Post on 31-Dec-2015
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– What makes “superglue” bond instantly while PVA glue does not?
– What factors determine how quickly food spoils?– Why do “glow sticks” last longer when stored in
the freezer?
Methods for measuring rate of reaction
Collect a gas over time Follow a change in mass over time
150g
Formation of a precipitate or colour change over time.
Factors affecting rate
• Concentration (pressure with gases)
• Temperature• Catalysts • Surface Area
Not all collisions are effective
• Paper burns• Paper + oxygen carbon dioxide + water +
nitrogen
• The paper in this room isn’t burning.• It doesn’t have enough energy to
burn.
• If we make it hotter it will catch fire.• Paper burns on its own at 250 ºC
A collision but with no effect
Reactant particlescollide
Product particles not formed as there is
not enough energy
Activation Energy • Reactions only happen if the particles
have enough energy.
• The minimum amount of energy needed to start a reaction is called the Activation Energy
• The amount of activation energy needed is different for each reaction.
• But
• Every reaction has activation energy, they all need a little push to get started.
The Collision Theory• Particles are constantly moving
• For a chemical reaction to take place the reactant particles must collide first
• For the collision to be successful the particles must have the right amount of energy
• The minimum amount of energy required for an effective collision is called the activation energy
Successful collisions
When two molecules collide a reaction might take place if:
- The molecules have enough energy to overcome the activation energy
- The molecules collide in the correct orientation
Concentration
• More particles in the same volume means they are closer together so greater chance of colliding.
• More collisions means more collisions with energy greater than activation energy.
• More frequent successful collisions means increased rate of reaction.
Pressure• When the pressure is
increased the same number of molecules occupy a smaller volume
• For gaseous reactants this has the same effect as increasing the concentration.
• More collisions take place, so more collisions with energy greater than activation energy.
• More frequent successful collisions means increased rate of reaction.
Catalysts
• Catalysts reduce the activation energy needed for a reaction
• They do this by offering an alternate route for the reaction to take (for example via an intermediate)
• Lower activation energy means more successful collisions
• More successful collisions means faster rate
Catalysts are very useful in industry
- They often require lower temperatures than uncatalysed reactions, which reduces the energy demand and carbon dioxide emissions from fossil fuels.
- They allow different reactions to be used with a better atom economy and reduced waste.
- They are often enzymes which allow processes to operate at lower temperatures and pressures