Download Rates of Reactions:

Rates of Reactions:
Reaction rate (definition): speed at which a chemical change takes place.
Some reactions take place so quickly we consider them to be instantaneous
i.e. - firecracker exploding
Some reactions take a long time to occur
i.e. - a nail rusting
Most reactions happen somewhere in between these 2 extremes.
Collision Theory:
In order for a reaction to take place particles must collide with enough energy to break and/or alter
the bonds.
1) The more energy in the collisions the more likely bonds will be broken and the faster the
reaction takes place. The more collisions that can take place the faster the reaction takes
place.
2) Reactions require some input of energy to get started. (ie. A match will not light without a
little energy from striking the match on the box.)
Activation Energy – Energy required to start a reaction.
(Slide 1)
The collision between particles creates an unstable transition state called the activated complex.
The activated complex is created by an effective collision and while it exists the chemical bonds are
being rearranged to form the products.
Factors affecting reaction rate:
1. Temperature
2. Concentration
3. Surface area
4. Pressure (a gas must be involved)
5. Catalysts/inhibitors
Temperature: The average kinetic energy in the motion of the particles in a substance. As
temperature increases the motion of the particles increases resulting in more collision, and more
energy in each collision.
General Pattern: For each 10C rise in temperature the reaction rate doubles. (Slide 2)
Concentration: the amount of a substance in a given space. (Slides 3 and 4)
As concentration increases, the number of collisions increases causing the reaction rate to increase.
Demo:
Zn + 2HCl  ZnCl2 + H2  in 2 set ups side by side.
Dilute HCl results in a slow reaction.
Concentrated HCl results in a fast reaction.
Surface Area (Particle Size): the amount of material directly available to take part in a reaction.
(Slides 5 – 8)
For a solid the smaller the pieces a substance is broken into, the greater the surface area, the more
collisions possible and the faster the reaction. Dissolving a substance will maximize its surface area.
Examples:
1)
A log – has small surface area so it burns slow.
Wood chips- have large surface area so they burn fast.
2) Bakery in Branford
Demo:
Zn + 2 HCl  ZnCl2 + H2
Powdered Zinc runs fast.
Mossy Zinc runs slower.
Pressure (gases):
An increase in pressure on a gas; results in a decrease in the volume; which results in an increase in
the concentration of the gas; resulting in an increase in the reaction rate.
P α 1/V
(Slides 9 – 12)
Example:
The pressure in a car’s cylinder is increased before the spark ignites the gas causing a more rapid,
complete burning.
A race car engine uses a higher pressure than a street vehicle.
Catalyst/inhibitor:
(Slides 13 – 14)
Catalyst: a substance which can speed up a reaction, but is unchanged in the reaction.
Demo: 1) Sugar cube and Ash
Sugar cube without catalyst will not burn. Sugar cube with ash burns
C12H22O11 + 12 O2  12 CO2 + 11 H2O
2) Alka seltzer in water (Active ingredients are Citric Acid and Sodium bicarbonate)
(this reaction does not take placed until the solids dissociate in the water)
H3C6H5O7 (aq) + 3 NaHCO3 (aq)  Na3C6H5O (aq) + 3 CO2  + 3 H2O (l)
Inhibitor: a substance which can slow down a reaction, but is unchanged in the reaction.
Example:
Burning wet wood, the water acts as an inhibitor slowing down the reaction.
Catalysts and Inhibitors work by changing the activation energy required to start a reaction.
(Slide 15)
Catalysts lower the activation energy making it easier for a reaction to take place.
Inhibitors raise the activation energy making it harder for the reaction to start and so the reaction
runs slower.
The same chemical can act as a catalyst in one reaction, but can be an inhibitor in a different
reaction.
Example:
A hand warmer contains iron filings, wood pulp and water in a packet made of a plastic polymer
that has small openings for Oxygen to pass into the packet and react with the chemicals.
Reaction 1: 4 Fe + 3 O2  2 Fe2O3 + Heat
In this reaction water acts as a catalyst allowing the iron to rust faster and generate more heat. The
heat being produced by this reaction is used as the activation energy for reaction #2.
Reaction2:
Wood + O2  CO2 + H2O + Heat
In this reaction water acts as an inhibitor slowing down the burning of the wood so it gives off the
heat slowly, in small amounts over time.