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Average Rate = quantity
time
The amount of increase or decrease depends
on mole ratios
Units =Molarity/s or mol/Ls
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As a reaction proceeds, there is a decrease in
concentration of reactants and an increase in
the concentration of the products
N2 + 3H2  2NH3
N2 and H2 decrease in concentration over
time, while NH3 increases in concentration
over time.
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Average reaction rate
Concentration M2 - Concentration M1
t2 - t1
[Concentration]
time
Reaction rates must always be positive
If you get a negative reaction rate, change the
sign to positive
1.
2.
3.
Reacting substances (atoms, ions, or
molecules) must collide.
Reacting substances must collide with the
correct orientation
Reacting substances must collide with
sufficient energy to form the
activated complex.
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The minimum amount of energy that reacting
particles must have to form the activated
complex and lead to a reaction
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Symbol: Ea
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Direct influence on the rate of a reaction
inc time
inc energy
inc time
inc energy
F
Given H2 + Cl2
Time (s)
0.00
5.00
[Concentration]
time
2HCl
[H2] (M)
0.060
0.025
[Cl2] (M)
0.070
0.035
[HCl] (M)
0.00
0.035
1. Calculate the average reaction rate expressed in
moles H2 consumed per liter per second
0.025 – 0.060
= 0.007 M/s
5.00 – 0.00
2. Calculate the average reaction rate expressed in
moles Cl2 consumed per liter per second
0.035 – 0.070
5.00 – 0.00
= 0.007 M/s
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An expression that relates rate of a reaction
and reactant concentration
Rate of reaction depends on reactant
concentrations
never includes products
Given: 2A + 3B
2C
◦ In general, rate = k [A]x [B]y
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Symbol: k (lowercase)
A constant specific and unique for every
reaction
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If k is large, products form quickly
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If k is small, products form slowly
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Exponents in rate law (x and y)
Determine how much the rate depends on the
respective concentration(s)
Can ONLY be determined by experiment
Overall order of a reaction is the sum of all
orders (x + y)
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2A + 3B
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Skeleton Rate = k [A]x [B]y
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2C
Overall order of reaction (x + y)
If x=1, 1st order in A
If y=2, 2nd order in B
Overall order 1 + 2 = 3rd order
Write the skeleton rate law
2H2O(l)
2H2(g) + O2(g)
rate = k [H2O]x
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Example 1: rate = k [A]
1st order
◦ If concentration of A doubles, rate doubles
◦ If concentration of A triples, rate triples
◦ If concentration of A is halved, rate halves
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**directly
proportional
Example 2: rate = k [A]2 2nd order
◦ If concentration of A doubles, rate quadruples
◦ If concentration of A triples, rate is 9 times as fast
**exponentially
proportional
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Example 3: rate = k [A]0 0th order
◦ Recall: anything raised to the 0th power = 1
◦ If the concentration of A changes in any way
NOTHING happens to the rate
◦ Rate is not dependent on concentration
◦ Rate = k