Rate of Reaction

Topics: Chemical reaction, Chemical kinetics, Reaction rate Pages: 75 (10674 words) Published: November 2, 2014
Kinetics

Kinetics
6.1

Rates of reaction

6.2

Collision theory

6

16.1 Rate Expression (AHL)
16.2 Reaction mechanism (AHL)
16.3 Activation energy (AHL)

6.1 Rates of reaction
6.1.1 Define the term rate of reaction.
6.1.2 Describe suitable experimental
procedures for measuring rates of
reactions.
6.1.3 Analyse data from rate experiments.
© IBO 2007

Figure 601  An explosion is a quick reaction

D

ifferent chemical reactions occur at different rates
(i.e. speeds). Some, such as the neutralisation of a
strong acid by a strong base in aqueous solution, take place very rapidly whilst others, such as the rusting of iron, take place far more slowly. Rates of reactions should not be
confused with how far a reaction goes - this is determined
by equilibrium.
The rate of a chemical reaction is a measure of the speed
at which products are formed, measured as the change in
concentration divided by the change in time, so reaction
rate has units of mol dm–3 s–1. This is equal to the rate at which the reactants are consumed, so for a reaction:

R

[ ]
∆​[ R  ]​ 
P, rate = ____
​ ∆​  P   ​ 
​ = –​ _____
​ 
∆t
∆t

Figure 602  Corrosion is a slow reaction

161
070809 Chem Chap 6-3 for correct161 161

7/12/2007 8:22:39 AM

Chapter 6

CORE

Note the minus sign for the reactants, which is necessary
as the concentrations of reactants decreases with time
whereas the concentrations of products increases. Rate is
always positive.
The numerical value will vary according to the amount of
the substance involved in the stoichiometric equation, so
that in the reaction:

concentration. It is most common to compare initial
rates, that is, the gradient of the tangent to the curve at
t = 0. At this time the concentrations of the reagents are
accurately known, as is the temperature of the system. It
is also easiest to draw tangents at this time as this section of the curve is the most linear. Typical curves obtained for the consumption of a reagent and formation of a product
are shown in Figure 603.

MnO4– (aq) + 8 H+ (aq) + 5 Fe2+ (aq)
Mn2+ (aq) + 4 H2O (l) + 5 Fe3+ (aq)
The rate of appearance of Fe3+ is five times as great as
the rate at which MnO4– is consumed. The rate is usually
considered to apply to a product that has a coefficient of
one as the equation is usually written:

Exercise
1.

∆​[ Mn​O4​–​ ]​ _1______
[ 3+ ]
Rate = – ________
​ 
   
 
​= ​ 5 ​​ ∆​  ​Fe​  ​  ​ 
​ 
∆t
∆t
Or more simply for a reaction: a A

The equation for a reaction is:

4 NO2 (g) + 2 H2O (g) + O2 (g)

Any property that differs between the reactants and the
products can be used to measure the rate of the reaction.
Refer to Section 6.2. Whichever property is chosen, a
graph is drawn of that property against time and the rate
of reaction is proportional to the gradient of the curve or
line ignoring the sign. Changes in the gradient of similar
graphs illustrate the effect of changing conditions on the
rate of reaction, without the need to convert the units to
mol dm–3 s–1.

Gradient = rate at time t = x

A

∆​[ N​O​2​ ]​
– __
​ 1 ​ _______
​ 
   
 

2 ∆t

B

∆​[ ​O​ ​ ]​
– __
​ 1 ​ _____
​   2  
​ 
2 ∆t

C

∆​[ ​H2​ ​O ]​
– _______
​ 
   
​ 
∆t

∆​[ HN​O3​ ​ ]​
1 ​ ________
D​ __
​ 
   
​ 
2
∆t
2.

Property

In most cases the rate of reaction decreases with time
because the concentration of the reactants decreases with
time and the reaction rate usually depends on the reactant

4 HNO3 (g)

Which one of the following is not numerically equal
to the others?

b B, then

[ ]
[ ]
1 ​_____
Rate = __
​ 1 ​_____
​ ∆​  B   ​ 
​ = –​ __
​ ∆​  A   ​ 
​ 
a
∆t
∆t
b

Property

6.1

Which of the curves on the following graph shows
the greatest initial reaction rate?

Gradient = – initial rate

Gradient = – rate at time t = x
Gradient = initial rate

x

Time

Product formation

x...
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