Overview
Introduction to chemical changes vs physical changes and the basics of Collision Theory affecting reaction rates in IGCSE Chemistry.
Physical vs Chemical Changes
- Physical changes alter form or appearance without changing chemical composition.
- No new substances are formed during physical changes; identities remain the same.
- Examples: melting ice; dissolving sugar in water; mixing sand and salt.
- Mixed substances in physical changes can be separated by physical methods like filtration.
- Physical changes are usually reversible, e.g., water freezing back into ice.
- Chemical changes form new substances with different chemical compositions.
- Examples: burning wood; rusting iron.
- Most chemical reactions are not reversible to original reactants due to new substances formed.
Evidence of Chemical Reactions
- Formation of a solid indicates a new product formed.
- Formation of gas seen as bubbling or fizzing.
- Color change suggests different products with new properties.
- Temperature change shows heat is released or absorbed during reaction.
- New odor formation indicates new substances produced.
Collision Theory
- For reactions, particles must collide and have sufficient energy.
- Successful collisions break bonds and form new substances.
- Collisions without enough energy are unsuccessful; no reaction occurs.
- Particles with enough energy but not colliding will not react.
- Reaction occurs only when collision energy ≥ activation energy.
Factors Affecting Reaction Rate
- Number of particles per unit volume increases chance of collisions.
- Higher collision frequency raises likelihood of successful reactions.
- Greater kinetic energy increases chances of collisions meeting required energy.
- Activation energy is minimum energy needed for reaction to occur.
Collision Theory: Key Factors and Effects
| Factor | Definition/Description | Effect on Successful Collisions | Effect on Reaction Rate |
|---|
| Number of particles per unit volume | Quantity of reactant particles in a given volume | More particle encounters | Increases |
| Collision frequency | How often particles collide | More opportunities for successful collisions | Increases |
| Kinetic energy of particles | Energy due to particle motion | Higher energy makes collisions more effective | Increases |
| Activation energy | Minimum energy threshold for reaction | Only collisions ≥ this energy react | Lower threshold would increase rate |
Key Terms & Definitions
- Physical change: Change in form/appearance without new substances; reversible.
- Chemical change: Process forming new substances with different compositions.
- Collision Theory: Model stating particles must collide with sufficient energy to react.
- Kinetic energy: Energy of motion possessed by particles.
- Activation energy: Minimum collision energy needed for a reaction to occur.
- Successful collision: Collision with energy ≥ activation energy leading to reaction.
- Unsuccessful collision: Collision lacking sufficient energy; no reaction.
Action Items / Next Steps
- Distinguish physical vs chemical changes using examples and reversibility.
- Memorize evidence indicating a chemical reaction has occurred.
- Explain Collision Theory and relate energy and collision frequency to reaction rate.
- Define and apply activation energy concept to predict reaction outcomes.