SECTION 6.1: RATES OF CHEMICAL REACTIONS:
EXPRESSING AND MEASURING RATES

How quickly a chemical reaction occurs is a crucial factor in how the reaction affects its surroundings.
What are some examples of fast and slow chemical reactions?
fast – nitroglycerine exploding
– gasoline burning

A reaction rate is the change in the amount of reactants or products over time. Reaction rates are
expressed in several ways:
Rate = Δquantity mol
Δtime
s

slow – car rusting
– food rotting
OR
Rate = Δconcentration mol/L or mol
Δtime
s
L•s
Reaction rates are always positive. The rate expressed in terms of the reactant represents the rate at
which the [ ] of the reactant is decreasing, while the rate expressed in terms of the product represents
the rate at which the [ ] of the product is increasing.
[Reactant]
mol/L
[Product]
mol/L
time (s)


time (s)
If reactions proceeded at a constant rate, we would just need to calculate the slope of the line (graph
would be linear). However, reaction rates are not usually constant.
They usually start out faster and slow down as the reactants are being used up.
Average Rate: is the average change in concentration of a reactant or product per unit time over a given
time interval. This is represented by the slope of a line that is drawn between two points on the curve (the
secant line)
Instantaneous Rate: is the rate of the reaction at a particular time. This is represented by the slope of a
tangent line to the curve at that particular point. The tangent line touches the curve at only one point. It
does not intersect the curve.
[Product]
mol/L
tangent
line
slope (m) = rise
run
secant line
time (s)
 Reaction rates can be expressed according to the [ ] of the reactants (as they are used up) or
the [ ] of the products (as they are formed). If you determine the rate of one of the reactants
or products, you can determine the rates of the others by looking at the mole ratios in your
balanced equation.
Ex)
Dinitrogen pentoxide, N2O5, decomposes to form nitrogen dioxide and oxygen:
2 N2O5 → 4 NO2 + O2
N2O5 is consumed at a rate of 2.5x10-6 mol/(L•s). What is the corresponding rate of
formation of (a) NO2 and (b) O2 ?
2 N2O5
2.5x10-6mol/L•s
→
a)
4 NO2
5x10-6mol/L•s
+
O2
-6
b) 1.25x10 mol/L•s
 There are a number of methods for measuring reaction rates, depending of the reaction under
study and the equipment available:
1) Monitoring Colour:
- in solutions that involve coloured reactants or products, the intensity of the colour of the
solution can be measured with a spectrophotometer – the absorption of light is directly
related to the [ ] of the compounds involved.
2) Monitoring Mass:
- you can tract the decrease in mass in a reaction with respect to time in reactions where a
product is lost to the surroundings (such as during the production of a gas)
Ex)
Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)
(decrease in mass can be plotted vs time as H2 gas is escaping to the surroundings)
3) Measuring pH:
- you can record changes in pH with respect to time in reactions where an acid or base are
involved.
(in above example, HCl is being consumed, therefore, the solution will become less acidic
and the pH will increase)
4) Monitoring Pressure and Volume
- When a reaction involves gases, chemists can measure the change in volume of gas or the
change in pressure
Ex) 2 N2O5(g) → 4 NO2(g) + O2(g)
2 mol

5 mol
Factors that Affect Reaction Rates:
a) Concentration: Increased concentration, increases
(as reactants are used up, reaction slows down)
rate of reaction
increases
rate of reaction
b) Temperature: Increased temperature,
c) Surface area: Increased surface area,
increases
rate of reaction
(increase surface are by breaking reactant into smaller pieces or powder form)
Why does this occur?
Collision Theory: molecules of reactants must collide with each other for reactants
to form products
# of collisions =


Rate of Reaction
temperature, molecules move faster, therefore more collisions
concentration and surface area, more molecules available to collide
d) Nature of Reactants:
 Generally, a more complicated reaction will react slower than a simple one
Ex)
Pb2+ + Cl- → PbCl2
H2O2 → H2O + ½ O2
H–O–O–H
O O=O
H H
fast
* ions attracting to form ionic bond
slow
* breaking/forming bonds
* the more bonds breaking/forming the SLOWER the reaction
e) Catalysts:
H2O2 → H2O + ½ O2
slow
H2O2 + MnO2 → H2O + ½ O2 + MnO2
catalyst
fast
catalyst
Catalyst: substance that increases the rate of reaction without being consumed.
 Enzymes are biological catalysts (eg. Lactase, protease)
 Enzymes and catalysts hold molecules in the correct orientation for the reaction to
occur (or holds reactants closer together)