Why do chemical reactions occur between some substances and not
in others?
Slide 3
Chemical reactions occur so that the atoms in the elements
involved attain a more stable state of being.
Slide 4
Collision Theory Collision theory molecules must collide with
the proper orientation and sufficient energy to react.
Slide 5
Slide 6
Activation Energy The activation energy is the amount of energy
required to break the bonds between the atoms of the
reactants.
Slide 7
Slide 8
Condition Necessary for Reactions to Occur 1)Collision:
Reactants must collide. 2) Orientation: The reactants must align
properly to react. 3) Energy: The activation energy must be
attained to react.
Slide 9
Energy in Chemical Reactions Many chemical reactions also
produce energy changes. Definitions: System the reactants and
products in the reaction Surroundings everything else around the
reaction (eg air in the room, reaction flask)
Slide 10
Heat of Reaction Heat of Reaction (H) the amount of heat lost
or gained in a reaction Heat of Reaction: H = H products H
reactants
Slide 11
Exothermic Reactions Exothermic Reactions energy is produced by
a reaction; energy flows from the system to the surroundings H is
negative because the reaction loses heat.
Slide 12
Exothermic Reaction Graph
Slide 13
Endothermic Reactions Endothermic Reactions energy is gained by
a reaction; energy flows from the surroundings into the system H is
positive because the reaction gains heat.
Slide 14
Endothermic Reaction Graph
Slide 15
Slide 16
Classify the reaction: 2H 2 O + 572kJ -> 2H 2 + O 2
1.Exothermic 2.Endothermic
Slide 17
H of the reaction = -560kJ 1.Exothermic 2.Endothermic
Slide 18
After the reaction, your hand gets burnt from the heat. The
reaction must be: 1.Exothermic 2.Endothermic
Slide 19
Heat Values in Chemical Reactions Heat of Reaction is a
stoichiometric value and is proportional to the coefficients of the
reactants and products. 2H 2 O + 572kJ -> 2H 2 + O 2 Therefore,
for every 2 moles of water that react, 572kJ of energy are
required.
Slide 20
Reaction Rate Reaction rate rate of disappearance of one of the
reactants ( or the rate of appearance of one of the products)
-Measured in = Molarity s Change in concentration (molarity) over
time.
Slide 21
Factors Affecting Reaction Rate What factors could affect the
rate that a reaction takes place?
Slide 22
Factors affecting reaction rate Temperature Pressure Surface
Area exposed Concentration of reactants and products Catalyst
Slide 23
Temperature Temperature as temperature goes up, the average
kinetic energy increases. As velocity goes up, more collisions
occur. Temperature, rate
Slide 24
Slide 25
Factors Affecting Reaction Rate Pressure as the volume of a
closed container decreases, the pressure of a gas increases,
causing more collisions (P, rate )
Slide 26
Surface Area Surface area exposed As more surface area is
exposed, more collisions can occur. Surface Area , rate Note:
crunching up a piece of candy or crushing a tablet into a powder
makes smaller pieces, which increases the surface area.
Slide 27
Concentration of Reactants If the reactants are more
concentrated, there are more reactants present = more collisions.
Concentration of Reactants , rate
Slide 28
Presence of a Catalyst a substance that increases the rate
without being permanently changed - lowers activation energy
Slide 29
Also used: inhibitors tie up a reaction so that it does not
occur (opposite of a catalyst) - preservatives - anti-rust
agents
Slide 30
Properties of an Equilibrium Reaction Equilibrium systems are
DYNAMIC (in constant motion) DYNAMIC (in constant motion)
REVERSIBLE REVERSIBLE can be approached from either direction can
be approached from either direction Equilibrium systems are DYNAMIC
(in constant motion) DYNAMIC (in constant motion) REVERSIBLE
REVERSIBLE can be approached from either direction can be
approached from either direction
Slide 31
The Concept of Equilibrium Chemical equilibrium occurs when a
forward reaction and its reverse reaction proceed at the same
rate.
Slide 32
Chemical Equilibrium Fe 3+ + SCN - FeSCN 2+ After a period of
time, the concentrations of reactants and products are constant.
After a period of time, the concentrations of reactants and
products are constant. The forward and reverse reactions continue
after equilibrium is attained. The forward and reverse reactions
continue after equilibrium is attained.
Slide 33
The Concept of Equilibrium As a system approaches equilibrium,
both the forward and reverse reactions are occurring. At
equilibrium, the forward and reverse reactions are proceeding at
the same rate.
Slide 34
A System at Equilibrium Once equilibrium is achieved, the
amount of each reactant and product remains constant even though
the reaction is still occurring.
Slide 35
Depicting Equilibrium In a system at equilibrium, both the
forward and reverse reactions are being carried out; as a result,
we write its equation with a double arrow N 2 O 4 (g) 2 NO 2
(g)
Slide 36
Le Chteliers Principle If a system at equilibrium is disturbed
by a change in temperature, pressure, or the concentration of one
of the components, the system will shift its equilibrium position
so as to counteract the effect of the disturbance.
Slide 37
Look at the equation: N 2 (g) + 3H 2 (g) 2NH 3 (g) + energy How
can changes in conc., temp, or pressure affect this reaction?
Slide 38
N 2 (g) + 3H 2 (g) 2NH 3 (g) + energy Increase reactant
concentration, there are more collisions between reactants, so
reaction shifts to the right, towards the products.
Slide 39
N 2 (g) + 3H 2 (g) 2NH 3 (g) + energy Increase in temperature
affects both sides of reaction, but it will increase one side more
than the other. Think of energy as a product, if we increase its
concentration, the reaction will lean in the other direction to
regain equilibrium.
Slide 40
N 2 (g) + 3H 2 (g) 2NH 3 (g) + energy Increase in pressure will
demand the fewest moles of gas. Occurs with a reduction in volume.
-In this example, products will be favored -If number of moles of
gas are equal, pressure has no effect. -Only has an effect on
gases. -Decrease in pressure will have the opposite effect.
Slide 41
N 2 (g) + 3H 2 (g) 2NH 3 (g) + energy Decrease in volume will
demand the fewest moles of gas. -In this example, products will be
favored -If number of moles of gas are equal, pressure has no
effect. -Only has an effect on gases. -Increase in volume will have
the opposite effect.