C6 - Chemical Reactions Savemyexams

CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award)

Chemical Change & Rate of Reaction

• Contents:

  • Physical & Chemical Changes

  • Rates of Reaction

  • Factors

  • Collision Theory

  • Explaining Rates Using Collision Theory

  • Investigating the Rate of a Reaction

  • Interpreting Data

  • Resources available at www.savemyexams.com

Physical & Chemical Changes

Physical Changes

• Do not produce new chemical substances.

• Characteristics:

  • Easy to reverse.

  • Relatively easy to separate.

• Examples:

  • Changing state: melting (solid → liquid).

  • Making a mixture from multiple substances.

  • Dissolving a solute in a solvent.

Chemical Changes

• Produce new chemical substances with different properties from the reactants.

• Usually difficult to reverse.

• Signs of chemical change may include:

  • Colour Changes: E.g., when copper displaces silver in silver nitrate, resulting in color changes: solid changes from orange-brown to silver and solution from colorless to blue.

  • Temperature Changes:

    • Exothermic Reactions: E.g., reaction of calcium oxide with water (produces heat).

    • Endothermic Reactions: E.g., photosynthesis and dissolving ammonium chloride (absorbs heat).

Rates of Reaction

Factors Affecting Rates of Reaction

• Key Factors:

  • Concentration of reactants or pressure of gases.

  • Surface area of solid reactants.

  • Temperature of the reaction.

  • Presence of a catalyst.

• Economic interest in faster rates due to increased production efficiency.

Increasing Concentration or Pressure

• Higher concentrations or pressures lead to:

  • More frequent collisions.

  • Steeper initial gradient on reaction graphs.

• Effects can be illustrated in graph format.

Increasing Surface Area

• More surface area leads to increased rates of reaction due to more exposed particles.

  • Crushed or powdered reactants yield faster reactions than larger chunks.

Collision Theory

Fundamentals

• Definition:

  • Reactions occur when particles collide with sufficient energy, overcoming activation energy (minimum energy necessary for reaction).

• Collisions can be:

  • Successful: Leads to product formation.

  • Unsuccessful: No reaction occurs.

Increasing Successful Collisions

• To increase the number of successful collisions, consider:

  • Number of particles per volume.

  • Frequency of collisions.

  • Kinetic energy of the particles.

Investigating Reaction Rates

Measurement Techniques

• Reaction rates can be gauged by measuring:

  • Reactants used up.

  • Products formed.

• Properties that can change during reactions:

  • Colour, mass, or volume.

Examples of Investigation

• Investigate effects of concentration, surface area, temperature, and catalysts on reaction rates using various methods (e.g., measuring gas volume, mass loss).

Interpreting Data

Data Analysis

• Data plotted to show reaction rates and trends over time.

• Quickest rate occurs at start due to highest concentration of reactants.

• Graphs show decreasing rate as reactants are consumed.

• Graphical Representation:

  • Drawing tangents to determine rate at specific points.

Worked Examples

Example of Concentration Impact

• Comparing two concentrations of hydrogen peroxide to determine effect on reaction rates by sketching trends on graph.

Calculating Reaction Rate

• Techniques for deriving rates from graph gradients using change in y and change in x.