54
Reaction Rates Ch. 13.1
Reaction Rates
Ch. 13.1
Kinetics and
Reaction Rate
• Kinetics is the area of chemistry
dealing with the speed or rate at
which reactions take place.
• Some reactions act quickly. (air bags)
• Some reactions react slowly. (rusting)
What is a Rate?
• Fixed ratio between two
things
• It measure how fast or slow a
reaction or an action takes
place.
Why Rates Important?
• It is important to understand
rates in order to control the
reaction.
Fires have fast rates,
that must be controlled
Fire Triangle
Understanding Rates
• Chemists, engineers, and
everyday chefs need to be
aware how reaction rates can
change when conditions for the
reaction change.
REACTION RATES ! !
RR = D [reactants ]
D t
RR = D [products ]
D t
Collision Theory
•Reactions and the rate of
reactions taking place
depend on two or more
molecules colliding in
such a way that old bonds
break and new bonds
form.
COLLISION THEORY
• C = collisions
• E = energy
• O = orientation
Collisions
• Reactants must be in contact in
order to react.
Energy
• Every reaction needs energy
to get things started.
(Activation Energy)
• Energy to form activated
complex
Energy
1. Exothermic Reaction
-A process that loses heat to
surrounding.
-surrounding get hotter
-Products are lower in energy than
reactants
2. Endothermic Reaction
-A process that absorbs heat from
surrounding.
-surrounding get colder
-Products are higher in energy
than reactants
Energy Diagram
Orientation
• Reactants must be lined up
properly when colliding.
Factors Affecting
RXN Rates
• Nature of Reactants
• Temperature
• Concentration
• Surface Area
• Catalysts/inhibitors
Nature of Reactants
• Not all materials react with
each other.
• If reactants don’t have
enough energy, no reaction
can take place.
• If reactants don’t collide in
the proper orientation, no
reaction takes place.
Nature of reactants
Collisions
Collisions
Energy
Collisions
Energy
Orientation
NO
NO
YES
Temperature• Higher the temperature the
faster the reaction.
• Higher temperature causes
particles to move faster, thus
they collide more often.
• Raising the temperature only
100C will double the reaction
rate
Concentration (M)
• The more concentrated a solution
the faster the reaction will be.
• If the number of reactant
particles per unit volume
increases, then the chance of
successful collisions increase.
• Double the concentration of
reactants can quadruple the rate.
Which has a higher concentration of reactants?
More concentrated
Surface Area
(particle exposure)
• The greater the surface area
the faster the reaction.
• More surface area means
more reactants are in
contact, which of course
leads to more collisions.
Surface AreaMore it can be broken down a more area
MORE
kindle
groundLESS Whole bean
Logs
a
a
Catalyst• A substance that changes
the rate of chemical
reactions without being
permanently changed itself.
• Brings energy to reaction
• Lowers the activation
energy required.
• Ex. yeast, enzymes
Inhibitors
•Inhibitors slow down
reactions by tying up
the reactants
•Ex. preservatives
Reversible
Reactions and
Equilibrium
Ch 13.2
Reactions Going
Completion
• products are removed from
the reacting system
(3 examples)
1. Forming a insoluble
precipitate (no ions left)
2. Forming a gas
3. Neutralized water
Removing a Product
• gas formed (g)
• 2H2O 2H
2(g) + O
2 (g)
• Precipitate formed (s)
CuSO4
+Na2SNa
2SO
4(aq) + CuS(s)
• Water is formed (H2O)
• NaOH + HCl NaCl + H2O
Reversible
Reactions
3H2
+ N2
↔ 2NH3
• the products remain in the system so
they may react back to original
reactants.
• Two reactions where the only
difference is the direction.
• Reactions are occurring at the same
time.
Equilibrium
•The state in which a
chemical reaction
and its reverse
reaction occur at the
same rate.
Examples:
Reversible Reactions.
• Unopened Soda
• Breathing cycle
• Rechargeable batteries
• Mood rings
1. Equilibrium
is reached
when rates
are equal
2. No change
in the
amounts.
Ch. 13.5
Le Chatelier’s Principle
Ch. 13.5
Le Chatelier’s Principle
•If a system at
equilibrium is
stressed, it will react
to undo the stress.
STRESS to SYSTEM
Three stress’s are
1.Temperature
2.Concentration
3. Pressure.
(change in volume)
Le Chatelier’s Principle
• If disturbed by a stress, a net
shift (left or right) will occur to
offset the change.
• A shift represents the
formation of more reactants or
products. (opposite to the shift
is consumed)
When a stress is applied the
reaction will reestablish equilibrium
Equilibrium = No change in amount over time
1. Concentration
Equilibrium
Reactants
Products
Equilibrium reached
when concentrations
become constant
2. Temperature
Heat +
3. Pressure
The side with
least moles
The side with
most moles
LEFT: RIGHT:
↔
Stress, Shift, Change In
Concentration
