Rates of Reaction
• Read 18.1 pg. 737-738
• Demonstration of variations in reaction rates:
Ca vs. Na vs. Li in water to produce H2 gas
Flour burning in air
8H+ + C2O42- + MnO4-  Mn2+ + 4H2O + CO2
H2SO4, H2C2O4 (oxalic), KMnO4 provide ions
• From the list predict the fastest (1) to slowest (4)
0.5 M
1.0 M water
Other
oxalic acid H2SO4
a 2.5 mL
2.5 mL 5.0 mL
b 2.5 mL
2.5 mL 5.0 mL at 60C
c 2.5 mL
2.5 mL 5.0 mL + manganous
sulfate
d 2.5 mL
5.0 mL 2.5 mL
Examples of factors affecting rate
• Read 18.2. For each of the 5 factors give one
example from today’s demonstrations
• Nature of reactants –
• Ability of the reactants to meet –
• The concentration of the reactants –
• The temperature of the system –
• The presence of catalysts –
Examples of factors affecting rate
• Nature of reactants - Na is more reactive than
Ca (it more easily loses its outer electron)
• Ability of the reactants to meet - Flour burns
faster as a fine powder because oxygen
surrounds it (heterogeneous reaction?)
• The concentration of the reactants - reaction d
has a higher H2SO4 (reactant) concentration,
and thus reacts faster; flour + pure O2
• The temperature of the system - reaction b is
faster due to an elevated temperature
• The presence of catalysts - manganous sulfate
(reaction c) is a catalyst for this reaction
Measuring Reaction Rates
Read 18.3 (740 – 42). Answer these questions:
1. What units are associated with concentration?
2. What units are associated with reaction rate?
3. What do the square brackets in [HI] indicate
(see figure 18.2, pg. 741)
4. Explain how the rate of a reaction is
determined (see fig. 18.2)?
5. Plot this data (include title, axes labels):
[HI] 0.100 0.072 0.056 0.046 0.039 0.034 0.030 0.026
Time
(s)
0
50
100
150
200
250
300
350
Measuring Reaction Rates
6. For the data, determine the rate of reaction at
i) 25 s, ii) 175 s, and iii) 325 s. Show your
work and calculations on the graph.
7. How does the rate at the beginning of a
reaction compare to the rate later in a
reaction? Explain why this makes sense
according to one of the 5 factors that affect
reaction rates (from 18.2)?
8. Explain how the rate of reaction of
2HI  H2 + I2 is determined experimentally.
9. Do PE 2 (pg. 742)
Measuring Reaction Rates
1. Concentration: mol/L or mol•L–1
2. Rate = concentration/time: (mol/L)/s or
mol•L–1•s–1
3. The square brackets "[ ]" is the symbol for
concentration (mol/L)
4. The rate of reaction is measured by:
Instantaneous slope; rise over run; slope of
the tangent at any point.
4, 5, 6
Concentration HI (mol/L)
0.12
Graph of [HI] vs. Time
0.10
0.08
rise
run
0.06
0.04
0.02
0.00
-50
0
50
100 150 200 250 300 350 400
Time (s)
Slope (rise/run) is the reaction rate in (mol/L)/s…
Measuring Reaction Rates cont
6. i) rate  (0.100-0.072)/50 = 0.00056 (mol/L)/s
= 5.6 x 10-4 mol•L-1•s-1
ii) rate  1.4 x 10-4 mol•L-1•s-1
iii) rate  0.8 x 10-4 mol•L-1•s-1
7. As the reaction proceeds, the rate decreases
because reactants are being used up (recall,
concentration of reactants affects rate)
8. HI concentration is measured indirectly by
measuring the production of I2(g) (purple) –
likely via a spectrometer
9. N2O4  2NO2, NO2 forms at 0.010 mol•L-1•s1, thus N O decomposes at 0.005 mol•L-1•s-1
2 4
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