Unit 12 : Unit 12 Rates of chemical reactions
What makes a reaction happen? : What makes a reaction happen? Spontaneous processes - processes that
happen by themselves without outside help.
Example:
water flowing downhill
phosphorus reacting with air
What makes a reaction happen? : What makes a reaction happen? Nonspontaneous processes - processes
that need help to occur.
Examples:
moving water uphill
making spring toy move
C(s,graphite)? C(s, diamond)
What makes a reaction happen? : High potential energy What makes a reaction happen? All matter possesses potential energy.
Potential energy is energy stored in matter. Low potential energy
What makes a reaction happen? : What makes a reaction happen? There is a tendency for things to move
from high energy to low energy. Potential
energy
What makes a reaction happen? : What makes a reaction happen? Potential energy is stored in the chemical
bonds of chemical compounds.
Spontaneous chemical reactions involve
high energy reactants forming lower energy
products Reactants Products
What makes a reaction happen? : What makes a reaction happen? Exothermic reactions - energy is given off
in form of heat when system moves to lower
energy. Reactants Products
What makes a reaction happen? : What makes a reaction happen? Endothermic reactions - energy is
absorbed when system moves from low
energy to high. Reactants Products
What makes a reaction happen? : What makes a reaction happen? What keeps a reaction from moving to
low energy? Reactants Products
What makes a reaction happen? : What makes a reaction happen? What keeps a reaction from moving to
low energy? Reactants Products
What makes a reaction happen? : What makes a reaction happen? Activation energy - minimum energy
colliding particles must have in order to react. Reactants Products
What makes a reaction happen? : What makes a reaction happen? Collision Theory –
molecules must collide to react.
some collisions result in reaction, others do
not.
Factors affecting rate of reaction : Factors affecting rate of reaction What must happen for a reaction to
occur? Molecules must collide with sufficient energy Molecules must collide with proper orientation
Factors affecting rate of reaction : Factors affecting rate of reaction What could you do to increase the
number of high energy, correctly-
oriented collisions? Increase temperature catalyst Increase surface area through crushing Increase concentration
Factors affecting rate of reaction : Increase temperature Factors affecting rate of reaction Increasing temperature increases the
kinetic energy of the reactant molecules,
leading to more high-energy collisions.
More high-energy collisions = faster rate
Factors affecting rate of reaction : Factors affecting rate of reaction What could you do to increase the
number of high energy, correctly-
oriented collisions? Increase temperature catalyst Increase surface area through crushing Increase concentration
Factors affecting rate of reaction : Increase concentration Factors affecting rate of reaction Increasing concentration increases the
number of reactant molecules in a given
volume, therefore leading to more high-
energy collisions.
More high-energy collisions = faster rate
Factors affecting rate of reaction : Factors affecting rate of reaction What could you do to increase the
number of high energy, correctly-
oriented collisions? Increase temperature catalyst Increase surface area through crushing Increase concentration
Factors affecting rate of reaction : Increase surface area through crushing Factors affecting rate of reaction Increasing surface area of reactant
molecules increases the number of
reactant molecules that can collide with
other molecules, therefore leading to
more collisions.
big surface area/small particles = faster rate
Factors affecting rate of reaction : Which will react faster with X, the: big particles (A), or small particles (B)?
Why? Explain your answer based on the following pictures: Factors affecting rate of reaction A B X
Factors affecting rate of reaction : Factors affecting rate of reaction Explain the following based on what you
know about the factors influencing rates of
reactions:
You put food in the fridge
You blow on a fire
You chop up logs of wood before throwing them on the fire
You put lighter fluid on charcoal to light it
Powdered metals are kept in special metal cabinets
What makes a reaction happen? : What makes a reaction happen? From last time:
Spontaneous reactions are _____________ .
Spontaneous reactions occur because there is a natural tendency for things to move from high energy to low energy.
What makes a reaction happen? : What makes a reaction happen? Compounds with high energy are unstable.
Compounds with low energy are stable.
Chemical compounds undergo reactions to
become more stable.
What makes a reaction happen? : What makes a reaction happen? We said in spontaneous processes, high
energy reactants become low energy
products Does this type of process give off or absorb heat? What do we call this type of process?
What makes a reaction happen? : What makes a reaction happen? While many spontaneous reactions are
exothermic, not all are. Consider the following example:
H2O(s) + 6.0kJ/mol ? H2O(l) Is this reaction exothermic or endothermic? Draw the energy diagram for this reaction.
What makes a reaction happen? : What makes a reaction happen? Although the melting of ice is endothermic
the process is still spontaneous. Ice when it melts becomes more disordered.
What makes a reaction happen? : Entropy is a measure of the disorder in a system? What makes a reaction happen? Disordered
high entropy Ordered
low entropy See section 2 of 13
Messy Room by Shel Silverstein : Messy Room by Shel Silverstein Whosever room this is should be ashamed! His
underwear is hanging on the lamp. His raincoat is there
in the overstuffed chair, And the chair is becoming
quite mucky and damp. His workbook is wedged in the
window, His sweater's been thrown on the floor. His
scarf and one ski are beneath the TV, And his pants
have been carelessly hung on the door. His books are
all jammed in the closet, His vest has been left in the
hall. A lizard named Ed is asleep in his bed, And his
smelly old sock has been stuck to the wall. Whosever
room this is should be ashamed! Donald or Robert or
Willie or-- Huh? You say it's mine? Oh, dear, I knew it
looked familiar!
What makes a reaction happen? : What makes a reaction happen? 2 Driving forces for reactions
Move to lower energy
Become more disordered
What makes a reaction happen? : What makes a reaction happen? Complete the following table to predict
whether a reaction will occur:
What makes a reaction happen? : What makes a reaction happen? Why do things get more disordered over
time? Statistically the disordered state is more
favorable or likely to occur.
Equilibrium : Equilibrium What does it mean to say a reaction is
reversible? See link
Equilibirium : Equilibirium Initially, only products are formed, however
after enough product has been formed, then it
is possible for some product to react to form
reactants. H2 + O2 H2O
Equilibirium : Equilibirium Eventually, the rate at which products and
reactants are formed is the same and so the
overall concentrations of reactants and
products remain the same. EQUILIBRIUM H2 + O2 H2O
Equilibirium : Equilibirium KEY POINT
Although the concentrations of a reactants and
products may reach a constant value, this does
not mean the reaction has stopped occuring. It
just means the forward and reverse processes
are happening at the same rate.
Equilibrium : Equilibrium What can we do to push a reaction in? Although a system is at equilibrium where
concentrations of reactants and products
remain constant, you can push the reaction
in the forward or reverse direction. Shifts in Equilibrium
Equilibrium : Equilibrium Pressure
Concentration
Temperature Factors affecting equilibirium How can we predict how changing pressure,
concentration and temperature will affect the
equilibrium?
Equilibrium : Equilibrium When a system at equilibrium is subjected to
a change in concentration, temperature, or
pressure, it will shift in such a way as to
minimize the change. Le Chatelier’s principle
Equilibrium : Equilibrium Imagine you have a reaction at equilbrium:
A ? B
At equilibrium you have 50 A’s and 50 B’s.
What will happen to the equilibrium when you
add 50 more B’s? What will be the new
amounts of A and B? EXAMPLE 1 50 50
Equilibrium : Equilibrium Adding more of a substance to one side of
the reaction will push the reaction in the
opposite direction
A ? B Le Chatelier’s principle B added to
product side Rxn shifts to
reactants
Equilibrium : Equilibrium Adding more of a substance to one side of
the reaction will push the reaction in the
opposite direction
A ? B Le Chatelier’s principle A added to
reactant side Rxn shifts to
products
Equilibrium : Equilibrium Imagine you have a reaction at equilbrium:
A(g) + B(g) ? C(g)
What would happen to the relative amounts of
A, B and C if you increased pressure? EXAMPLE 2 50 50 50
Equilibrium : Equilibrium Adding pressure to a reaction will push the
reaction in the direction of less moles of gas.
A(g) + B(g) ? C(g) Le Chatelier’s principle Pressure increased Rxn shifts to products 2 moles of
reactants 1 mole of products
Equilibrium : Equilibrium Imagine you have a reaction at equilbrium:
2NH3(g) ? N2(g) + 3H2(g)
Which way would the equilibrium shift if you
increased the pressure of this reaction? Try this
Equilibrium : Equilibrium Imagine you have a reaction at equilbrium:
A+ 10kJ ? B
What would happen to the relative amounts of
A, B if you increased the temperature of the
reaction? EXAMPLE 3 50 50
Slide 46 : Adding heat to a reaction will push endothermic
processes forward and exothermic processes in
reverse.
A+ 10kJ ? B Le Chatelier’s principle Reaction is
endothermic Rxn shifts to
products Temperature increased
Equilibrium : Equilibrium For different reactions you can expect
different amounts of reactants and products
at EQ
Equilibrium : Equilibrium The ratio of products to reactants at EQ is
given by the Equilibrium constant (K).
When K is big (>1) you get more product
than reactant.
When K is small (<1) you get more reactant
than product.
Equilibrium : Equilibrium Imagine you have determined the equilibrium
for several reactions. In which of these
reactions are the products favored over
the reactants?
Rxn 1: K = 1 X 102
Rxn 2: K = 0.003
Rxn 3: K = 3.5
Rxn 4: K = 6 X 10-4 Example
Equilibrium : Equilibrium For the following reaction:
aA + bB cC + dD
K = [C]c [D]d
[A]a [B]b
Equilibrium : Equilibrium Imagine you have a reaction at equilbrium:
2NH3(g) ? N2(g) + 3H2(g)
Write the equilibrium expression for this
reaction. Example
Equilibrium : Equilibrium Imagine you have a reaction at equilbrium:
H2(g) + I2(g) ? 2HI(g)
Write the equilibrium expression for this
reaction. Example
Equilibrium : Equilibrium Imagine you have a reaction at equilbrium:
N2O4(g) ? 2NO2(g)
Write the equilibrium expression for this
reaction. Example
Equilibrium : Equilibrium Imagine you have a reaction at equilbrium:
2NH3(g) ? N2(g) + 3H2(g)
At equilibrium you have the following
concentrations of reactants and products:
[H2] = 0.15M [N2] = 0.25M [NH3] = 0.10M
Calculate the equilibrium constant for this
reaction. Example
Equilibrium : Equilibrium Imagine you have a reaction at equilbrium:
N2O4(g) ? 2NO2(g)
At equilibrium you have the following
concentrations of reactants and products:
[N2O4] = 0.0045M [NO2] = 0.030M
Calculate the equilibrium constant for this
reaction. Example
Rates of reactions : Rates of reactions Review from last time
1) What is the relationship between a reaction’s rate and the concentration of its reactants?
2) What is a rate law?
3) What does the order of a reaction tell
you?
Rate of reaction : Rate of reaction The rate of a reaction and its relationship
with the concentration of the reactants in a
reaction is given by its rate law.
Rate = k [reactant]x Rate constant order
Rate of reaction : Rate of reaction Rate constant (k) - adjusts rate for different
conditions (temperatures).
Order - gives relationship between
concentration and rate.
Rate of reaction : Rate of reaction Order - gives relationship between
concentration and rate.
If you doubled the concentration of reactant
in each of the following 3 reactions, what
would be the affect on rate for each?
RXN 1: Rate = k [reactant] 1
RXN 2: Rate = k [reactant] 2
RXN 3: Rate = k [reactant] 3
Rate of reaction : Rate of reaction RXN 1: Rate = k [reactant] 1 (1st order rxn)
RXN 2: Rate = k [reactant] 2 (2nd order rxn)
RXN 3: Rate = k [reactant] 3 (3rd order rxn)
IMPORTANT Order can only be determined
experimentally, it does not come from the
coefficients in the reaction equation.
Rate of reaction : Rate of reaction For a simple reaction involving 1 reactant:
aA bB
The rate law for a reaction like this would be:
rate = k[A]x
Where x is the order of the reaction with
respect to A.
Rate of reaction : Rate of reaction Imagine you determine the rate laws for
several reactions to be:
rxn 1: rate = k[A]
rxn 2: rate = k[Z]2
rxn 3: rate = k[N]3
a) What is the order for each reaction?
b) For which reaction does doubling
concentration affect the reaction rate most?
Rate of reaction : Rate of reaction Many reactions involve 2 reactants:
aA + bB cC + dD
The rate law for a reaction like this would be:
rate = k[A]x[B]y
Where x is the order for A and y is the order
for B; x + y gives the overall order for the rxn
Rate of reaction : Rate of reaction Consider the following reactions and their rate
laws:
rxn 1: rate = k[A][B]
rxn 2: rate = k[D]2[E]
rxn 3: rate = k[X][Y]2
For each reaction give: a) the order with
respect to each reactant and b) the overall
order of the reaction.
Rate of reaction : Rate of reaction Write rate laws for the following reactions:
1) The reaction between F2 and ClO2 is first order with respect to F2 and first order with respect to ClO2.
2) The reaction between S2O82- and I- is 2nd order with respect to S2O82- and first order with respect to I-
Review from last time : Review from last time 1) Imagine you have a reaction at equilbrium:
2N2O5(g) ? 4NO2(g) + O2(g)
Write the equilibrium expression for this
reaction.
Review from last time : Review from last time 2) What does the rate law of a reaction tell you?
3) Consider the following rate law:
rate = k[A]
a) What order is this reaction?
b) If the concentration of A is doubled what will happen to the rate of reaction?
Review from last time : Review from last time 4) Consider the following rate law:
rate = k[A][B]
a) What is the overall order of this reaction?
b) If the concentration of A is doubled, while the concentration of B is held constant what will happen to the rate of reaction?
c) If the concentration of B is doubled, while the concentration of A is held constant what will happen to the rate of reaction?
Review from last time : Review from last time 4) Consider the following rate law:
rate = k[A][B]2
a) What is the overall order of this reaction?
b) If the concentration of A is doubled, while the concentration of B is held constant what will happen to the rate of reaction?
c) If the concentration of B is doubled, while the concentration of A is held constant what will happen to the rate of reaction?
Rate of reaction : Rate of reaction Determining the order of a reaction is
important because it can help a chemist figure-
out the mechanism of a reaction.
A mechanism is the set of steps that a reaction
goes through in order to make product.
Rate of reaction : Rate of reaction The order of a reaction can be determined by
performing a number of experiments where
the concentrations of the reactants are
changed and the effect on rate is measured.
Rate of reaction : Rate of reaction An example of this type of data is shown here
for the reaction aA bB: Write the rate law for this reaction.
Rate of reaction : Rate of reaction Solution
step 1: Write the general form of the rate law
for the reaction.
step 2: Determine the order by comparing 2
experiments and seeing how rate and
concentration are changing.
Rate of reaction : Rate of reaction Solution
step 1:
for the reaction aA bB the general form of
the rate law should be:
rate = k[A]x
Now we need to determine the order, x for the
reaction.
Rate of reaction : Rate of reaction Pick 2 rows of data to look at 1st. By what factors do concentration and rate change?
Rate of reaction : Rate of reaction By what factors do concentration and rate change? X2 X4 If conc. doubles and rate quadruples what is the order?
Rate of reaction : Rate of reaction Order = 2nd X2 X4
Rate of reaction : Rate of reaction Pick another row of data to look at to confirm your first answer.
Rate of reaction : Rate of reaction By what factors do concentration and rate change? X2 X4 If conc. doubles and rate quadruples what is the order? Order = 2nd
Rate of reaction : Rate of reaction Solution
We determined the order of the reaction to be
2nd order with respect to [A].
Now we write our rate law with the order:
Answer rate = k[A]2
Rate of reaction : Rate of reaction Try this An example of this type of data is
shown here for the reaction cC dD: Write the rate law for this reaction.
Rate of reaction : Determine the rate law for the reaction:
aA + bB cC + dD
You are given the following data: Rate of reaction
Rate of reaction : Rate of reaction Solution
step 1: Write the general form of the rate law
for the reaction.
step 2: Determine the order for each reactant
by looking at 2 experiments where
concentration of only 1 reactant is changing.
Rate of reaction : Rate of reaction Solution
step 1: Write the general form of the rate law
for the reaction.
rate = k[A]x[B]y
Rate of reaction : 2 determine order with respect to A, look at 2
experiments where concentration of A
changes but concentration of B remains the
same. Rate of reaction
Rate of reaction : 2 determine order with respect to A, look at 2
experiments where concentration of A
changes but concentration of B remains the
same. Rate of reaction
Rate of reaction : Rate of reaction By what factors do concentration and rate change?
Rate of reaction : Rate of reaction By what factors do concentration and rate change? X2 X2
Rate of reaction : Rate of reaction By what factors do concentration and rate change? X2 X2 If conc. doubles and rate doubles what is the order?
Rate of reaction : Rate of reaction If conc. doubles and rate doubles what is the order? Order = 1st
Rate of reaction : Rate of reaction Solution
step 1: Write the general form of the rate law
for the reaction.
step 2: Determine the order for each reactant
by looking at 2 experiments where
concentration of only 1 reactant is changing. Figured out the order of reaction with respect to reactant A.
Rate of reaction : 2 determine order with respect to B, look at 2
experiments where concentration of B
changes but concentration of A remains the
same. Rate of reaction
Rate of reaction : 2 determine order with respect to B, look at 2
experiments where concentration of B
changes but concentration of A remains the
same. Rate of reaction
Rate of reaction : Rate of reaction By what factors do concentration and rate change?
Rate of reaction : Rate of reaction By what factors do concentration and rate change? X4 X4
Rate of reaction : Rate of reaction If conc. quadruples and rate quadruples what is the order? X4 X4
Rate of reaction : Rate of reaction If conc. quadruples and rate quadruples what is the order? Order = 1st X4 X4
Rate of reaction : Rate of reaction Solution
We determined the order of the reaction to be
1st order with respect to [A] and 1st order with
respect to [B].
Now we write our rate law with the order:
Answer rate = k[A][B]
Rate of reaction : Determine the rate law for the reaction:
S2O82- + 3 I- 2SO42- + I3-
You are given the following data: Rate of reaction
Rate of reaction : Rate of reaction RECALL Determining the order of a reaction is
important because it can help a chemist figure-
out the mechanism of a reaction.
A mechanism is the set of steps that a reaction
goes through in order to make product.
Rate of reaction : Rate of reaction Consider the following reaction:
2N2O(g) ? 2N2(g) + O2(g)
This is the overall reaction, it shows you what
you start with (reactants) and what you end up
with at the end (products).
Rate of reaction : Rate of reaction In reality, this reaction occurs in a 2 steps.
1st step: N2O(g) ? N2(g) + O(g)
2nd step: N2O(g) + O(g) ? N2(g) + O2(g)
reaction mechanism
Rate of reaction : Rate of reaction When you add these rxns together, you get
the overall reaction.
How to ADD equations - Add all compounds
on reactants sides together .
1st step: N2O(g) ? N2(g) + O(g)
2nd step: N2O(g) + O(g) ? N2(g) + O2(g)
Rate of reaction : Rate of reaction How to ADD equations - Add all compounds
on reactants sides together .
1st step: N2O(g) ? N2(g) + O(g)
2nd step: N2O(g) + O(g) ? N2(g) + O2(g) 2N2O(g) + O(g)
Rate of reaction : Rate of reaction Now - Add all compounds on products sides
together .
1st step: N2O(g) ? N2(g) + O(g)
2nd step: N2O(g) + O(g) ? N2(g) + O2(g) 2N2O(g) + O(g) ? 2N2(g) + O(g) + O2(g)
Rate of reaction : Rate of reaction Now - Cancel out terms appearing on both sides
1st step: N2O(g) ? N2(g) + O(g)
2nd step: N2O(g) + O(g) ? N2(g) + O2(g) 2N2O(g) + O(g) ? 2N2(g) + O(g) + O2(g) Compounds that are in the mechanism but not in the overall reaction are called intermediates
Rate of reaction : Rate of reaction Now - Rewrite equation without the
intermediates 2N2O(g) + O(g) ? 2N2(g) + O(g) + O2(g) 2N2O(g) ? 2N2(g) + O2(g)
overall reaction
Rate of reaction : Rate of reaction Example
For the following reaction mechanism
determine: 1) reaction’s intermediates, 2)
overall reaction equation.
1st step : 2NO(g) ? N2O2 (g)
2nd step: N2O2 (g) + O2(g) ? 2NO2(g)
Rate of reaction : Rate of reaction Example
For the following reaction mechanism
determine: 1) reaction’s intermediates, 2)
overall reaction equation.
1st step : NO(g) + Cl2(g)? NOCl2 (g)
2nd step: NOCl2 (g)+ NO(g) ? 2NOCl(g)
Rate of reaction : Rate of reaction The rates of each individual step can be
different.
1st step : 2NO(g) ? N2O2 (g) [fast]
2nd step: N2O2 (g) + O2(g) ? 2NO2(g) [slow]
The rate of the slow step determines how fast
or slow the overall process is. Do assembly line demo
Rate of reaction : Rate of reaction The rate law for the overall reaction comes
from the rate determining step (slow step)
1st step : 2NO(g) ? N2O2 (g) [fast]
2nd step: 1N2O2 (g) + 1O2(g) ? 2NO2(g)[slow]
rate = k[N2O2]1[O2] 1
rate = k[N2O2][O2]
The orders for each reactant come from the
coefficients in the slow step.
Rate of reaction : Rate of reaction Demonstration
We will produce 1 lego cat in 3 steps, 2 fast
and 1 slow.
Time how long it takes to produce 1 Lego
Cat.
Does it matter in which order we put the
steps?
Rate of reaction : Rate of reaction Write the rate law and overall reaction from the
following reaction mechanism.
1st step: NO2(g) + F2(g) ? NO2F(g) [slow]
2nd step: F(g) + NO2(g) ? NO2F(g) [fast]
Rate of reaction : Rate of reaction Write the rate law and overall reaction from the
following reaction mechanism.
1st step: NO2(g) + NO2(g) ? NO3(g) + NO(g) [slow]
2nd step: NO3(g) + CO(g) ? NO2(g) + CO2(g) [fast]
Rate of reaction : Rate of reaction Write the rate law and overall reaction from the
following reaction mechanism.
1st step: H2(g) + 2NO(g) ? N2O (g) + H2O(g) [slow]
2nd step: N2O (g) + H2(g) ? N2(g) + H2O(g) [fast]
Rate of reaction : Rate of reaction Write the rate law and overall reaction from the
following reaction mechanism.
1st step: H2(g) + NO(g) ? N (g) + H2O(g) [slow]
2nd step: N (g) + NO(g) ? N2(g) + O(g) [fast]
3rd step: H2(g) + O(g) ? H2O(g) [fast]