chemical kinetics
DESCRIPTION
Chemical Kinetics. Rates of chemical reactions. Diamonds are made by converting Carbon from its graphite state. takes millions years to consume the graphite (the reactant) to form a diamond (the product). this is why we place such a high value on diamonds. - PowerPoint PPT PresentationTRANSCRIPT
Every chemical reaction has its own signature rate
• Diamonds are made by converting Carbon from its graphite state.
• takes millions years to consume the graphite (the reactant) to form a diamond (the product).
• this is why we place such a high value on diamonds
• the chemical reaction of photosynthesis (a plants ability to make its own food, which is the reaction's product along with Oxygen) takes place on a time scale as short as... ...10^-12 seconds (0.000000000001 s)
Chemical Kinetics
• We can think of reaction rate in terms of the change of concentration with time.
• IF A→B, then...
rate = - ΔA/Δt or rate = ΔB/Δt
...wherein ΔA and ΔB are changes in the concentration (the molarity) over a time period.
• The rate of reaction A→B, represented as the decrease of A molecules with time and as the increase of B molecules with time.
B molecules
A molecules
Deriving rates of reactions: you must first determine the concentration of the reactant at different intervals
of time
• Average rate – the mean rate of chan
ge in the concentration of the reactant over the whole duration of the reaction
- ΔA/Δt
• Instantaneous rate– a specific rate at a part
icular moment in time– If temperature is the s
ame and the initial concentration of the reactant used in the reaction is the same, the rate will always be the same at that moment in time every time you perform the reaction
Reactant concentration is proportional to the reaction rate...
rate is directly proportional to Br2 concentration...
)(2)()()()(2 22 gaqaqaqaq COHBrHCOOHBr
2
2
k rate
constant a provide
must we),(equation an
intoality proportion make to
α rate
Br
Br
rate constant (k)• a constant of proportiona
lity between the reaction rate and the concentration of reactant.
• plot the data in a graph:rate vs. Br2 concentration• Is it a straight line or
a curved line?The fact that the graph is a straight line shows that the rate is directly
proportional to concentration.
time (s) Br2 M reaction rate (M/s)
0.0 0.0120 4.2 x 10^-5
50.0 0.0101 3.52 x 10^-5
100.0 0.00846 2.96 x 10^-5
150.0 0.00710 2.49 x 10^-5
units for rate constant
• Because reaction rate has the units M/s, and Br2 is in M, the unit of k is...
1/s or s^-1• k is not affected by the concentration of Br
2. • To be sure, the rate is greater at a higher c
oncentration and smaller at a lower concentration of Br2, but the ratio of rate/Br2 remains the same provided the temp. remains constant.
Find the rate constant for the following reaction
time (s) Br2 M reaction rate (M/s)
0.0 0.0120 4.2 x 10^-5
50.0 0.0101 3.52 x 10^-5
100.0 0.00846 2.96 x 10^-5
150.0 0.00710 2.49 x 10^-5
)(2)()()()(2 22 gaqaqaqaq COHBrHCOOHBr
k = rate/Br2
Use the chart to determine the rate con
stant
3.49 x 10^-3 s^-1
Reaction Rates and Stoichiometry
• STOICHIOMETRY - The quantitative study of reactants and products in a chemical reaction – Balancing equations and using molar masses
to figure the amount of each substance being used as reactants to determine how much product is being converted during a reaction
– Unbeknownst to you, we have been operating this concept, but we have yet to define it...
Reaction Rates and Stoichiometry• I have introduced to you simple situations...A→B• what if...2A→B• Uh-ohhhh... What to do? What to do?• Well, this expression is stating that 2 moles of A
disappear for each mole of B that forms.• The rate of disappearance of A is twice as fast
as the rate of appearance of B. • How could we express this rate?
Reaction Rates and Stoichiometry
trate
trate
B
or A
2
1How about like this...
THE RULE:take the reciprocal of every product's coefficient and eve
ry negative reciprocal of every reactant's coefficient
YOUR TURN...
)(22(g)2(g)4(g)
(g)2(g)2(g)3(g)
OH2COO2CH
OH6NO4O54NH
.products.. of appearance
and reactants of ncedisappeara
of in terms reactions following
for these sexpression rate theWrite
g
The next step is to find the actual rate
)(22)(2)(2 22 aqgl OHOOH Molecular oxygen is reacting at the rate of 0.02
4 M/s. At what rate is H2O2?
sMt
OH
sMt
OHt
OH
t
O
sMt
Ot
OH
t
O
t
OHrate
/0048.0
)/024.0(2)(
2
1
/024.0
2
1
2
1
22
22
222
2
2222
HOMEWORK CHECK!
• 13.7• 2NO + O2 → 2NO2
• Suppose that at a particular moment during the reaction nitric oxide (NO) is reacting at the rate of 0.066 M/s.
a) At what rate is NO2 being formed?
b)At what rate is molecular oxygen reacting?
2NO + O2 → 2NO2
NO is reacting at the rate of 0.066 M/s• To express the rate of reaction we need to do three th
ings...– Use proper sign (- or +)– Determine the reciprocal of the stoichiometric coeffi
cient for each molecule in the reaction– Make all molecular rates equal to one another
t
NO
t
O
t
NO
22
2
1
1
1
2
1
2NO + O2 → 2NO2
NO is reacting at the rate of 0.066 M/s
sM
t
NO066.0
t
NO
t
NO
t
NO
sM
2
2
2
1066.0
2
1
2
1
2
1
t
NOsM
2066.0
• At what rate is NO2 being formed?
t
NO
t
O
t
NO
22
2
1
1
1
2
1GIVEN
-
_
2NO + O2 → 2NO2
NO is reacting at the rate of 0.066 M/s
sM
t
NO066.0
• At what rate is molecular oxygen reacting?
t
NO
t
O
t
NO
22
2
1
1
1
2
1GIVEN
t
O
t
O
t
NO
sM
2
2
1
1066.0
2
1
1
1
2
1
t
O
t
O
sM
sM
2
2
033.0
066.02
1
-
_
_
_
N2 + 3H2 → 2NH3Molecular hydrogen reacts at a rate of 0.074 M/s
• 13.8• Suppose that at a particular moment during the
reaction molecular hydrogen is reacting at the rate of 0.074 M/s.
a) At what rate is ammonia being formed?b) At what rate is molecular nitrogen reacting?
t
NH
t
H
t
N
322
2
1
3
1
1
1
N2 + 3H2 → 2NH3Molecular hydrogen reacts at a rate of 0.074 M/s
• At what rate is ammonia being formed? s
M
t
H074.02
t
NH
t
H
t
N
322
2
1
3
1
1
1
t
NH
t
NH
t
H
sM
3
32
2
1074.0
3
1
2
1
3
1t
NHsM
3074.0
3
2
_
-
_
t
NHsM
3049.0
GIVEN
N2 + 3H2 → 2NH3Molecular hydrogen reacts at a rate of 0.074 M/s
• At what rate is molecular nitrogen reacting? s
M
t
H074.02
t
NH
t
H
t
N
322
2
1
3
1
1
1
-
GIVEN
t
N
t
N
t
H
sM
2
22
1
1074.0
3
1
1
1
3
1
t
NsM
2074.0)1(
3
1
t
NsM
2025.0
-