Rate: pressure, surface area (AQA GCSE Chemistry Combined Science): Revision Notes
Rate: pressure, surface area
How surface area affects reaction rate
When we break up a solid reactant into smaller pieces, we increase something called the surface area to volume ratio. This has a big effect on how fast reactions happen.
Understanding surface area to volume ratio
Comparing Surface Area: Large vs Small Pieces
Think about a large cube compared to the same amount of material broken into smaller cubes:
Large single cube:
- Has 6 faces exposed to other reactants
- Less surface area compared to its volume
- Surface area to volume ratio = 3
Same material as 8 small cubes:
- Has many more faces exposed (48 total face areas)
- Same total volume but much more surface area
- Surface area to volume ratio = 6
Why smaller pieces react faster
When a solid is broken into smaller pieces, several key changes occur that speed up the reaction:
- More surface area is exposed for other reactants to collide with
- More frequent collisions happen between reactant particles
- Higher collision frequency means the reaction goes faster
This explains why powdered substances react much faster than large lumps of the same material. The dramatic increase in surface area can make reactions happen hundreds of times faster!
How pressure affects reaction rate
Pressure has a big effect on reactions involving gases. When we increase the pressure of gases, we change how often the gas molecules bump into each other.
What happens when pressure increases
Pressure Effects on Gas Molecules
Low pressure:
- Gas molecules are spread out
- Fewer collisions between reactant molecules
- Slower reaction rate
High pressure:
- Gas molecules are squashed closer together
- More frequent collisions between reactant molecules
- Faster reaction rate
The pressure rule
In many cases, doubling the pressure doubles the collision frequency, which doubles the reaction rate. This happens because there are twice as many reactant molecules in the same space.
Key Relationship: Pressure ∝ Collision Frequency ∝ Reaction Rate
This direct relationship means that small increases in pressure can lead to significant improvements in reaction speed.
Real examples you might see
Worked Example: Marble Chips Experiment
When marble chips react with hydrochloric acid:
- Smaller chips produce gas faster than large chips
- The graph shows a steeper curve for smaller pieces
- Half the mass of chips produces half the volume of gas
This demonstrates the surface area effect in a measurable way.
Worked Example: Gas Reactions
For the reaction :
- Three times as many molecules in the same volume means three times more collisions
- This makes the reaction happen much faster
- The time taken to form the same amount of product is much shorter
This shows how pressure directly affects gas-phase reactions.
Key Points to Remember:
- Smaller pieces of solid reactants have more surface area and react faster
- Higher pressure squashes gas molecules closer together
- More collisions between particles means faster reactions
- Breaking up solids and increasing gas pressure both use collision theory
- These effects are really important in industrial processes to speed up reactions