Rates of Reactions
What affects the rate of a chemical reaction?
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High rate of reaction
Low rate of reaction
Explosion in car air bags
Rust
Collision Theory
• A chemical reaction may occur as a result of collisions between
particles
• Particles that collide must fulfil 2 criteria
• Correct orientation of reactant particles
• Must have enough energy to break the chemical bonds of the reactants.
• If these criteria are not met the collision will not result in a
reaction.
Analogy for collision theory
Animation of collision theory
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Activation Energy
• Activation energy is the minimum
amount of energy that a collision
must possess for a reaction to
occur.
• It is given the symbol
• For a collision to result in a
reaction, the collision must have
energy which is greater than or
equal to the activation energy.
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Controlling reaction rate
• Reaction rate can be controlled by:
• Altering the temperature of the system
• Altering the surface area of the reactant(s)
• Altering the concentration of the reactant(s)
• Using a catalyst
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Temperature and reaction rate
• Temperature is a measure of the average kinetic energy of
particles.
• High temperature means:
 Faster particles with more kinetic energy
More chance of collision with enough energy to overcome
activation energy
Faster reaction rate
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Surface area and Reaction Rate
• For a solid reactant, only the particles on the surface are
available for the reaction.
• If the surface area is increased by breaking up the solid into
smaller pieces, more surface is exposed so the reaction rate will
increase.
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• Example: magnesium + nitric acid
• Write the equation?
• What would be the signs of chemical reaction?
• Which reacts faster – large piece or small pieces? Why?
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Concentration of reactants
High concentration:
 More particles
More chance of collision
Faster reaction rate
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The effect of pressure
• Higher pressure (compressed
gas) has less volume
• Collision frequency between
particle increase
• Increase the effective collision
frequency
• Rate of reaction increase
Catalysts and Rate of Reaction
• Catalyst: Substance which speeds up a chemical reaction and
remains unchanged at the end of the reaction
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• A catalyst provides an
alternative route for the
reaction, with a lower
activation energy.
• It does not “lower the
activation energy of the
reaction”
• Model / Analogy: You
provided an alternate route
(tunnel). You did NOT lower
the hill.
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Example- Manufacture of nitric acid
The manufacture of nitric oxide, NO, is an important
step in the industrial manufacture of nitric acid. The first
step of this process used a platinum–rhodium catalyst.
The overall reaction is:
4NH3(g) + 5O2(g)
𝑐𝑎𝑡.
4NO(g) + 6H2O(g)
The model shows a simplified view of the reactions that break and reform bonds on the surface of the metallic catalyst in this initial step.
Reactions at the surface of the platinum–rhodium catalyst
Effect of copper on the rate of the reaction between zinc granules
and hydrochloric acid
Determine whether copper is
acting as a catalyst in the
reaction.
Image Sources
Slide
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http://spmchemistry.onlinetuition.com.my/2013/01/finding-instantaneous-rate-of-reaction.html
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https://tenor.com/view/airbag-boom-accident-wow-gif-5749934
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http://www.sheilazellerinteriors.com/articles/for-the-love-of-rusty-old-things
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https://sites.google.com/site/mrcopilchemistry/energy-changes-and-reversible-reactions
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https://s3-us-west-2.amazonaws.com/courses-images-archive-read-only/wpcontent/uploads/sites/887/2015/04/23205758/CNX_Chem_12_05_COandO2.jpg
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http://www.thecasabouquet.com/glow-stick-experiment/
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http://changeinmatter.tripod.com/main/factors_affecting_rate_of_reaction.htm
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http://biology.reachingfordreams.com/chemistry-cheat-sheet/energy-changes-and-rates-of-reaction/37reaction-rates.html
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http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr_gateway/chemical_economics/reaction2rev3.shtml
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http://science-dfjhs.web.nebo.edu/home/8th-grade/chemistry/chem3/c8
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https://www.thoughtco.com/kid-friendly-elephant-toothpaste-demo-604164
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http://tfssmypscience.pbworks.com/w/page/53182925/RateReaction
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