

Definition of Rate of Reaction

  • The rate of reaction indicates how fast or how slow a reaction takes place.

  • Chemists and chemical engineers seek ways to control chemical reaction rates for various applications:

    • Painkillers require quick action.

    • Slow-release insulin must operate more gradually.

  • Faster burning materials have higher reaction rates which can increase danger and require stringent safety measures for firefighters.

Five Main Ways to Alter the Rate of Reaction

  1. Surface area of the reactants.

  2. Temperature at which the reaction is carried out.

  3. Concentration of the reactants.

  4. Use of a catalyst, including enzymes.

  5. Gas pressure.

Measuring Rate of Reaction

  • The rate changes as the reaction progresses. An example is:

    • Reaction: Calcium carbonate + dilute hydrochloric acid.

    • Observation: Carbon dioxide gas is released, leading to a decrease in the flask's mass.

    • Details:

    • At the start: Mass decreases quickly.

    • As the reaction continues: The mass decreases more slowly.

Graphical Representation

  • A graph can illustrate the rate of reaction at any given time:

    • Slope/gradient of the line indicates reaction speed:

    • Steeper slope → Faster reaction.

Example Reaction

  • Reaction: Magnesium + hydrochloric acid → Magnesium chloride + hydrogen.

  • The volume of hydrogen gas is measured at regular intervals until three readings of the same value are obtained.

  • A graph plotting the volume of hydrogen gas produced against time is constructed.

Characteristics of the Graph

  • Volume of hydrogen gas produced in cm³

    • Shows decreasing gradient indicating slowing reaction.

    • At gradient = 0, the reaction stops.

    • Steepest gradient indicates the fastest reaction.

Measuring Rate from a Graph

  • Objective: Determine the average volume of carbon dioxide gas produced per second between 10s and 30s.

Steps to Measure Rate

  1. Draw a vertical line at 10 seconds on the horizontal axis up to the graph (label A).

  2. Draw another vertical line at 30 seconds up to the graph (label B).

  3. Draw a horizontal line C between points A and B (represents time taken: 20 seconds).

  4. Identify line D showing the amount of carbon dioxide produced in this period (20 cm³).

Calculating the Rate

  • Formula: Rate of reaction = Volume of CO₂ produced / Time taken

    • Calculation: 20 cm³ ÷ 20 s = 1 cm³ per second (average rate of reaction).

Reflection Questions

  1. Write the word equation for the reaction between copper carbonate and hydrochloric acid.

  2. Analyze the graph to determine the average rate of reaction between 60 to 80 seconds and compare it with the rate from 10 to 30 seconds.

Why Does the Rate of Reaction Change?

  • Explanation through Particle Theory:

    • Chemical reactions occur when reactant particles collide with sufficient energy to react.

Surface Area and the Rate of Reaction 8.2

  • Investigation: Effects of altering the surface area of a reactant on the rate of reaction.

  • Example using Calcium carbonate and dilute hydrochloric acid.

  • Considerations: How changing the shape of a material affects its surface area.

Effect of Surface Area on Reaction Rate

  • Scenario: Which dissolves faster? Whole candy vs. powdered candy?

Principles of Surface Area

  • Large lumps of reactants only allow surface particles to collide with others.

  • Breaking reactants into smaller pieces increases total surface area available for collisions:

    • More particles available to collide = Increased number of successful collisions = Increased rate of reaction.

Summary of Surface Area Effect

  • Increased surface area → Increased collisions → Faster rate of reaction.

  • Decreased surface area → Reduced collisions → Slower rate of reaction.

Temperature and the Rate of Reaction 8.3

  • Investigation: Effects of temperature changes on the rate of reaction.

Effect of Temperature

  • Concept: Why is food stored in a refrigerator?

    • Rate of decay is slower at lower temperatures.

  • General Observation: Most chemical processes slow down with decreased temperature.

Increased Temperature Dynamics

  • Temperature increases lead to:

    • Reactant particles gaining kinetic energy and moving faster.

    • Increased collision frequency due to faster particle movement.

    • Greater force of collisions, leading to more effective collisions and faster reaction rates.

Summary of Temperature Effects

  • At low temperature:

    • Low kinetic energy → Less collision frequency → Slower rate of reaction.

  • At high temperature:

    • High kinetic energy → Increased collision frequency → Faster rate of reaction.

Concentration and the Rate of Reaction 8.4

  • Investigation: Effects of reactant concentration on the rate of reaction.

How Concentration Affects Reactions

  • Analogy: Imagine a bumper car arena:

    • Fewer cars present leads to fewer collisions; more cars lead to frequent collisions.

Concentration Effects Explained

  • Higher Concentration:

    • More reactant particles in the same volume.

    • Closer particle proximity → More frequent collisions → Increased rate of reaction.

  • Lower Concentration:

    • Fewer reactant particles in the same volume.

    • Greater distance between particles → Reduced collision frequency → Decreased rate of reaction.

Summary of Concentration Effects

  • Low Concentration:

    • Less molecules → Particles spaced apart → Fewer collisions → Slower rate of reaction.

  • High Concentration:

    • More molecules → Particles closer together → More collisions → Faster rate of reaction.

Questions on Experimental Observations

  1. For the reaction of calcium carbonate and hydrochloric acid at varied concentrations:

    • How can the faster reaction be identified on a graph?

  2. Hypothetical scenario: If the experiment were conducted at an even lower acid concentration, predict how the graph would differ.