Patterns of Chemical Change

Exothermic Reaction
A reaction that gives out energy, usually in the form of heat.
Endothermic Reaction
A reaction that takes in more energy than it gives out.
Activation Energy
The energy needed to start a reaction – the energy needed to break the bonds.
Positive energy change
Endothermic reaction, more energy is stored.
Bond making energy is greater than bond breaking energy.
Negative energy change
Exothermic reaction, more energy is given out.
Bond breaking energy is greater than bond making energy.
Catalyst
Lowers the activation energy needed for a reaction to occur, more atoms have the energy required and the rate of reaction increases.
Rate of reaction – changing concentration
Increasing the concentration increases the number of particles and the chance of successful collisions, and so increases the rate of reaction.
Rate of reaction – changing surface area
Increasing the surface area of the solid increases the number of particles available and thus increases the chance of successful collisions and so the rate of reaction increases.
Rate of reaction – changing pressure
Increasing the pressure of a gas squashes the particles into a smaller space and effectively increases the concentration. The particle of gas have a greater chance of successful collisions and the rate of reaction increases.
Rate of Reaction – Catalyst
Using a catalyst can provide a surface for the reaction to occur on, it also reduces the activation energy needed for ther reaction to occur. The catalyst is specific to the reaction and does not get used up.
Rate of reaction – changing temperature
Increasing the temperature of the reaction increases the rate of reaction because the particles have more energy and more about more increasing the chance of successful collisions.
Haber process
The manufacture of ammonia
3H2 + N2 –> 2NH3
Uses a temperature of approx 400oC and 200 atm, with an iron catalyst
Reversible reaction
Reactions that can go forwards and backwards, an example is the Haber process. One reaction will be exothermic and the other endothermic.
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