Chemical Kinetics. An Introduction. CO(g) + NO 2 (g) CO 2 (g) + NO. H 2 O 2 (aq) H 2 O(l) + O 2 (g). S 2 O 8 2- + 2 I - I 2 + 2 SO 4 2-. Questions about reactions. ?. What’s happening?. The chemical equation.
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CO(g) + NO2(g) CO2(g) + NO
H2O2(aq) H2O(l) + O2(g)
S2O82- + 2 I- I2 + 2 SO42-
The chemical equation
2 IO3- + 5 HSO3-→ I2 + 5 SO42- + 3 H+ + H2O
2.How fast is it happening?
H2O2(aq) → H2O(l) + O2(g) -- slow
H2O2(aq) + catalyst → H2O(l) + O2(g) -- FAST
3.To what extent does it occur?
HC2H3O2 + H2O C2H3O2- + H3O+
The study of reaction rates and the sequence of steps by which a reaction occurs
Speed of a car:
For a chemical reaction we want to track concentration of products or reactants over time:
Concentration of reactants decreases
Concentration of products increases
Expressing Reaction Rate Cont’d
For the reaction A → B
Rate of reaction =
Note the sign:
A is disappearing
General Rate of Reaction
a A + b B → c C + d D
Rate of reaction = rate of disappearance of reactants
= rate of appearance of products
C2H4 (g) + O3 (g) C2H4O(g) + O2 (g)
Rate = - = -
Rate = -
(1.10x10-5mol/L) - (3.20x10-5mol/L)
Rate = -
= -3.50 x 10-7mol/L·s
60.0 s - 0.0 s
The reaction between ethylene and ozone:
This rate is the average rate for a time period
Does not show that rate is changing with time
Does not show rate at a given instant
Rate is dependent on concentration!
Can see this experimentally
Plot of [C2H4] and [O2] vs. Time
Reaction Rate Law
For a chemical reaction:
aA + bB + . . . →cC + dD + . . .
The rate law for the forward reaction has the form:
Rate = k [A]m[B]n . . .
For example, if the rate doubles when the concentration of A doubles, the rate depends on [A]1, so m = 1; if the rate quadruples when the concentration of B doubles, the rate depends on [B]2, so n = 2.
More on this next week
Molecules must collide in order to react.
Reaction rate is proportional to the
concentration of reactants.
Rate = k (collision frequency) = k (concentration)
Molecules must mix in order to collide.
The physical state (solid, liquid, gas) will affect frequency of collisions, as well as the physical size of droplets (liquid) or particles in the case of solids.
Molecules must collide with enough energy to react.
Raising the temperature increases the reaction rate by increasing the number of collisions per time unit, and especially, the energy of the collisions.
Nature of the reactants
Some species are more reactive than others.
You have seen this with the periodicity of reactivity. For example the reactivity of the group 1 metals.
Presence of a catalyst
Catalysts can provide alternate, lower energy, reaction pathways.
Catalysts generally reroute the pathway of a chemical reaction so that this “alternate” path, although perhaps more circuitous, has a lower activation energy for reaction than the un-catalyzed reaction.