Download 3 - Rates

Consider this reaction:
N2(g) + 3H2(g) ----> 2 NH3(g) + 92 kJ
DG298K = -16.8 kJ
What does this equation tell us?
N2(g) + 3H2(g) ----> 2 NH3(g) + 92 kJ
DG298K = -16.8 kJ





The reaction is spontaneous at 298K
It is exothermic
entropy is decreasing
enthalpy is decreasing
the names and states of reactants and
products are given
 for every 3 mol of H2(g) and 1 mol of
N2(g) consumed 2 mol of NH3(g) is
produced
 92 kJ of energy is produced
N2(g) + 3H2(g) ----> 2 NH3(g) + 92 kJ
DG298K = -16.8 kJ
What does this equation NOT tell us?
 How much of each substance we started
with
 how much of each substance is consumed
 How fast the reaction is
 Whether or not the reaction proceeds
directly as shown in the equation or if
intermediate steps occur (reaction
mechanism)
Consider this reaction:
4 HBr(g) + O2(g) --> 2 H2O + 2 Br2(g)
This reaction proceeds through these steps
HBr + O2 ---> HOOBr (slow)
HOOBr + HBr ---> 2 HOBr (fast)
2 HOBr + 2 HBr --> 2 H2O + 2 Br2(g) (fast)
In this case the equation simply tells us the
reactants and products. It doesn’t tell us the
series of steps by which the reaction goes.
When we refer to reaction rates we are
referring to how fast a reaction goes. Rates
can be expressed both qualitatively and
quantitatively.
In a qualitative way one would describe a
reaction as fast or slow.
H2(g) + O2(g) ----> H2O (g)
fast
Fe + 1/2O2 + H2O ---> Fe(OH)2 slow
Quantitatively rates are expressed by
observing the rate at which a reactant
disappears or a product appears.
It may be expressed as the change in
mol/unit time or
the change in concentration/unit time
Mg(s) + 2 HCl(aq) ---> MgCl2(aq) + H2(g)
2.4 g/24 g/mol
# mol of Mg consumed
Rate =
=
Time required to disappear
=
2.2 x 10-3 mol/s
45 s
See Saunders - Chapter 15Lesson 2 For Measuring
Rates then conduct an
exercise using burettes
Chemical reactions only occur when reacting
particles collide with sufficient energy, and at a
favourable geometry.
H2
H2
Cl2
Cl2
H2
Cl2
Fast enough, but the wrong geometry
Fast enough, but the wrong geometry
Fast enough, but the wrong geometry
Fast enough, but the wrong geometry
Fast enough, but the wrong geometry
Fast enough, but the wrong geometry
Fast enough, but the wrong geometry
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, too slow
Right Geometry, sufficient speed
Activation energy reached
Right Geometry, sufficient speed
Activation energy reached
Right Geometry, sufficient speed
Activation energy reached
Right Geometry, sufficient speed
Activation energy reached
Right Geometry, sufficient speed
Activation energy reached
Right Geometry, sufficient speed
Activation energy reached
Potential
Energy
4 - If the molecules have sufficient
energy to react a short lived
activated complex is formed. At
this point no bond breaking or
bond making is occuring.
3 - At the same time new bonds are
starting to form between H and
Cl as H nuclei attract Cl
electrons and Cl nuclei attract H
electrons
2 - As the
reacting molecules5 - If the reaction
approach their electrons startproceeds
to
the
1 - The reacting
repel so the H-H and Cl-Cl bonds between
molecules are
bonds stretch, Ek decreases and
H-Cl continue
sufficientlyEp
farincreases.
to shrink until
apart so they
they reach a
have no influence
stable state
on one another
Reaction Coordinate
2.
7.6
Eaf = energy of activation, forward
Eaf= 7.6 kJ - 4.0 kJ = 3.6 kJ
Potential
Energy (kJ)
4.0
Ear= 7.6 kJ - 2.0
kJ = 5.6 kJ
1.
DH = 2.0 kJ - 4.0 kJ = - 2.0 kJ
3.
2.0
Reaction Coordinate
Construct a potential energy vs.
reaction coordinate curve for an
endothermic reaction
5.6
2.
Eaf = energy of activation, forward
Potential
Energy (kJ)
Eaf= 5.6 kJ - 1.0 kJ = 4.6 kJ
Ear= 5.6 kJ - 3.0
kJ = 2.6 kJ
3.0
3.
DH = 3.0 kJ - 1.0 kJ = 2.0 kJ
1.0 1.
Reaction Coordinate