WARM UP
2 NO + 2 H2  N2 + 2 H2O
Trial
1
2
3
4
[NO]initial
.60
.60
.60
1.20
[H2]initial
.15
.30
.60
.60
ΔN2/Δt
.076
.15
.30
1.21
1. Determine the rate law expression.
(hint: order for NO and H2…..and k)
Reaction Order
Chemistry II
Reaction Order
• Sum of the orders of each
– [A]1[B]1 = 2nd order
– [A]0[B]1 = 1st order
– [A]0[B] 0 = Zero order
• Impacts the rate over time, as [ ] changes
Half – Life
• Time it takes for ½ the reactants to
disappear
– Radiometric dating of rocks, cavemen, etc.
– Initial is [.28] and half-life is 10 minutes…..
[.14] will remain after 10 minutes
• Another way to describe rate
– Longer half-life = slower rate
Types of Reactions
• Zero order reactions
– Constant rate,
independent of [ ]
• rate = k
– Half-life
• [A]0 / 2k
• Decreases over time
– [ ]-t equation
• [A]t = -kt + [A]0
– Rare
• Metal surface reactions
Types of Reactions
• First order reactions
– Rate decreases with [ ]
• rate = k[A]
– Half-life
• Constant
• .693 / k
– [ ]-t equation
• [A]t = [A]0e-kt
– Common
• Radioactive decay / dating
Types of Reactions
• Second order reactions
– Rate DROPS with [ ]
• rate = k [A]2
– Half-life
• 1 / k[A]0
• Increases over time
– [ ]-t equation
• 1/[A]t = kt + 1/[A]0
– Occasional
• Time-release meds
WRAP UP
• Draw a graph of concentration vs. time for
each of the following reaction orders:
– Zero order
– 1st order
– 2nd order
– 3rd order
WARM UP
• A decomposition reaction has a rate
constant of 0.12 / year at a certain
temperature. What is the half-life of this
reaction (it is 1st order)?
WRAP UP
• A decomposition reaction has a rate
constant of 0.12 / year at a certain
temperature. How long will it take for the
concentration of reactant reach 20% of its
original value (it is 1st order)?
WARM UP
1. Compare and contrast zero-order,
first-order, and second-order
reactions.
2. Give a real-world example of
zero-order, first-order, and
second-order chemical reactions.
WRAP UP
• List 5 conditions that might
make a chemical reaction
occur faster.
Hint: Think about collision theory, rate
law, and reaction order.