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OBJECTIVES:
•What is a chemical reaction?
•Definition-Rates of reaction

•Collision theory for reaction
•Factors affecting reaction rates

•Rate curves
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What is a chemical reaction?
A chemical reaction is a change occurring when two or
more substances react to form new substances with
different characteristics and properties,
i
...
change of reactants to products
...
e
...


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Rate of a reaction
This is a measured change in the amount of a specific
reactant or product per unit time
...
e
...


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FORMATION OF STALACTITE AND STALAGMITE

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Measuring reaction rates
Property changes that can be measured easily, include:
 The volume of a gaseous product

 Colour intensity
 Amount of a precipitate formed and temperature
 Decrease in mass (if a gas is evolved)
 Electrical conductivity
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Syringe apparatus-used to
measure the volume of a gas

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Used to measure the decrease in mass of a substance
...


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Collision Theory (contd)
1
...


(Must have successful collision)

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Collision Theory (contd)
2
...

This energy is called the activation energy
...


It is the energy that reactants must be given, in excess of
the energy they normally possess, in order to start
forming products
...


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Collision Theory (contd)
3
...


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The three (3) conditions described on the previous
slides, can be used to explain the change(s) in the
reaction rate, as the reaction proceeds
...
e
...
Since the concentration of reactants is greatest
then, the molecules will collide frequently thus

causing a fast reaction
...
The reaction will

stop when one or both of the reactants is/are used up
...

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c
con

of a reactants (contd)

An increase in the conc of the reactants or pressure
for gases increases the rate of a reaction,

i
...
increasing conc increases the number of particles
per unit volume-thus frequency of collision

increases
...

 Molecular equ
...
S2O32- (aq) +2H+ (aq) → S(s) +SO2 (g) +

H2O (l)
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EFFECT OF TEMPERATURE
When the temperature of a reaction mixture is
raised more heat is supplied to the reactant
particles resulting in greater internal energy

and faster movement of particles
...
More particles will have energy equal to or
greater than the activation energy
...
The particles will collide more frequently due to an
increase in the kinetic energy
...
e
...


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Examples include:
1
...


2
...

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Effect of surface area/ particle size
In reactions involving solids, reactions can only take
place when the surface of the solid comes in contact
with the other reacting particles,

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i
...
surface area only affects reactants which are in the
solid state
...
e
...


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Effect of surface area/ particle size
(contd)
Minutely divided particles have very large surface
areas
...


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Effect of surface area/ particle size
(contd)
i
...
an increase in the surface area (i
...
the smaller the
particle size) of the solid increases the rate of reaction
...


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Presence of a catalyst (contd)
A catalyst is a substance which will alter the rate of a
chemical reaction, but remains unchanged chemically,
at the end of the reaction
...
e
...


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40

Presence of a catalyst (contd)
Enzymes are biological catalysts which
speed up chemical reactions in living

organisms
...


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Presence of a catalyst (contd)
In the case of the negative catalyst (reaction
inhibitor or inhibitor- a substance that
prevents or decreases the rate of a chemical
reaction),

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the activation energy is probably raised and

decreases the rate of the reaction
...
e
...

A negative catalyst or inhibitor decreases the rate of a

reaction
...
There are
certain reactions which are influenced

by the presence of light
...
e
...


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© Conversion of silver halides to grey
metallic silver in the presence of light as is
done in photography
...


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Graph showing rate of reaction between magnesium and
hydrochloric acid
...

a
...

b
...


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Question contd
...
If 4g of magnesium ribbon and 50cm3 of hydrochloric
acid of concentration 0
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d
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 The total volume of gas evolved?

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Question contd
...
What effect, if any would using 100cm3 of
hydrochloric acid of concentration 0
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Question contd
...


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Rate Curves
When the change in concentration of a
reactant or product is plotted against

time a rate curve is obtained
...


Ms
...
Smith

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Note that the curve passes through the origin and this
implies that at the beginning there is no carbon
dioxide
...


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The rise is steepest near the beginning because the
concentration of reactants is at its highest at the start
of the reaction, therefore, the number of collisions

between particles is at its highest
...


This causes the rate to slow down until the reaction
stops
...
e
...


This suggests that there are no more particles of the
reacting substances left to collide
...
e
...


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Rate curve showing the effect of any
factor which increases the reaction
rate
...


Curve B has a steeper slope- the rate of reaction is
faster
...
Curve B becomes horizontal sooner- the reactants

are used up faster
...
e
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It is the conc
...


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Limiting reactant or limiting
reagent
 The reaction will stop when all of the limiting

reactant is consumed
...


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Question contd
...
Draw a diagram of the apparatus that could be used

to carry out this investigation
...
Write an equation for the reaction between

magnesium carbonate and dilute sulphuric acid
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c
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t
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, using 200cm3 of 2M
sulphuric acid and 1 mole of magnesium carbonate
...

d
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Label this graph A
...


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Question contd
...
On the same graph, draw another curve to represent
the results that you would expect if 400cm3 (that is,
twice the volume of 2M sulphuric acid) were used to

react with the magnesium carbonate
...

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End

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