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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?. Rates of reaction Rate = change in concentration (of reactant or product) time. Methods for measuring rate of reaction. - PowerPoint PPT Presentation
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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
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