In I/GCSE Chemistry, we will learn about how fast energy changes are.
Molecular collisions
H2O and O2 gas molecules are constantly colliding – but not all the collisions lead to reactions. This is because the molecules only have enough energy to merely bump into each other; they don’t have enough energy for the collisions to break bonds. Bonds between atoms (intra-molecular) must be broken before new bonds can be formed.
- If every collision lead to a reaction --> explosive reaction
Activation energies
In I/GCSE Chemistry, bonds cannot break though weak collisions. Thus, an external source of energy needs to be provided (activation energy) to the molecules so that they gain enough energy to break the bonds.
Activation energy (AE) - the minimum amount of energy required for the collision to break the bonds between atoms, and start the reaction.
- Low AE means molecules need less energy to break bonds
- High AE means molecules need more energy to break bonds
- If the energy provided is lower than AE, the reaction won’t start; nothing will happen
‘High-jump bar’ Theory
When it comes to I/GCSE Chemistry, if the high-jump bar (HJB) is low, many athletes will be successful in jumping over it.
Likewise, if the AE is low, a high proportion of collisions will release enough energy to break bonds.
- The reaction will be fast despite low temps. This is b/c the low temp. provides more than enough energy to surpass the AE boundary
If the HJB is high, fewer athletes will be successful in jumping over it.
Likewise, if the AE is high, a lower proportion of collisions will release enough energy to break bonds.
- Heating gives the reaction more energy --> increases no. of successful collisions
Catalysts
Catalysts have three jobs:
- Speed up reactions (not used up in process)
- Lowers the AE
- Provides alternative routes in energy graphs (dotted red line)
The overall energy change of the product doesn’t change but the AE is lowered so that there is an increase in the no. of successful collisions to break the bonds.
Explosives
Refer to I/GCSE Chemistry, Primary explosives have a low AE --> more sensitive to heat/ energy stimulus --> more dangerous
Secondary explosives have a higher AE --> less sensitive --> less dangerous
Detonators used to be used with electric current, but not anymore b/c electromagnetic radiation makes it unsafe to handle. As a result, fibre optic cables with lasers have been introduced (safer).
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