Knowt
6 - Chemistry Reactions
Physical and Chemical Changes
Definition of Physical Change:
A physical change is a transformation that can be easily reversed without any new substances being formed. Examples include:
Freezing: Water to ice
Boiling: Water to vapor
Melting: Ice to waterCommon characteristics of physical changes include:
Changes in state (solid, liquid, gas)
Changes in form (such as bending or breaking)
Changes in appearance (color changes that do not alter the substance fundamentally)
Definition of Chemical Change:
A chemical change involves a process that is often irreversible and results in the formation of new products with different properties. Indicators of a chemical change include:
Color change
Gas production (bubbles or odor)
Temperature change (exothermic or endothermic reactions)
Precipitation formation
Examples:
Baking a cake (creating gases and solid structures)
Combustion of fuels
Differences between Physical and Chemical Changes
Feature | Physical Changes | Chemical Changes |
|---|---|---|
Product Formation | No new products are formed | New products with different chemical properties formed. |
Reversibility | Easily reversible | Often irreversible; difficult to revert to originals. |
Change in Chemical Composition | No change; same molecules remain | Transfer of electrons; chemical bonds are altered. |
Examples: | - Melting (ice to water) | - Combustion (burning of substances) |
- Freezing (water to ice) | - Rusting (oxidation of iron) | |
- Boiling (water to steam) | - Digestion (food breakdown) | |
- Sublimation (dry ice to gas) | - Fermentation (sugar to alcohol) |
Collision Theory
For chemical reactions to take place, the reacting particles must collide with sufficient energy (activation energy). Key points include:
Successful Collision:
A collision that exceeds activation energy and leads to a reaction.
Factors Affecting Success:
Speed of particles
Orientation of particles
Energy level of particles
Factors Affecting Rate of Reaction
Temperature:
Increases kinetic energy of particles
Leads to more frequent and energetic collisions
Effect: Reaction rate increases.
Surface Area:
Larger surface areas provide more area for collisions.
Example: Powdered sugar reacts faster than a sugar cube.
Effect: Increased likelihood of reactions.
Concentration (Liquids) & Pressure (Gases):
Higher reactant concentration produces more moles in the reaction.
Increased pressure on gases reduces volume, bringing particles closer.
Effect: Enhanced collision frequency.
Catalysts:
Substances that speed up reactions without being consumed.
Function: Lower activation energy; provide alternative pathways.
Example: Enzymes in metabolic processes.
Effect: Accelerated reaction rates.
Reversible Reactions and Equilibrium
Definition:
A reversible reaction allows products to convert back to reactants under certain conditions.
Equilibrium State:
Rates of the forward and reverse reactions are equal.
Concentrations of reactants and products remain constant.
Dynamic Aspect: Continuous reactions occur, but no net change in concentration.
Le Châtelier's Principle:
If there’s a change in concentration, pressure, or temperature, the system will shift to counteract the change.
Example: Increasing reactants favours the forward reaction.
Redox Reactions:
Process | Description |
|---|---|
Reduction | A process where a substance gains electrons. |
Oxidation | A process where a substance loses electrons. |
Both processes are crucial in energy flow in ecosystems and are integral in reactions like combustion and respiration.
